After time, temperature is the variable most frequently measured. The three most common types of contact electronic temperature sensors in use today are thermocouples, resistance temperature detectors (RTDs), and thermistors. This article will examine the negative temperature coefficient (NTC) thermistor.

 

Photo 1. NTC thermistors are manufactured in a variety of sizes and configurations. The chips in the center of the photo can be used as surface mount devices or attached to different types of insulated or uninsulated wire leads. The thermistor element is usually coated with a phenolic or epoxy material that provides protection from environmental conditions. For applications requiring sensing tip dimensions with part-to-part uniformity and/or smaller size, the devices can be encapsulated in PVC cups or polyimide tubes.

General Properties and Features

NTC thermistors offer many desirable features for temperature measurement and control within their operating temperature range. Although the word thermistor is derived from THERMally sensitive resISTOR, the NTC thermistor can be more accurately classified as a ceramic semiconductor. The most prevalent types of thermistors are glass bead, disc, and chip configurations (see Photo 1), and the following discussion focuses primarily on those technologies.

Temperature Ranges and Resistance Values. NTC thermistors exhibit a decrease in electrical resistance with increasing temperature. Depending on the materials and methods of fabrication, they are generally used in the temperature range of -50°C to 150°C, and up to 300°C for some glass-encapsulated units. The resistance value of a thermistor is typically referenced at 25°C (abbreviated as R25). For most applications, the R25 values are between 100 and 100 k . Other R25 values as low as 10 and as high as 40 M can be produced, and resistance values at temperature points other than 25°C can be specified.

Accurate and Repeatable R/T Characteristic. The resistance vs. temperature (R/T) characteristic (also known as R/T curve) of the NTC thermistor forms the "scale" that

allows its use as a temperature sensor. Although this characteristic is a nonlinear, negative exponential function, several interpolation equations are available that very accurately describe the R/T curve [1,2,3]. The most well known is the Steinhart-Hart equation: 1/T = A + B(lnR) + C(lnR)3

where: T = kelvin temperature R = resistance at temperature T

Coefficients A, B, and C are derived by calibrating at three temperature points and then solving the three simultaneous equations. The uncertainty associated with the use of the Steinhart-Hart equation is less than ±0.005°C for 50°C temperature spans within the 0°C-260°C range, so using the appropriate interpolation equation or lookup table in conjunction with a microprocessor can eliminate the potential nonlinearity problem.

Sensitivity to Changes in Temperature. The NTC thermistor's relatively large change in resistance vs. temperature, typically on the order of -3%/°C to -6%/°C, provides an order of magnitude greater sensitivity or signal response than other temperature sensors such as thermocouples and RTDs. On the other hand, the less sensitive thermocouples and RTDs are a good choice for applications requiring temperature spans >260°C and/or operating temperatures beyond the limits for thermistors.

 

  Figure 1. Over the range of -50°C to 150°C, NTC thermistors offer a distinct advantage in sensitivity to temperature changes compared to other temperature sensors. This graph illustrates the R/T characteristics of some typical NTC thermistors and a platinum RTD.

Interchangeability.

Another important feature of the NTC thermistor is the degree of interchangeability that can be offered at a relatively low cost, particularly for disc and chip devices. Interchangeability describes the degree of accuracy or tolerance to which a thermistor is specified and produced, and is normally expressed as a temperature tolerance over a temperature range. For example, disc and chip thermistors are commonly specified to tolerances of ±0.1°C and ±0.2°C over the temperature ranges of 0°C to 70°C and 0°C to 100°C. Interchangeability helps the systems manufacturer or thermistor user reduce labor costs by not having to calibrate each instrument/system with each thermistor during fabrication or while being used in the field. A health care professional, for instance, can use a thermistor temperature probe on one patient, discard it, and connect a new probe of the same specifications for use on another patient--without recalibration. The same holds true for other applications requiring reusable probes.

Small Size.

The small dimensions of most bead, disc, and chip thermistors used for resistance thermometry make for a very rapid response to temperature changes. This feature is particularly useful for temperature monitoring and control systems requiring quick feedback.

Remote Temperature Sensing Capability.

Thermistors are well suited for sensing temperature at remote locations via long, two-wire cable because the resistance of the long wires is insignificant compared to the relatively high resistance of the thermistor.

Ruggedness, Stability, and Reliability.

As a result of improvements in technology, NTC bead, disc, and chip thermistor configurations are typically more rugged and better able to handle mechanical and thermal shock and vibration than other temperature sensors.

Materials and Configurations

Most NTC thermistors are made from various compositions of the metal oxides of manganese, nickel, cobalt, copper, and/or iron. A thermistor's R/T characteristic and R25 value are determined by the particular formulation of oxides. Over the past 10 years, better raw materials and advances in ceramics processing technology have contributed to overall improvements in the reliability, interchangeability, and cost-effectiveness of thermistors.

Of the thermistors shown in Figure 2, beads, discs, and chips are the most widely used for precise temperature measurements. Although each configuration is produced by a unique method, some general ceramics processing techniques apply to most thermistors:

formulation and preparation of the metal oxide powders; milling and blending with a binder; forming into a "green" body; heat-treating to produce a ceramic material; addition of electrical contacts (for discs and chips); and, for discrete components, assembly into a usable device with wire leads and a protective coating.

 

Figure 2. A variety of manufacturing processes are used to make NTC thermistors configured as beads (A), chips (B), discs (C), rods (D), and washers (E).

Historical Note on the Thermistor

Michael Faraday (1791-1867), the British chemist and physicist, is best known for his work in electromagnetic induction and

electrochemistry. Less familiar is his 1833 report on the semiconducting behavior of Ag2S (silver sulfide), which can be considered the first recorded NTC thermistor [9].

Because the early thermistors were difficult to produce and applications for the technology were limited, commercial manufacture and use of thermistors did not begin until 100 years later. During the early 1940s, Bell Telephone Laboratories developed techniques to improve the consistency and repeatability of the manufacturing process [10]. Some of the first commercial thermistors were the disc type, and by today's standards, their tolerances were quite broad. These devices were used primarily for regulation, protection, and temperature compensation of electronic circuits.

In the 1950s and 1960s, the expanding aerospace industry's requirement for more accurate and stable devices led to several improvements in the materials used to manufacture glass bead and disc thermistors. During the 1960s and 1970s, the demand for tight-tolerance devices in high volumes at a lower cost led to the development of the chip thermistor [11].

As the reliability of these devices improved during the 1980s, the use of electronic thermometers in the health care industry increased. The rising costs of sterilization and concerns about cross-infection among patients led to the demand for low-cost disposable temperature probes, for which chip thermistors were well suited. Throughout the 1980s and 1990s, the use of NTC thermistors has continued to grow in the automotive, food processing, medical, HVAC, and telecommunications markets.

