1 Measurement of Temperature Practical Temperature Measurement Temperature Measurement Presentation...

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1 Measurement of Temperature Practical Temperature Measurement Temperature Measurement Presentation Defining and measuring temperature Thermal Time Constant Measurement Errors RTD’s Thermistors I.C. Sensors Thermocouples

Transcript of 1 Measurement of Temperature Practical Temperature Measurement Temperature Measurement Presentation...

Page 1: 1 Measurement of Temperature Practical Temperature Measurement Temperature Measurement Presentation Defining and measuring temperature Thermal Time Constant.

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Measurement of Temperature

•Practical Temperature Measurement

•Temperature Measurement Presentation

•Defining and measuring temperature•Thermal Time Constant•Measurement Errors•RTD’s•Thermistors•I.C. Sensors•Thermocouples

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Defining Temperature

• A scalar quantity that determines the direction of heat flow between two bodies

• A statistical measurement• A difficult measurement• A mostly empirical measurement

http://www.m-w.com/dictionary.htm Empirical: originating in or based on observation or experience

http://www.m-w.com/dictionary.htm Temperature: degree of hotness or coldness measured on a definite scale

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Temperature Systems

The Reaumur temperature scale is named after the French scientist (1683-1757). He proposed his temperature scale, in 1731. Reaumur divided the fundamental interval between the ice and steam points of water into 80 degrees, fixing the ice point at 0 Degrees and the steam point at 80 degrees. The reaumur scale, although of historical significance, is no longer in use.

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Measuring Temperature

• Don't let the measuring device change the temperature of what you're measuring.

• Response time is a function of– Thermal mass (mass of the device e.g large

Thermistor vs small Thermistor)– Measuring device (type of device e.g. RTD or

Thermocouple)• The Thermal Time Constant for a thermistor is the

time required for a thermistor to change its body temperature by 63.2% of a specific temperature span when the measurements are made under zero-power conditions in thermally stable environments.

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Thermal Time Constant

The dominant factors that affect the T.C. of a thermistor are: – The mass and the thermal mass of the thermistor itself. – Custom assemblies and thermal coupling agents that

couple the  thermistor to the medium being monitored. – Mounting configurations such as a probe assembly or

surface mounting. – Thermal conductivity of the materials used to assemble

the thermistor  in probe housings. – The environment that the thermistor will be exposed to

and  the heat transfer characteristics of that environment. • Typically, gases are less dense than liquids so

thermistors have greater time constants when monitoring temperature in a gaseous medium than in a liquid one.

http://www.betatherm.com/t_c.html

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6Thermal Time Constant

• Example: A thermistor is placed in an oil bath at 25°C and allowed to reach equilibrium temperature. The thermistor is then rapidly moved to an oil bath at 75°C. The T.C. is the time required for the thermistor to reach 56.6°C (63.2% of the temperature span [difference]).

250C

750C

1τ 5τ4τ3τ2τ

56.60C

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Temperature Errors

• What is YOUR normal temperature?

• Thermometer accuracy, resolution

• Contact time• Thermal mass of

thermometer, tongue• Human error in

reading

http://www.amstat.org/publications/jse/v4n2/datasets.shoemaker.html

95%Confidenceinterval

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The Resistance Temperature Detector (RTD)

• RTD: Most accurate, Most stable, Fairly linear– Expensive (platinum)– Slow (relative)– Needs I source (changing resistance)– Self-heating (don’t change the measurement due

to the internal current!)– 4-wire measurement (must take the resistance of

the leads into account)

http://www.minco.com/sensorsg.php

http://www.temperatures.com/sensors.html

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RTD’s• RTDs are among the most precise

temperature sensors commercially used. They are based on the positive temperature coefficient of electrical resistance.

http://www.sensorsmag.com/articles/article_index/index.htm http://www.omega.com/

http://www.efunda.com/designstandards/sensors/rtd/rtd_intro.cfm

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RTD Linearity

R=RRef[1+α(T-TRef)]

R=100[1+.00385(70-60)]

=103.85 ohms

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RTD Measurement

DDC RTD Measurement

To balance the bridge: R1R3=R2R4

Dissipation ConstantThe power in milliwatts required to raise a thermistor 1°C above the surrounding temperature is the dissipation constant.

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124-wire circuit

http://www.tiptemp.com/sense/Sense_RTD_TechData.pdf

To estimate leadwire error for a 2-wire configuration, multiply the total length of the extension leads by the resistance per foot in the table shown below. Then divide by the sensitivity of the RTD, given in the table below to obtain an error in C°.

Example: You are using a 100 platinum RTD with a TCR of 0.00385 and 50 ft. of 22 AWG leadwire.

