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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
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