Bead thermistors, which have lead wires that are embedded in the ceramic material, are made by combining the metal oxide powders with a suitable binder to form a slurry. A small amount of slurry is applied to a pair of platinum alloy wires held parallel in a fixture. Several beads can be spaced evenly along the wires, depending on wire length. After the beads have been dried, the strand is fired in a furnace at 1100°C-1400°C to initiate sintering. During sintering, the ceramic body becomes denser as the metal oxide particles bond together and shrink down around the platinum alloy leads to form an intimate physical and electrical bond. After sintering, the wires are cut to create individual devices. A glass coating is applied to provide strain relief to the lead-ceramic interface and to give the device a protective hermetic seal for long-term stability. Typical glass bead thermistors range from 0.01 in. to 0.06 in. (0.25 mm to 1.5 mm) in dia.

Disc thermistors are made by preparing the various metal oxide powders, blending them with a suitable binder, and then compressing small amounts of the mixture in a die under

several tons of pressure. The discs are then fired at high temperatures to form solid ceramic bodies. A thick film electrode material, typically silver, is applied to the opposite sides of the disc to provide the contacts for the attachment of lead wires. A coating of epoxy, phenolic, or glass is applied to each device to provide protection from mechanical and environmental stresses. Typical uncoated disc sizes range from 0.05 in. to 0.10 in. (1.3 mm to 2.5 mm) in dia.; coated disc thermistors generally measure 0.10 in. to 0.15 in. (2.5 mm to 3.8 mm) in dia.

Chip thermistors are manufactured by tape casting, a more recent technique borrowed from the ceramic chip capacitor and ceramic substrate industries. An oxide-binder slurry similar to that used in making bead thermistors is poured into a fixture that allows a very tightly controlled thickness of material to be cast onto a belt or movable carrier. The cast material is allowed to dry into a flexible ceramic tape, which is cut into smaller sections and sintered at high temperatures into wafers 0.01 in. to 0.03 in. (0.25 mm to 0.80 mm) thick. After a thick film electrode material is applied, the wafers are diced into chips. The chips can be used as surface mount devices or made into discrete units by attaching leads and applying a protective coating of epoxy, phenolic, or glass. Typical chip sizes range from 0.04 in. by 0.04 in. (1 mm by 1 mm) to 0.10 in. by 0.10 in. (2.5 mm by 2.5 mm) in square or rectangular shapes. Coated chip thermistors commonly measure from 0.08 in. to 0.10 in. (2.0 mm to 2.5 mm) in diameter. Very small coated chip thermistors 0.02 in. to 0.06 in. (0.5 mm to 1.5 mm) in dia. are available for applications requiring small size, fast response, tight tolerance, and interchangeability.

Washer-shaped thermistors are essentially a variation of the disc type except for having a hole in the middle, and are usually leadless for use as surface mount devices or as part of an assembly. Rod-shaped thermistors are made by extruding a viscous oxide-binder mixture through a die, heat-treating it to form a ceramic material, applying electrodes, and attaching leads. Rod thermistors are used primarily for applications requiring very high resistance and/or high power dissipation.

 

Photo 2. NTC thermistors can be attached to extension leads or jacketed cable and assembled into various types of housings. The optimum materials, dimensions, and configuration for a probe assembly are determined by careful review of the application requirements.

Comparison of Thermistor Configurations

One of the problems the thermistor industry has faced over the years is that some manufacturers have claimed their particular style or configuration of thermistor is better than other configurations made by their competitors, without regard to other, more

pertinent factors. These thermistor "politics," more harmful than beneficial to the industry, can confuse engineers and purchasing agents who are looking for reliable information to help them choose the appropriate product for their application. Although some thermistor qualities or capabilities, including interchangeability, repeatability, size, responsiveness, and stability, can either be enhanced or limited by style or geometry, these characteristics are much more dependent on a manufacturer's ability to understand the ceramics technology being used and to maintain control of the manufacturing process.

Glass-coated beads feature excellent long-term stability and reliability for operation at temperatures up to 300°C. Studies at the National Institute of Standards and Technology (NIST) and other laboratories indicate that some special bead-in-glass probes have measurement uncertainties and stabilities (better than ± 0.003°C for temperatures between 0°C and 100°C) that approach those of some standard platinum resistance thermometers [3,4,5]. The relatively small size of glass bead thermistors gives them a quick response to temperature changes, but for some applications this small size can make the devices hard to handle during assembly and have the effect of limiting their power dissipation. It is also more difficult and more expensive to produce glass beads with close tolerances and interchangeability. Individual calibration and R/T characterization, resistor network padding, or use of matched pairs are among the methods used to achieve interchangeability.

Chip and disc thermistors are noted for their tight tolerances and interchangeability at a relatively low cost compared to bead thermistors. These qualities are inherent in the manufacturing processes. The thermistors' larger size permits power dissipation higher than that of beads, although at some expense of response times. Larger size can be a disadvantage in some applications. Because of their geometry, disc thermistors normally have larger coated diameters and higher power dissipation capabilities than chip thermistors. On the other hand, chip thermistors typically can be produced to smaller coated diameters and are better suited for applications requiring smaller size and faster response times. More recent designs of chip thermistors allow the production of sizes and response times approaching those of glass beads. In some cases, chip and disc thermistors with equivalent physical and electrical characteristics can be used in the same applications without any noticeable difference in performance.

Thermistors, thermocouples, RTDs, and other sensors and electronic components exhibit a phenomenon called drift, a gradual, predictable change in certain properties over time. For a thermistor, drift results in a change in resistance from its initial value, typically after being continuously exposed to or cycled to an elevated temperature. Thermistor drift is expressed as a percent change in resistance and/or as a change in temperature that occurs at a given exposure temperature for a certain length of time. As the exposure temperature increases, so do the drift and the drift rate [4,5,6].

Chip and disc thermistors with soldered leads and an epoxy or phenolic coating have potential limitations in their maximum operating temperatures, typically 150° C for short-term exposures (1-24 hours) and 105°C for long-term exposures (1-12 months). When subjected to environmental conditions above their recommended maximum operating

temperatures, epoxy- or phenolic-coated chips and discs can begin to exhibit an undesirable, excessive amount of drift. When such thermistors are used at temperatures below the specified maximum operating temperatures, drift is minimal, on the order of 0.02°C to 0.15°C after 12 months of continuous exposure to temperatures between 25°C and 100°C, respectively. Recent advances in the techniques used to manufacture chip and disc thermistors with a glass coating have produced devices that combine the interchangeability advantage of chips and discs with the stability of glass beads [5,6]. For applications that require operating temperatures up to 200°C, these new devices offer a lower cost alternative to the conventional glass bead thermistors.

These comparisons can help determine whether a thermistor supplier is objectively evaluating an application in terms of the appropriate thermistor, or simply promoting the configuration it manufactures. For an example of the latter approach, see [7], where a manufacturer of disc thermistors stated that "Loose-tolerance thermistors are usually mass-produced by tape casting," and that "These devices . . . are designed for applications requiring neither interchangeability nor a high degree of accuracy," implying that all chip thermistors are loose tolerance. On the contrary, millions of precision chip thermistors with superior long-term stability are produced annually to an interchangeable tolerance of ±0.1°C, and they are available with an interchangeability of ±0.05°C. In reality, broad-tolerance and tight-tolerance thermistors are available in each of the three major thermistor configurations discussed above.