R = 50 ft. x 0.0165/ft. = 0.825Approximate error = 0.825 / 0.385 = 2.14°C

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Thermistors

• Advantages:– High output– Fast– 2-wire

measurement

• Disadvantages– Very nonlinear– Limited range– Needs I source– Self-heating– Fragile

http://www.embedded.com/story/OEG20020125S0100

NTC Thermistor Shown

RT

R25

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14Thermistors

• Commonly used for sensing air and liquid temperatures in pipes and ducts, and as room temperature sensors.  Unlike RTD's, the temperature-resistance characteristic of a thermistor is non-linear, and cannot be characterized by a single coefficient.  

• The following is a mathematical expression for thermistor resistance1: R(T) = R0 exp[b (1/T - 1/T0)]

• Where: R(T) = the resistance at temperature T, in K, R0 = the resistance at reference temperature T0, in K, b = a constant that varies with thermistor composition T = a temperature, in K, T0 = a reference temperature (usually 298.15 K)

• Because the lead resistance of most thermistors is very small in comparison to sensor resistance, three and four wire configurations have not evolved.  Otherwise, sensing circuits are very similar to RTD's, using the Wheatstone bridge

DDC Thermistors

1Beckwith, Thomas G., Roy D. Marangoni, and John H. Lienhard V. Mechanical Measurements. New York: Addison and Wesley, 1993. Pp. 673

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15ThermistorEquation

http://www.omega.com/Temperature/pdf/44000_THERMIS_ELEMENTS.pdf

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Thermistor Curvehttp://www.workaci.com/pdf/t-19.pdf

ACI Thermistor Data

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

0 20 40 60 80 100

Degrees Centigrade

Oh

ms

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17Thermistor Circuit

The Omega Thermistor equation is:1/T =A+B*Ln(R)+C*(Ln(R))3

.003661=A+8.903B+705.65C

.0030945=A+6.698B+300.52C

.0026799=A+5.0293B+127.21C

273.15

323.15

373.15

To use this equation you write 3 simultaneouseqs. In 3 unknowns and solve. The eqs. Used the values at 0, 50 and 100 Celsius, with the Kelvin values shown on the left.

-0.01241

9.990243

14.9908

19.99065

24.99121

29.99101

34.99036

39.98917

44.98954

49.98872

54.98818

59.988

64.98751

69.98734

74.98829

79.98832

84.9899

89.99019

94.99223

99.99313

Eq. Temp

The resulting temperatures from the equationare shown here and are almost identical to thegiven values.

The resulting graph from the Eq. is indistinguishable From the original graph from the table.

The final equation is:1/T =A+B*Ln(R)+C*(Ln(R))3 with A = 1.472E-3, B=237.5E-6, and C=105.9E-9

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18I.C. Sensors

• Advantages– High output– Most linear– Inexpensive

•Disadvantages–Limited variety–Limited range–Needs V source–Self-heating

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I.C. Sensors

• AD590 (Analog Devices)– Current Output – Two Terminal IC Temperature Transducer– Produces an output current proportional to absolute

temperature. For supply voltages between +4 V and +30 V the device acts as a high impedance, constant current regulator passing 1 µA/K.

• LM34 (National Semiconductor)– The LM34 is a precision integrated-circuit temperature

sensor, whose output voltage is linearly proportional to the Fahrenheit temperature.

LM34: $2.33 from DigiKeyAD590: $5.24 from Analog Devices

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AD590 & LM34Circuits

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Conversion from Kelvin to Fahrenheit

We know that 273.150K = 00C = 320F AND 373.150K=1000C=2120F so

we can write two linear equations in two unknowns.32 = 273.15m + b212=373.15m + b

Solving these for m and b yields:

0F = 1.8*(0K) – 459.67

the linear conversion equation is

m = 1.8b = -459.67

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AD590 Conversion to Fahrenheit in mV

0F = -1.8*(0K) + 459.67 in mvolts

AD590

+10

1KΩ +

-100KΩ

180KΩ180KΩ-.45967volts

Use an inverting amplifier to get positive output

1mV/0K -1mV/0F

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Thermocouples

• Advantages:– Wide variety– Cheap– Wide T. range– No self-heating

• Disadvantages– Hard to measure– Relative T. only– Nonlinear– Special connectors

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Seebeck and Peltier Effects

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25Thermocouples

Seebeck coefficient   in a circuit exhibiting the Seebeck effect, the ratio of the open-circuit voltage to the temperature difference between the hot and cold junctions.

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27Thermocouples

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29Thermocouples

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Summary

• Defining and measuring temperature• Thermal Time Constant• Temperature Errors• RTD’s• Thermistors• I.C. Sensors• Thermocouples• Next