Temperature Measurement with Thermistors Portland State University Department of Mechanical Engineering ME 121: Engineering Problem Solving

Temperature Measurement Temperature can be measured with many devices  Liquid bulb thermometers  Gas bulb thermometers  bimetal indicators  RTD: resistance temperature detectors (Platinum wire)  thermocouples  thermistors  IC sensors  Optical sensors  Pyrometers  Infrared detectors/cameras  liquid crystals ME 121: Engineering Problem Solvingpage 1

IC Temperature Sensors (1) Semiconductor-based temperature sensors for thermocouple reference-junction compensation Packaged suitable for inclusion in a circuit board Variety of outputs: analog (voltage or current) and digital More useful for a manufactured product or as part of a control system than as laboratory instrumentation. Examples (circa 2010) Manufacturer Part number Analog Devices Dallas Semiconductor Maxim National Instruments AD590, AD22103, TMP35, TMP36, TMP37 DS1621, DS18B20 Max675, REF-01, LM45 LM35, LM335, LM75, LM78 ME 121: Engineering Problem Solvingpage 2

Example: TMP36 from Analog Devices Don’’ t confuse the TO-92-3 package with a transistor! See, e.g., part number TMP36GT9Z-ND from $1.42 each (Qty 1) in Feb 2013 See tmp36-temperature-sensor/ overview for instructions on how to use the TMP36. page 3 IC Temperature Sensors (2)

Thermistors (1) A thermistor is an electrical resistor used to measure temperature. A thermistor is designed such that its resistance varies with temperature in a repeatable way. A simple model for the relationship between temperature and resistance is ∆T = k∆R A thermistor with k > 0 is said to have a positive temperature coefficient (PTC). A thermistor with k < 0 is said to have a negative temperature coefficient (NTC). Photo from YSI web site: ME 121: Engineering Problem Solvingpage 4

Thermistors (2)  NTC thermistors are semiconductor materials with a well-defined variation electrical resistance with temperature  Mass-produced thermistors are interchangeable: to within a tolerance the thermistors obey the same T = F (R) relationship.  Measure resistance, e.g., with a multimeter  Convert resistance to temperature with calibration equation Note:The Arduino cannot measure resistance. We will use a voltage divider to measure the change in resistance with temperature. ME 121: Engineering Problem Solvingpage 5

Thermistors (3) Advantages  Output is directly related to absolute temperature – no reference junction needed.  Relatively easy to measure resistance  Sensors are interchangeable (±0.5 ◦ C) Disadvantages  Possible self-heating error  Each measurement applies current to resistor from precision current source  Measure voltage drop ∆V, then compute resistance from known current and ∆V.  Repeated measurements in rapid succession can cause thermistor to heat up  Can be more expensive than thermocouples for comparable accuracy: $10 to $20/each versus $1/each per junction. Thermistors costing less than $1 each are available from electronic component sellers, e.g. Digikey or Newark.  More difficult to apply for rapid transients: slow response and self-heating ME 121: Engineering Problem Solvingpage 6

Thermistors (4) Calibration uses the Steinhart-Hart equation T = 1 c 1 + c 2 ln R + c 3 (ln R) 3 Nominal resistance is controllable by manufacturing. Typical resistances at 21 ◦ C: 10 kΩ, 20 kΩ, kΩ. ME 121: Engineering Problem Solvingpage 7

Resistance Measurement Resistancecanbemeasuredifaprecisioncurrent source is available. If I is known and Vis measured, then R is obtained with Ohm’s law R =R = VIVI RV I For a typical ohmmeter, the current source and voltage measurement are inside the device, and leads connect the current source to the resistance element. RV I leads ohmmeter ME 121: Engineering Problem Solvingpage 8

Direct Resistance Measurement of Thermistors (1) Two-wire resistance measurement: R T =. V I Ohmmeter Thermistor V R T Resistance in the lead wires can lead to inaccurate temperature measurement. ME 121: Engineering Problem Solvingpage 9

Direct Resistance Measurement of Thermistors (2) Four-wire resistance measurement eliminates the lead resistance 1 Ohmmeter R lead Thermistor R lead R T R lead V 1 Sketch adapted from Hints for MakingBetter DigitalMultimeter Measurements,Agilent TechnologiesCorporation, ME 121: Engineering Problem Solvingpage 10

A Voltage Divider for Thermistors (1) Using an Arduino, we do not have ready access to a precision voltage source. We could assemble a board using high precision voltage sources, but for less effort we could just buy a temperature measurement chip like the LM334 or TMP36. Instead, we will use our familiar strategy of measuring resistance with a voltage divider. thermistor 10 kΩ 5V Analog input ME 121: Engineering Problem Solvingpage 11

Arduino code for Thermistor measurement int thermistor_reading( int power_pin, int read_pin) { int reading; digitalWrite(power_pin, HIGH); delay(100); reading = analogRead(read_pin); digitalWrite(power_pin, LOW); return(reading); } float thermistor_reading_ave( int power_pin, int read_pin, int nave) { inti, reading; float sum; digitalWrite(power_pin, HIGH); delay(10); for (i=1; i<=nave; i++) { sum += analogRead(read_pin); } digitalWrite(power_pin, LOW); return(sum/float(nave)); } ME 121: Engineering Problem Solvingpage 12