After determining the appropriate specifications, the engineer and purchasing agent need to evaluate which configuration and supplier will best meet the requirements for process control, quality, on-time delivery, and value at a reasonable price. An important part of the evaluation process is to perform some basic tests on the design and quality of the thermistor and, wherever possible, include simulation of the actual environmental conditions of the intended application. To achieve optimum performance, thermistors are usually mounted into protective housings or probe assemblies (see Photo 2). For additional information on sensor assembly design, see [8]. An informed decision can then be made as to which product and supplier will provide the best value for the application requirement. Part II of this article will examine the ways to perform these tests.

The full five-part article on Negative Temperature Coefficient Thermistors can be found at the Sensors Magazine web site..

References

1. J.S. Steinhart and S.R. Hart. 1968. "Calibration Curves for Thermistors," Deep Sea Research 15:497.

2. M. Sapoff et al. 1982. "The Exactness of Fit of Resistance-Temperature Data of Thermistors with Third-Degree Polynomials," Temperature, Its Measurement and Control in Science and Industry, Vol. 5, James F. Schooley, ed., American Institute of Physics, New

York, NY:875.

3. W.R. Siwek et al. 1992. "A Precision Temperature Standard Based on the Exactness of Fit of Thermistor Resistance-Temperature Data Using Third Degree Polynomials," Temperature, Its Measurement and Control in Science and Industry, Vol. 6, James F. Schooley, ed., American Institute of Physics, New York, NY:491.

4. S.D. Wood et al. 1978. "An Investigation of the Stability of Thermistors," J Res of the Nat Bur of Stds:83, 247.

5. W.R. Siwek et al. 1992. "Stability of NTC Thermistors," Temperature, Its Measurements and Control in Science and Industry, Vol. 6, James F. Schooley, ed., American Institute of Physics, New York, NY:497.

6. J.A. Wise. 1992. "Stability of Glass-Encapsulated Disc-Type Thermistors," Temperature, Its Measurement and Control in Science and Industry, Vol. 6, James F. Schooley, ed., American Institute of Physics, New York, NY:481.

7. C. Faller and F. Arment. Jun. 1996. "NTC Thermistor Update," Sensors:22.

8. D. McGillicuddy. Dec. 1993. "NTC Thermistor Basics and Principles of Operation," Sensors:42.

9. D. Hill and H. Tuller. 1991. "Ceramic Sensors: Theory and Practice," Ceramic Materials for Electronics, R. Buchanan, ed., Marcel Dekker, Inc., New York, NY:272.

10. U.S. Patent No. 2,258,646, 14 Oct. 1941.

11. R.L. Winter, former chairman and CEO, Western Thermistor Corp., private communication. Jul. 1996.

Gregg Lavenuta is President, Cornerstone Sensors, Inc., 1304 N. Melrose Dr., Ste. C, Vista, CA 92083; 760-732-0111, fax 760-732-0163.

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NTC Engineering Notes

Introduction

A negative temperature coefficient (NTC) thermistor is a two terminal solid state electronic component that exhibits a large, predictable change in resistance corresponding to changes in absolute body temperature. This change in body temperature of the thermistor can be brought about either externally via a change in ambient temperature or internally by heat resulting from current passing through the device or by a combination of these effects.

NTC thermistors are manufactured using metallic oxides of manganese, nickel, cobalt, copper, iron and other metals. They are fabricated using a mixture of two or more metallic oxides and a binder material and are then pressed into the desired configuration. The resulting material is then sintered at elevated temperatures. By varying the types of oxides, the sintering time and temperature as well as the atmosphere, a wide variety of curves and resistance values can be manufactured.

Thermistor Terminology

Thermistors exhibit a large negative change in resistance with respect to temperature, on the order of -3%/C to -6%/ºC at 25ºC. This relationship between resistance and temperature follows an approximately exponential-type curve as shown below. A few parameters will help to describe the curve and how it changes over temperature.

Figure 20: Resistance vs Temperature Graph

Resistance at 25ºC (R25)

The most common temperature used to measure the thermistor resistance and the one temperature that is most often used to reference the resistance value of the thermistor is 25ºC. For NTC thermistors, this value can vary from less than 100 to greater than 1Meg. The value at 25ºC is normally measured in a temperature controlled bath where very low power is used to measure the resistance value. When a resistance value for a thermistor is mentioned, it is the value at 25ºC that is usually being used.

Temperature Coefficient of Resistance ()

One way to describe the curve of an NTC thermistor is to measure the slope of the resistance versus temperature (R/T) curve at one temperature. By definition, the coefficient of resistance is given by:

where:

T = Temperature in ºC or K R = Resistance at Temp T

The temperature coefficient is expressed in ohms/ohms/ºC or more commonly %/ºC.

As can be seen from Figure 20, the steepest portion of the NTC curve is at colder temperatures. Depending upon the type of NTC material, the temperature coefficient at -40ºC can be as high as -8%/ºC. The flattest portion of the curve occurs at higher temperatures where, at temperatures of 300ºC, a can be less than 1%/ºC.

The temperature coefficient is one method that can be used to compare the relative steepness of NTC curves. It is important that the temperature coefficient be compared at the same temperature because, as was noted previously, a varies widely over the operating temperature range.

Resistance Ratio (Slope)

The resistance ratio, or slope, for thermistors is defined as the ratio of resistance at one temperature to the resistance at a second higher temperature. The resistance ratio is one method of describing the NTC curve. It is sometime used to compare the relative steepness of two curves. There is no industry standard for the two temperatures that are used to calculate the ratio, although some common temperature ranges are:

The value obtained by taking the resistance ratio at different temperatures will vary greatly depending upon the temperatures used. Therefore, resistance ratios cannot be used to compare thermistor curves unless the same temperature ranges are used. For example, for ATP Curve “Z”, the following ratios are obtained:

Beta value ()

A simple approximation for the relationship between the resistance and temperature for a NTC thermistor is to use an exponential approximation between the two. This approximation is based on simple curve fitting to experimental data and uses two points on a curve to determine the value of . The equation

relating resistance to temperature using is:

R = Ae(/T)

Where:

R = thermistor resistance at temp TA = constant of equation = Beta, the material constantT = Thermistor temperature (K)

To calculate Beta for any given temperature range, the following formula applies:

can be used to compare the relative steepness of NTC thermistor curves. However, as with resistance ratios, the value of will vary depending upon the temperatures used to calculate the value, although not to the extent that resistance ratio does. For example, to calculate for the temperature range of 0ºC to 50ºC for ATP curve “Z”:

T1 = 0ºC + 273.15ºC = 273.15KT2 = 50ºC + 273.15ºC = 323.15KR1 = 3.265R2= 0.3601

This value of would be referenced as 0ºC/50ºC . Using other temperatures to calculate b for curve “Z” would yield the following results:

25ºC/50ºC = 3936K25ºC/85ºC = 3976K

As you can see, it is important to know what temperatures were used to calculate the value of before it is used to compare thermistor curves. b can be used to calculate the resistance of the curve at other temperatures within the range that b was calculated once the constant A is determined. However, the accuracy of this equation is only approximately ±0.5ºC over a 50ºC span.

