Temperature Measurement with Thermistors Portland State University Department

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 Ø Pyrometers Ø Infrared detectors/cameras Ø liquid crystals ME 121: Engineering Problem Solving page 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 Analog Devices Dallas Semiconductor Maxim National Instruments ME 121: Engineering Problem Solving Part number AD 590, AD 22103, TMP 35, TMP 36, TMP 37 DS 1621, DS 18 B 20 Max 675, REF-01, LM 45 LM 35, LM 335, LM 78 page 2

IC Temperature Sensors (2) Example: TMP 36 from Analog Devices Don’’ t confuse the TO-92 -3 package with a transistor! See, e. g. , part number TMP 36 GT 9 Z-ND from www. digikey. com. $1. 42 each (Qty 1) in Feb 2013 See http: //learn. adafruit. com/ tmp 36 -temperature-sensor/ overview for instructions on how to use the TMP 36. page 3

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). ME 121: Engineering Problem Solving Photo from YSI web site: www. ysitemperature. com page 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 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 Solving page 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 Ø Each measurement applies current to resistor from precision current source Ø Measure voltage drop ∆ V , then compute resistance from known current and ∆ V. Ø Repeated measurements in rapid succession 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 Solving page 6

Thermistors (4) Calibration uses the Steinhart-Hart equation 40 1 30 ° c 1 + c 2 ln R + c 3 ( l n R ) 3 35 T ( C) T = Nominal resistance is controllable by manufacturing. Data Curve Fit 45 25 20 15 10 Typical resistances at 21 ◦ C : 10 kΩ, 20 kΩ, . . . 100 kΩ. ME 121: Engineering Problem Solving 5 0 5 10 15 20 Resistance (kΩ) 25 30 page 7

Resistance Measurement Resistance can be measured if a precision current source is available. I If I is known and V is measured, then R is obtained with Ohm’s law V R = 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. ME 121: Engineering Problem Solving R I V V ohmmeter R leads page 8

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

Direct Resistance Measurement of Thermistors (2) Four-wire resistance measurement eliminates the lead resistance 1 Ohmmeter Rlead V Rlead Thermistor RT Rlead 1 Sketch adapted from Hints for Making Better Digital Multimeter Measurements, Agilent Technologies Corporation, www. agilent. com. ME 121: Engineering Problem Solving page 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 LM 334 or TMP 36. Instead, we will use our familiar strategy of measuring resistance with a voltage divider. ME 121: Engineering Problem Solving 5 V thermistor Analog input 10 kΩ page 11

Arduino code for Thermistor measurement int thermistor_reading( int power_pin, int read_pin) { int reading; digital. Write(power_pin, HIGH); delay(100); reading = analog. Read(read_pin); digital. Write(power_pin, LOW); return(reading); } float thermistor_reading_ave( int power_pin, int read_pin, int nave) { int i, reading; float sum; digital. Write(power_pin, HIGH); delay(10); for (i=1; i<=nave; i++) { sum += analog. Read(read_pin); } digital. Write(power_pin, LOW); return(sum/float(nave)); } ME 121: Engineering Problem Solving page 12