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PHYS 352

Transducers for Temperature Measurements

Temperature Transducers   temperature is a key concept for almost every scientific

or engineering process   let’s look at temperature transducers both as important

instruments on their own…   and considering them from the perspective of

measurement system design → selection of transducer based upon performance/physical characteristics

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Resistance Thermometry   electrical resistance in a material is temperature

dependent why?   most material properties are temperature dependent

  measuring resistance lets you measure temperature   after you calibrate versus standards (e.g. triple point of

water 271.13K at partial pressure of 611.73 Pa)   called RTD (resistive temperature device) if a metal   called a thermistor if a semiconductor

Why Does R(T)?   in a semiconductor, electrons can be

thermally excited above the band gap. The lower the temperature, the fewer the electrons in the conduction band, therefore the higher the resistance

  in metals, the dominant loss of energy by conducting electrons is through scattering off phonons – the thermal vibration of the atoms in the solid lattice. This scattering increases as temperature increases (more vibrations, larger amplitude), therefore resistance increases with temperature

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Resistance Thermometers   RTD – resistive temperature device

chemical stability is important, purity of material, consistency of mechanical assembly (small size) to achieve stability and reproducibility

Platinum RTD   typical design is platinum wire, wound in a package, or thin

film on a substrate with 100Ω resistance at a reference temperature of T0 = 0°C   known in industry as PT100

  platinum is very linear over a wide range

  works out to about 0.385 Ω/°C over the range from 0-100°C

  time (chemical) stability, reproducibility (pure metal fabrication)   use Pt RTD for critical applications

  thin film packaging less rugged, but better thermal contact possible   resistance can be made much higher (1000Ω devices common) why?

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Pt RTD Specs   international standards exist for

platinum (the only resistance thermometer with such)

  thermal expansion coefficient (linear)?   9 × 10−6 /°C L = L0 (1 + aT)   small in comparison, not a concern

  sensitivity?: other devices higher   not very small hence largish heat capacity and moderately

slow response speed   expensive, not just for Pt, but for the care required in

assembly   durability of packaging (for reproducibility: don't smack it

around and avoid extreme thermal shock)

Pt RTD Considerations

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  ohmmeters send a known current through a resistor and measure the voltage

  1 mA sense current, through 100 Ω   P = I2R or 100 µW

  self-heating causes a rise in temperature   worst case is RTD in still air

  up to 1°C higher reading per mW of heat produced   a 1 mA sense current through 100Ω is 100 mV signal (not a

huge signal)   0.385 Ω/°C → 0.385 mV/°C resulting in fairly poor sensitivity

RTD: Self-Heating Effect

Thermistor   semiconductor RTD   most have negative temperature coefficient (NTC) but

some can be positive (PTC)   less reproducible than Pt RTD   calibration is less stable

  chemical stability not as good   long term drift from thermal cycling

  RT can be Ω to MΩ   sensitivity is high (steep slope)

  3% change/°C   non-linear   calibrate at many points   range is small

  choose in your range of interest   can be tiny size → fast thermal response time   cheaper than Pt RTD

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  typical NTC: α = (−0.03 to −0.05) /°C   rugged, compact, inexpensive   self-heating problem, same as RTD

  1-2°C temperature change per mW in still air at 25°C   0.13°C per mW in a well-stirred oil bath at 25 °C

  long-term drift (stability problem) due to exposure to high temperature:   +0.05% ΔR (per year at 75 °C)

Thermistor Features

thermistor

Thermistor R(T) Function

Steinhart-Hart Equation -  T in Kelvin -  a,b,c,d given by manufacturer

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  metal oxide semiconductor   e.g. doped polycrystalline BaTiO3   resistance is through grains and across grain

boundaries   material is ferroelectric   above Curie temperature, depolarization

results in large potential barriers at grain boundaries

  resistance shoots way up   use of PTC materials:

  self-limiting heater is more common than thermometry

  e.g. rear window defroster   e.g. hair dryers, overload or overheat fuse

PTC Thermistors just for interest!

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Accurately Measuring Resistance   the RTD or thermistor is the sensor  what about the resistance of the wires

used to connect to the RTD or thermistor?  what about the temperature dependence

of the resistance of those wires?

  2-wire: for when you just don’t care!   maybe be just fine for thermistors with high R   just accept the error introduced by the

resistance of the leads (and their temperature dependence)

How to Measure Resistance?

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How to Measure Resistance?   4-wire: simple in concept, doubles the amount

of wire you use   one pair of wires supplies the sense current   one pair of wires measures the voltage with very

high input impedance   little current flows in this pair

measure resistance (1) across top wire and bottom wire: 2rL+R (2) across top two: 2rL (provided the three wires are identical) rL1 = rL2 = rL3

subtract (2) from (1) = R

How to Measure Resistance?   3-wire: coolest in concept   “industrial” modules exist that are designed for

3-wire connection to the temperature sensor   they do the subtraction for you

switchable internal to voltmeter

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  for very sensitive measurements of resistance   if the ratio R2/R1 = Rx/R3, then the voltage reading is

zero   R2 is a variable resistor; Rx is the unknown

  adjust until the voltage reading is zero   cannot do exactly since Rx is function of T   measure the unbalanced signal

  look for deviation about a null reading

  note: does not overcome problems such as resistance of leads, temperature variation of the lead wires, contact resistance, etc.   although one can make the current through R3 and Rx very

small, reducing self-heating effects   this is just a way to sense small resistance changes,

perhaps smaller than what an ordinary ohmmeter could resolve accurately

Wheatstone Bridge

this is the RTD