Steinhart-Hart Thermistor equation

The Steinhart-Hart equation is an emperically derived polynomial formula which best represents the resistance versus temperature relationship of NTC thermistors. The Steinhart-Hart equation is the best method used to describe the RvT relationship and is accurate over a much wider range of temperature than is . To solve for temperature when resistance is known, yields the following form of the equation:

1/T = a + b(LnR) + c(LnR)3

Where:

T = temperature in Kelvins (K = ºC + 273.15)a, b and c are equation constantsR = resistance in at temp T

To solve for resistance when the temperature is known, the form of the equation is:

Where:

The a, b and c constants can be calculated for either a thermistor material or for individual values of thermistors within a material type. To solve for the constants, three sets of data must be used. Normally, for a temperature range, values at the low end, middle and high end are used to calculate the constants. This will ensure the best fit for the equation over the range. Using the Steinhart-Hart equation allows for an accuracy as good as ±0.001ºC over a 100ºC temperature span. See the Steinhart-Hart Equation Constants for ATP Thermistor Chart for values of the constants. Print outs that contain resistance versus temperature data for individual parts are available by contacting ATP.

Thermistor tolerance and temperature accuracy

There are two factors to consider when discussing thermistors and their ability to measure temperature. The first is resistance tolerance and this is defined as the amount of resistance that any part will vary from its nominal value. The tolerance on the resistance at any temperature is the sum of:

a) the closest tolerance at any specified temperatureb) the additional tolerance due to deviation from the nominal curve for the material

In any application where the thermistor is to be used to measure temperature it is more appropriate to discuss the temperature accuracy for the device. The accuracy can be calculated if the resistance tolerance and are known.

There are two generally accepted methods of describing the tolerance or accuracy of a thermistor. The first is point matched. This describes a thermistor that has its tightest resistance tolerance at one temperature, the reference temperature, which is normally 25ºC. At temperatures below and above the reference temperature the resistance tolerance will become larger due to the uncertainty in the material curve. The other type of thermistor tolerance is known as curve matched or interchangeable. These thermistors are normally defined to have a certain accuracy over a range, typically ±0.2ºC from 0ºC to 70ºC.

A simple equation is used to describe the relationship between resistance tolerance and temperature accuracy. When one is known the other can be calculated.

For example, for ATP part number A1004Z-2, the resistance tolerance is ±2% @ 25ºC. Looking at the data for curve “Z” shows that the a at 25ºC is 4.4 %/ºC. Therefore, the accuracy at 25ºC can be calculated to be (±2% / 4.4%/ºC) = ±0.45ºC.

Similarly, for ATP part number A1004Z-C3, the temperature accuracy is expressed as ±0.2ºC from 0ºC to 70ºC. To calculate the resistance tolerance at 25ºC divide the temperature accuracy at the temperature by the a at that temperature. For 25ºC, the resistance tolerance would be (±0.2ºC * 4.4%/ºC) = ±0.88%.

In the data section for NTC thermistors, ATP also provides the curve deviation for parts that are point matched at 25ºC. Using this information and the value of , allows for the temperature accuracy to be calculated at any temperature. For example for curve “Z” at 50ºC for ATP part number A1004Z-2, the resistance tolerance at 25ºC is ±2%. The deviation due to the curve uncertainty is listed as ±1.2%. Therefore, the total resistance tolerance would be:

(±2%) + (±1.2%) = ± 3.2% @ 50ºC

The at 50ºC for this material is listed as -3.8%/ºC. Therefore, to calculate the temperature accuracy at 50ºC for A1004Z-2:

(±3.2%) / (-3.8%/ºC) = ±0.84ºC

The a at 50ºC for this material is listed as -3.8%/ºC. An example of how the accuracy of a thermistor changes with respect to temperature can be seen in the following graph.

Figure 21: Typical Temperature Tolerance of NTC Thermistors (Point Matched vs Interchangeable

NTC Thermistor self-heated parameters

Self-heating occurs in a thermistor when current passing through the device is such that the internal heat generated is sufficient to raise the thermistor body temperature above that of its environment. For temperature sensing applications, it is not desirable to self-heat the thermistor to any extent. Other NTC thermistor applications utilize the self heated characteristics inherent to the parts. The ability of a thermistor to dissipate power is a function of the size of the part, its geometry, lead material and size, method of mounting and any other factor that would contribute to the ability of the part to dissipate heat.

Dissipation Factor ()

The dissipation factor, , defines the relationship between the applied wattage and the thermistor self heating in terms of temperature rise. This relationship is defined as follows:

where:

P = power dissipated in watts

T = the rise in temperature (ºC

The dissipation factor () is expressed in units of mW/ºC. A particular value of will correspond to the amount of power necessary to raise the body temperature of the thermistor by 1ºC. Because the dissipation factor, , is dependent upon a number of factors, the values listed in the data sheets are for reference only.

Time constant ()

The thermal time constant for a thermistor is defined as the time required for a thermistor to change 63.2% of the difference between the initial temperature of the thermistor and that of its surroundings when no power is being dissipated by the thermistor. The value of defines a response time for the thermistor

when it has been subjected to a step change in temperature. For example, a thermistor that has been in an ambient temperature of 25ºC for a period of time long enough for it to reach equilibrium, is then moved to an environment where the temperature is 75ºC. The thermistor will not immediately indicate a resistance corresponding to the new temperature but rather will exponentially approach the new resistance value. For measurement purposes, the resistance value that corresponds to the temperature for will correspond to 63.2% of the temperature span, i.e.

T = 0.632 (70-25) = 31.6 + 25 = 56.6ºC

Therefore the temperature that the part must reach is 56.6ºC. The resistance of the part at that temperature can be calculated using the Steinhart-Hart equation or an approximation can be used. For example for ATP part number A1004Z-C3, using the equation, the value at 56.6ºC should be 2814. Therefore, to find the value for , we would monitor the resistance value of the part using a multimeter or similar instrument. The part should start at 25ºC where the resistance should be 10,000. The time that the part takes to reach 2814 once the part is moved to the new temperature of 75ºC will correspond the value of and will have the units of seconds. The factors that affect are similar to those that affect and include the mass of the thermistor, mounting, environment and other factors.

Temperature Sensor - The Thermistor

You are at:  Elements - Sensors - Thermistors Return to Table of Contents

         Thermistors are inexpensive, easily-obtainable temperature sensors.  They are easy to use and adaptable.  Circuits with thermistors can have reasonable outout voltages - not the millivolt outputs thermocouples have.  Because of these qualities, thermistors are widely used for simple temperature measurements.  They're not used for high temperatures, but in the temperature ranges where they work they are widely used.

        Thermistors are temperature sensitive resistors.  All resistors vary with temperature, but thermistors are constructed of semiconductor material with a resistivity that is especially sensitive to temperature.  However, unlike most other resistive devices, the resistance of a thermistor decreases with increasing temperature.  That's due to the properties of the semiconductor material that the thermistor is made from.  For some, that may be counterintuitive, but it is correct.  Here is a graph of resistance as a function of temperature for a typical thermistor.  Notice how the resistance drops from 100 k, to a very small value in a range around room temperature.  Not only is the resistance change in the opposite direction from what you expect, but the magnitude of the percentage resistance change is substantial.

        In this lesson you will examine some of the characteristics of thermistors and the circuits they are used in.

Why Use Thermistors To Measure Temperature? o They are inexpensive, rugged and reliable. o They respond quickly

What Does A Thermistor Look Like? o Here it is.

What Does A Thermistor Do? o A Thermistor is a temperature dependent resistor.  When

temperature changes, the resistance of the thermistor changes in a predictable way.

How Does A Thermistor's Resistance Depend Upon Temperature? o The Steinhart-Hart equation gives the reciprocal of

absolute temperature as a function of the resistance of a thermistor.

o Using the Steinhardt-Hart equation, you can calculate the temperature of the thermistor from the measured resistance.

o The Steinhardt-Hart equation is:

1/T = A + B*ln(R) + C*(ln(R))3 R in , T in oKo The constants, A, B and C can be determined from

experimental measurements of resistance, or they can be calculated from tabular data.

        Here are some data points for a typical thermistor from "The Temperature Handbook" (Omega Engineering, Inc., 1989).  (By the way, when you refer to this thermistor, you would say it has 5k at room temperature.)  

T (oC) R ()

0 16,330

25 5000

50 1801

        Using these values, we can get three equations in A, B and C.

(1/273)   = A + B ln(16330) + C (ln(16330))3

(1/298)   = A + B ln(5000) + C (ln(5000))3

(1/323)   = A + B ln(1801) + C (ln(1801))3

        This set of simultaneous linear equations can be solved for A, B and C.  Here are the values computed for A, B and C.

 A = 0.001284  B = 2.364x 10-4  C = 9.304x 10-8

Using these values you can compute the reciprocal, and therefore the temperature, from a resistance measurement.

 Using these values for A, B and C we obtain a plot of resistance vs. Kelvin temperature.

Getting the temperature from resistance

        If you have a resistance value - and that is what you will measure electrically - you then need to solve for the temperature.  Use the reciprocal of the equation above, and you will get:

T = 1/[A + B*ln(R) + C*(ln(R))3] R in , T in oK

However, if the thermistor is embedded in a circuit - like a voltage divider, for example - then you will have to measure electrical quantities - usually voltage - and work back from that electrical measurement.

      There will be situations where you need to measure a higher temperature than a thermistor can work in.  Or you may need more precision than a thermistor can give.  Consider a thermocouple or and integrated circuit sensor like the LM35.

How Do You Use A Thermistor?

        Thermistors are most commonly used in bridge circuits like the one below.  Bridge circuits are discussed in more detail in the lesson on bridge circuits.

In this bridge circuit, three resistors are constant, Ra, Rb, and Rc, while the resistive sensor, Rs, varies depending upon some physical variable - like temperature, light level, etc.  That's where the thermistor can be used.

        The thermistor can be placed anywhere in the bridge with three constant resistors, but different placements can produce different behavior in the bridge.  For example, different placements might cause the output voltage to go in different directions as the temperature changes.

Links to Related Lessons Temperature Sensors

o Thermistors o Thermocouples o LM35s

Other Sensors o Strain Gages o Temperature

Sensor Laboratories

RTD

History

The same year that Seebeck made his discovery about thermoelectricity, Sir Humphrey Davy announced that the resistivity of metals showed a marked temperature dependence. Fifty years later, Sir William Siemens proffered the use of platinum as the element in a resistance thermometer. His choice proved most propitious, as platinum is

used to this day as the primary element in all high-accuracy resistance thermometers. In fact, the Platinum Resistance Temperature Detector, or PRTD, is used today as an interpolation standard from the oxygen point (-182.96°C) to the antimony point (630.74°C).

Platinum is especially suited to this purpose, as it can withstand high temperatures while maintaining excellent stability. As a noble metal, it shows limited susceptibility to contamination.

The classical resistance temperature detector (RTD) construction using platinum was proposed by C.H. Meyers in 1932. He wound a helical coil of platinum on a crossed mica web and mounted the assembly inside a glass tube. This construction minimized strain on the wire while maximizing resistance.

Although this construction produces a very stable element, the thermal contact between the platinum and the measured point is quite poor. This results in a slow thermal response time. The fragility of the structure limits its use today primarily to that of a laboratory standard.

Another laboratory standard has taken the place of Meyers’ design. This is the bird-cageelement proposed by Evans and Burns. The platinum element remains largely unsupported, which allows it to move freely when expanded or contracted by temperature variations.

Strain-induced resistance changes over time and temperature are thus minimized, and the bird-cage becomes the ultimate laboratory standard. Due to the unsupported structure and subsequent susceptibility to vibration, this configuration is still a bit too fragile for industrial environments.

A more rugged construction technique is shown in Figure 37. The platinum wire is bifilar wound on a glass or ceramic bobbin. The bifilar winding reduces the effective enclosed area of the coil to minimize magnetic pickup and its related noise. Once the wire is wound onto the bobbin, the assembly is then sealed with a coating of molten glass. The sealing process assures that the RTD will maintain its integrity under extreme vibration, but it also limits the expansion of the platinum metal at high temperatures. Unless the coefficients of expansion of the platinum and the bobbin match perfectly, stress will be placed on the wire as the temperature changes, resulting in a strain-induced resistance change. This may result in a permanent change in the resistance of the wire.

There are partially supported versions of the RTD which offer a compromise between the bird-cage approach and the sealed helix. One such approach uses a platinum helix threaded through a ceramic cylinder and affixed via glass-frit. These devices will maintain excellent stability in moderately rugged vibrational applications.

TYPICAL RTD’s (FIgures 36 and 37)

Metal Film RTD’s

In the newest construction technique, a platinum or metal-glass slurry film is deposited or screened onto a small flat ceramic substrate, etched with a lasertrimming system, and sealed. The film RTD offers substantial reduction in assembly time and has the further advantage of increased resistance for a given size. Due to the manufacturing technology, the device size itself is small, which means it can respond quickly to step changes in temperature. Film RTD’s are presently less stable than their hand-made counterparts, but they are becoming more popular because of their decided advantages in size and production cost. These advantages should provide the impetus for future research needed to improve stability.

Metals - All metals produce a positive change in resistance for a positive change in temperature. This, of course, is the main function of an RTD. As we shall soon see, system error is minimized when the nominal value of the RTD resistance is large. This implies a metal wire with a high resistivity. The lower the resistivity of the metal, the more material we will have to use.

Table 6 lists the resistivities of common RTD materials.

METAL  RESISTIVITY OHM/CMF(cmf = circular mil foot)

 GoldSilverCopperPlatinumTungsten    

AuAgCuPtwNi

  13.008.8

9.2659.0030.0036.00

Nickel

Table 6

Because of their lower resistivities, gold and silver are rarely used as RTD elements. Tungsten has a relatively high resistivity, but is reserved for very high temperature applications because it is extremely brittle and difficult to work.

Copper is used occasionally as an RTD element. Its low resistivity forces the element to be longer than a platinum element, but its linearity and very low cost make it an economical alternative. Its upper temperature limit is only about 120ºC.

The most common RTD’s are made of either platinum, nickel, or nickel alloys. The economical nickel derivative wires are used over a limited temperature range. They are quite non-linear and tend to drift with time. For measurement integrity, platinum is the obvious choice.

Resistance Measurement

The common values of resistance for a platinum RTD range from 10 ohms for the bird-cage model to several thousand ohms for the film RTD. The single most common value is 100 ohms at 0ºC. The DIN 43760 standard temperature coefficient of platinum wire is α = .00385. For a 100 ohm wire, this corresponds to + 0.385 ohms/ºC at 0ºC. This value for α is actually the average slope from 0ºC to 100ºC. The more chemically pure platinum wire used in platinum resistance standards has an α of +.00392 ohms/ohm/ºC.

Both the slope and the absolute value are small numbers, especially when we consider the fact that the measurement wires leading to the sensor may be several ohms or even tens of ohms. A small lead impedance can contribute a significant error to our temperature measurement.

A ten ohm lead impedance implies 10/.385 ≈ 26ºC error in measurement. Even the temperature coefficient of the lead wire can contribute a measurable error. The classical method of avoiding this problem has been the use of a bridge.

The bridge output voltage is an indirect indication of the RTD resistance. The bridge requires four connection wires, an external source, and three resistors that have a zero temperature coefficient. To avoid subjecting the three bridge-completion resistors to the same temperature as the RTD, the RTD is separated from the bridge by a pair of extension wires:

These extension wires recreate the problem that we had initially: The impedance of the extension wires affects the temperature reading. This effect can be minimized by using a three-wire bridge configuration:

If wires A and B are perfectly matched in length, their impedance effects will cancel because each is in an opposite leg of the bridge. The third wire, C, acts as a sense lead and carries no current.

The Wheatstone bridge shown in Figure 41 creates a non-linear relationship between

resistance change and bridge output voltage change. This compounds the already non-linear temperature-resistance characteristic of the RTD by requiring an additional equation to convert bridge output voltage to equivalent RTD impedance.

4-Wire Ohms - The technique of using a current source along with a remotely sensed digital voltmeter alleviates many problems associated with the bridge.

The output voltage read by the dvm is directly portional to RTD resistance, so only one conversion equation is necessary. The three bridge-completion resistors are replaced by one reference resistor. The digital voltmeter measures only the voltage dropped across the RTD and is insensitive to the length of the lead wires.

The one disadvantage of using 4-wire ohms is that we need one more extension wire than the 3-wire bridge. This is a small price to pay if we are at all concerned with the accuracy of the temperature measurement.

3-Wire Bridge Measurement Errors

If we know VS and VO, we can find Rg and then solve for temperature. The unbalance voltage Vo of a bridge built with R1 = R2 is:

If Rg = R3, VO= 0 and the bridge is balanced. This can be done manually, but if we don’t want to do a manual bridge balance, we can just solve for Rg in terms of VO:

This expression assumes the lead resistance is zero. If Rg is located some distance from the bridge in a 3-wire configuration, the lead resistance RL will appear in series with both Rg and R3:

Again we solve for Rg:

The error term will be small if Vo is small, i.e., the bridge is close to balance. This circuit works well with devices like strain gauges, which change resistance value by only a few percent, but an RTD changes resistance dramatically with temperature. Assume the RTD resistance is 200 ohms and the bridge is designed for 100 ohms:

Since we don’t know the value of RL, we must use equation (a), so we get:

The correct answer is of course 200 ohms. That’s a temperature error of about 2.5ºC.

Unless you can actually measure the resistance of RL or balance the bridge, the basic 3-wire technique is not an accurate method for measuring absolute temperature with an RTD. A better approach is to use a 4-wire technique.

Resistance to Temperature Conversion

The RTD is a more linear device than the thermocouple, but it still requires curve-fitting. The Callendar-Van Dusen equation has been used for years to approximate the RTD curve:

Where:

RT = Resistance at Temperature TRo = Resistance at T = 0ºC

α =Temperature coefficient at T = 0ºC ((typically +0.00392Ω/Ω/ºC))

δ = 1.49 (typical value for .00392 platinum)β = 0    T > 0

0. 11    (typical) T < 0

The exact values for coefficients α , β, and δ are determined by testing the RTD at four temperatures and solving the resultant equations. This familiar equation was replaced in 1968 by a 20th order polynomial in order to provide a more accurate curve fit. The plot of this equation shows the RTD to be a more linear device than the thermocouple.

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RTD2

A resistance temperature detector (RTD) is a temperature sensor that senses temperature by means of changes in the magnitude of current through, or voltage across, an element whose electrical resistance varies with temperature. These types of sensors provide a change in resistance proportional to a change in temperature. Resistance temperature detectors have been used for making accurate temperature measurements. They utilize a resistance element whose resistance

changes with the ambient temperature in a precise and known manner. The resistance temperature detector may be connected in a bridge circuit which drives a display calibrated to show the temperature of the resistance element. Most metals become more resistant to the passage of an electrical current as the metal increases in temperature. The increase in resistance is generally proportional to the rise in temperature. Thus, a constant current passed through a metal of varying resistance produces a variation in voltage that is proportional to the temperature change. Platinum is the most commonly used metal for RTDs due to its stability and nearly linear temperature versus resistance relationship. Platinum also has the advantages of chemical inertness, a temperature coefficient of resistance that is suitably large in order to provide readily measurable resistance changes with temperature, and a resistance which does not drastically change with strain. Other types of RTDs include copper, nickel and nickel alloys. Resistance temperature detectors are usually fabricated by winding a fine diameter wire on a bobbin or mandrel which is made of a ceramic material such as aluminum oxide. Then, the wire-wound bobbin is coated with an cement-like insulating material and is installed in a protective tube.

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← Thermocouple ← Heat detector ← Temperature controller ← Temperature probe ← Temperature sensor ← Temperature transmitter ← Thermistor sensor

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Resistance Temperature Detectors (RTDs)

Features

Excellent long-term stability

  CustomizeThis Sensor!

-60ºC to 250ºC operation for Nickel

-200ºC to 600ºC operation for Platinum

Values from 100Ω to 1,000Ω Small size, fast response time Resistant to vibration and

thermal shock

Available in standard DIN class accuracies

Description

Resistance Temperature Detectors (RTDs) are characterized by a linear change in resistance with respect to temperature. RTDs exhibit the most linear signal with respect to temperature of any sensing device. RTDs are specified primarily where accuracy and stability are critical to the application. RTDs operate through the principal of electrical resistance changes in pure metal elements. Spectrum Sensors & Controls offers elements manufactured with platinum, the most common element, as well as nickel. The RTD element consists of a thin film of platinum or nickel which is deposited onto a ceramic substrate and laser trimmed to the desired resistance. Thin-film elements attain higher resistances with less metal and, thus, tend to be loss costly then the equivalent wire-wound element.

 

[ Resistance Temperature Detectors Datasheet (394K) ]

[ Create a Custom RTD Solution ]

[ Click Images to Enlarge ]

RTD Type   The table below compares some of the different types of base metals that are used in the construction of RTD elements.

Temperature Range

Benefits∝

(%/ºC)

-200ºC to Best 0.375

R@0ºC (Ω)

DIN Class  

Temperature (ºC)

Class A Limit

Class B Limit

-200 ±0.55ºC ±1.3ºC

-100 ±0.35ºC ±0.8ºC

0 ±0.15ºC ±0.3ºC

 

Class± Temperature Accuracy

t < 0ºC

1/2 DIN0.2 + 0.014 [T]

DIN 0.4 + 0.028

rangeBest stabilityGood linearity

to 0.385

-100ºC to Lost costBest sensitivity

0.618

-100ºC to Low costHigh sensitivity

0.518 to 0.527

Note: is normally used to distinguish between RvT ∝curves of the same element or those of different material.

100 ±0.35ºC ±0.8ºC

200 ±0.55ºC ±1.3ºC

300 ±0.75ºC ±1.8ºC

400 ±0.95ºC ±2.3ºC

500 ±1.15ºC ±2.8ºC

600 ±1.35ºC ±3.3ºC

350 ±1.45ºC ±3.6ºC

43760 [T]

2 x DIN0.8 + 0.028 [T]

Platinum 100 A, B

Platinum 500 A, B

Platinum 1,000 A, B

Nickel 100 1/2 DIN, DIN 43760

Nickel 500 1/2 DIN, DIN 43760

Nickel 1,000 1/2 DIN, DIN 43760

Research Paper

RTD characteristics for micro-thermal sensors

Gwiy-Sang Chung , a, and Chael-Han Kima

aSchool of Electrical Engineering, University of Ulsan, San 29, Mugeodong, Namgu, Ulsan 680-749,

Republic of Korea

Received 9 January 2008; 

accepted 25 February 2008. 

Available online 15 April 2008.

Abstract

The physical and electrical characteristics of MgO (medium layer) and Pt (sensor material) thin films

deposited by a reactive RF sputtering method and a magnetron sputtering method, respectively, were

analyzed as a function of the annealing temperature and time by using a four-point probe, SEM, and XRD.

After being annealed at 1000 °C for 2 h, the MgO layer showed good adhesive properties on both layers (Pt

and SiO2 layers) without any chemical reactions, and the surface resistivity and the resistivity of the Pt thin

film were 0.1288 Ω/□ and 12.88 μΩ cm, respectively. Pt resistance patterns were made on MgO/SiO2/Si

substrates by the lift-off method, and Pt resistance thermometer devices (RTDs) for micro-thermal sensor

applications were fabricated by using Pt-wire, Pt-paste, and spin-on-glass (SOG). From the Pt RTD samples

having a Pt thin film thickness of 1.0 μm, we obtained a temperature coefficient of resistor (TCR) value of

3927 ppm/°C, which is close to the Pt bulk value, and the ratio variation of the resistance value was highly

linear in the temperature range of 25–400 °C.

Keywords: Pt-RTD; Thermal sensor; MgO; TCR

Resistance Elements and RTD’sDavid J. King

( Printable Version )

INTRODUCTION

Resistance elements come in many types conforming to different standards, capable of different temperature ranges, with various sizes and accuracies available. But they all function in the same manner: each has a pre-specified resistance value at a known temperature which changes in a predictable fashion. In this way, by measuring the resistance of the element, the temperature of the element can be determined from tables, calculations or instrumentation. These resistance elements are the heart of the RTD (Resistance Temperature Detector). Generally, a bare resistance element is too fragile and sensitive to be used in its raw form, so it must be protected by incorporating it into an RTD.

Resistance Temperature Detector is a general term for any device that senses temperature by measuring the change in resistance of a material. RTD’s come in many forms, but usually appear in sheathed form. An RTD probe is an assembly composed of a resistance element, a sheath, lead wire and a termination or connection. The sheath, a closed end tube, immobilizes the element, protecting it against moisture and the environment to be measured. The sheath also provides protection and stability to the transition lead wires from the fragile element wires.

Some RTD probes can be combined with thermowells for additional protection. In this type of application, the thermowell may not only add protection to the RTD, but will also seal whatever system the RTD is to measure (a tank or boiler for instance) from actual contact with the RTD. This becomes a great aid in replacing the RTD without draining the vessel or system.

Thermocouples are the old tried and true method of electrical temperature

measurement. They function very differently from RTD’s but generally appear in the same configuration: often sheathed and possibly in a thermowell. Basically, they operate on the Seebeck effect, which results in a change in thermoelectric emf induced by a change in temperature. Many applications lend themselves to either RTD’s or thermocouples. Thermocouples tend to be more rugged, free of self-heating errors and they command a large assortment of instrumentation. However, RTD’s, especially platinum RTD’s, are more stable and accurate.

RESISTANCE ELEMENT CHARACTERISTICS

There are several very important details that must be specified in order to properly identify the characteristics of the RTD:

1. Material of Resistance Element (Platinum, Nickel, etc.)2. Temperature Coefficient3. Nominal Resistance4. Temperature Range of Application5. Physical Dimensions or Size Restrictions6. Accuracy

1. Material of Resistance ElementSeveral metals are quite common for use in resistance elements and the purity of the metal affects its characteristics. Platinum is by far the most popular due to its linearity with temperature. Other common materials are nickel and copper, although most of these are being replaced by platinum elements. Other metals used, though rarely, are Balco (an iron-nickel alloy), tungsten and iridium.

2.Temperature CoefficientThe temperature coefficient of an element is a physical and electrical property of the material. This is a term that describes the average resistance change per unit of temperature from ice point to the boiling point of water. Different organizations have adopted different temperature coefficients as their standard. In 1983, the IEC (International Electrotechnical Commission) adopted the DIN (Deutsche Institute for Normung) standard of Platinum 100 ohm at 0ºC with a temperature coefficient of 0.00385 ohms per ohm degree centigrade. This is now the accepted standard of the industry in most countries, although other units are widely used. A quick explanation of how the coefficient is derived is as follows: Resistance at the boiling point (100ºC) =138.50 ohms. Resistance at ice point (0ºC) = 100.00 ohms. Divide the difference (38.5) by 100 degrees and then divide by the 100 ohm nominal value of the element. The result is the mean temperature coefficient (alpha) of 0.00385 ohms per ohm per ºC.

Some of the less common materials and temperature coefficients are:

Pt TC   =   .003902 (U.S. Industrial Standard)Pt TC = .003920 (Old U.S. Standard)Pt TC = .003923 (SAMA)Pt TC = .003916 (JIS)Copper TC = .0042Nickel TC = 0.00617 (DIN)

Nickel TC =.00672 (Growing Less Common in U.S.)

Balco TC = .0052Tungsten = 0.0045

TC  

Please note that the temperature coefficients are the average values between 0 and 100ºC. This is not to say that the resistance vs. temperature curves are truly linear over the specified temperature range.

3. Nominal ResistanceNominal Resistance is the prespecified resistance value at a given temperature. Most standards, including IEC-751, use 0ºC as their reference point. The IEC standard is 100 ohms at 0ºC, but other nominal resistances, such as 50, 200, 400, 500, 1000 and 2000 ohm, are available.

4.Temperature Range of ApplicationDepending on the mechanical configuration and manufacturing methods, RTD’s may be used from -270ºC to 850ºC. Specifications for temperature range will be different, for thin film, wire wound and glass encapsulated types, for example.

5. Physical Dimensions or Size RestrictionsThe most critical dimension of the element is outside diameter (O.D.), because the element must often fit within a protective sheath. The film type elements have no O.D. dimension.To calculate an equivalent dimension, we need to find the diagonal of an end cross section (this will be the widest distance across the element as it is inserted into a sheath).

Permissible deviations from basic values

Class A

TemperatureºC

Deviation

ohms ºC

-200 ±0.24 ±0.55

-100 ±0.14 ±0.35

0 ±0.06 ±0.15

100 ±0.13 ±0.35

       Class B

TemperatureºC

Deviation

ohms

-200 ±056

-100 ±0.32

0 ±0.12

100 ±0.30

200 ±0.20 ±0.55

300 ±0.27 ±0.75

400 ±0.33 ±0.95

500 ±0.38 ±1.15

600 ±0.43 ±1.35

650 ±0.46 ±1.45

200 ±0.48

300 ±0.64

400 ±0.79

500 ±0.93

600 ±1.06

650 ±1.13

700 ±1.17

800 ±1.28

850 ±1.34

For example, using an element that is 10 x 2 x 1.5 mm, the diagonal can be found by taking the square root of (22 + 1.52).Thus, the element will fit into a 2.5 mm (0.98") inside diameter hole. For practical purposes, remember that any element 2 mm wide or less will fit into a 1/8" O.D. sheath with 0.010" walls, generally speaking. Elements which are 1.5 mm wide will typically fit into a sheath with 0.084" bore. Refer to Figure 1.

6. AccuracyIEC 751 specifications for Platinum Resistance Thermometers have adopted DIN 43760 requirements for accuracy. DIN-IEC Class A and Class B elements are shown in the chart on this page.

7. Response Time50% Response is the time the thermometer element needs in order to reach 50% of its steady state value. 90% Response is defined in a similar manner. These response times of elements are given for water flowing with 0. 2 m/s velocity and air flowing at 1 m/s.They can be calculated for any other medium with known values of thermal conductivity. In a 1/4" diameter sheath immersed in water flowing at 3 feet per second, response time to 63% of a step change in temperature is less than 5.0 seconds.

8. Measurement Current and Self HeatingTemperature measurement is carried out almost exclusively with direct current. Unavoidably, the measuring current generates heat in the RTD.The permissible measurement currents are determined by the location of the element, the medium which is to be measured, and the velocity of moving media. A self-heating factor, “S”, gives the measurement error for the element in ºC per milliwatt (mW). With a given value of measuring current, I, the milliwatt value P can be calculated from P = I2R, where R is the RTD’s resistance value. The temperature measurement error Δ T (ºC) can then be calculated from Δ T = P x S.

RESISTANCE ELEMENT SPECIFICATIONS

Stability: Better than 0.2ºC after 10,000 hours at maximum temperature (1 year, 51 days, 16 hours continuous).

Vibration Resistance: 50 g @ 500ºC; 200 g @ 20ºC; at frequencies from 20 to 1000 cps.

Temperature Shock Resistance: In forced air: over entire temperature range. In a water quench: from 200 to 20ºC.

Pressure Sensitivity: Less than 1.5 x 10-4 C/PSI, reversible.

Self Heating Errors & Response Times: Refer to specific Temperature Handbook pages for the type of element selected.

Self Inductance From Sensing Current: Can be considered negligible for thin film elements; typically less than 0.02 microhenry for wire wound elements.

Capacitance: For wire wound elements: calculated to be less than 6 PicoFarads; for film-type elements: capacitance is too small to be measured and is affected by lead wire connection. Lead connections with element may indicate about 300 pF capacitance.

LEAD WIRE CONFIGURATIONS

As stated previously, a Resistance Temperature Detector (RTD) element generally appears in a sheathed form. Obviously, all of the criteria applicable to resistance elements also apply here, but rather than element size, the construction and dimensions of the entire RTD assembly must be considered. Since the lead wire used between the resistance element and the measuring instrument has a resistance itself, we must also supply a means of compensating for this inaccuracy. Refer to Figure 2 for the 2-wire configuration.

FIGURE 2. 2-WIRE CONFIGURATION (STYLE 1)

The circle represents the resistance element boundaries to the point of calibration. 3- or 4-wire configuration must be extended from the point of calibration so that all uncalibrated resistances are compensated.

The resistance RE is taken from the resistance element and is the value that will supply us with an accurate temperature measurement. Unfortunately, when we take our resistance measurement, the instrument will indicate RTOTAL:

Where

RT = R1 + R2 + RE

This will produce a temperature readout higher than that actually being measured.

Many systems can be calibrated to eliminate this. Most RTD’s incorporate a third wire with resistance R3. This wire will be connected to one side of the resistance element along with lead 2 as shown in Figure 3.

This configuration provides one connection to one end and two to the other end of the sensor. Connected to an instrument designed to accept 3-wire input, compensation is achieved for lead resistance and temperature change in lead resistance. This is the most commonly used configuration.

FIGURE 3. 3-WIRE CONFIGURATION (STYLE 2)

If three identical type wires are used and their lengths are equal, then R1 = R2 = R3. By measuring the resistance through leads 1, 2 and the resistance element, a total system resistance is measured (R1 + R2 + RE ). If the resistance is also measured through leads 2 and 3 (R2 + R3), we obtain the resistance of just the lead wires, and since all lead wire resistances are equal, subtracting this value (R2 + R3) from the total system resistance (R1 + R2 + RE) leaves us with just RE, and an accurate temperature measurement has been made. A 4-wire configuration is also used. (See Figure 4.) Two connections are provided to each end of the sensor. This construction is used for measurements of the highest precision.

FIGURE 4. 4-WIRE CONFIGURATION (STYLE 3)

With the 4-wire configuration, the instrument will pass a constant current (I) through the outer leads, 1 and 4.

The voltage drop is measured across the inner leads, 2 and 3. So from V = IR we learn the resistance of the element alone, with no effect from the lead wire resistance. This offers an advantage over 3-wire configurations only if dissimilar lead wires are used, and this is rarely the case.

Still another configuration, now rare, is a standard 2-wire configuration with a closed loop of wire alongside (Figure 5). This functions the same as the 3-wire configuration, but uses an extra wire to do so. A separate pair of wires is provided as a loop to provide compensation for lead resistance and ambient changes in lead resistance.

FIGURE 5. 2-WIRE CONFIGURATION PLUS LOOP (STYLE 4)

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