Temperature Measurement with Thermistors

Size: px

Start display at page:

Download "Temperature Measurement with Thermistors"

Maud Allison
5 years ago
Views:

1 Temperature Measurement with Thermistors Gerald Recktenwald Portland State University Department of Mechanical Engineering March 3, 2019 ME 121: Introduction to Systems and Control

2 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: Introduction to Systems and Control page 1

3 Thermocouples: Overview Principle of operation Wire types: B, E, J, K, N, R, S, T Formats: prefab, homemade, fast response, slow response Circuit diagrams: reference junction compensation Good practice ME 121: Introduction to Systems and Control page 2

4 Seebeck Effect (1) Temperature gradient in a conductor induces a voltage potential T 2 Voltmeter E 12 T 1 E 12 = σ(t 2 T 1 ) (1) where σ is the average Seebeck coefficient for the range T 1 T T 2. ME 121: Introduction to Systems and Control page 3

5 EMF Relationships for Thermocouples (2) Nominal values of Seebeck Coefficient Type Metal + Seebeck Coefficient Temperature Range J Iron Constantan 50 µv/ C 210 to +760 C K Nickel- Chromium Nickel 39 µv/ C 270 to C T Copper Constantan 38 µv/ C 270 to +400 C σ values are small, so the voltage output from thermocouples is small, typically on the order of 10 3 V. ME 121: Introduction to Systems and Control page 4

6 Thermocouple Amplifier Although it s possible, and in come cases preferable, to measure thermocouple signals with a laboratory-grade DMM, for our purposes in ME 120, 121, 122, we can use a simple thermocouple amplifier chip. ME 121: Introduction to Systems and Control page 5

7 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 Part number AD590, AD22103, TMP35, TMP36, TMP37 DS1621, DS18B20 Max31885, Max675, REF-01, LM45 LM35, LM335, LM75, LM78 ME 121: Introduction to Systems and Control page 6

o 5.5 V) FUNCTIONAL BLOCK DIAGRAM #.-)/*01 Low Voltage Temperature Sensors " *-) TMP35/TMP36/TMP37!" # $%&'("$)*$+'+", )234+5 )23465 )234( Figure 1.

" Don t # & )*3$"9:0 confuse the TO-92-3 package with a transistor! " *-) C1/ 4 %1;<$<;$#=>?@, + 8 $B$1*$A*11:A) 7 & 4 8 C1/ #.-)/*01 Figure 2. RJ-5 (SOT-23) )*3$"9:0 %1;<$<;$#=>?

8 o 5.5 V) FUNCTIONAL BLOCK DIAGRAM #.-)/*01 Low Voltage Temperature Sensors " *-) TMP35/TMP36/TMP37!" # $%&'("$)*$+'+", )234+5 )23465 )234( Figure IC Temperature Sensors (2) FUNCTIONAL BLOCK DIAGRAM!" # $%&'("$)*$+'+", C on TMP37) re (typ) ads eration to +150 C rent s PIN CONFIGURATIONS Example: TMP36 from Analog Devices " *-) 7!" Don t # & )*3$"9:0 confuse the TO-92-3 package with a transistor! " *-) C1/ 4 %1;<$<;$#=>?@, + 8 $B$1*$A*11:A) 7 & 4 8 C1/ #.-)/*01 Figure 2. RJ-5 (SOT-23) )*3$"9:0 %1;<$<;$#=>?@, D ( 6 + $B$1*$A*11:A) e low voltage, precision centiy provide a voltage output that elsius (centigrade) temperature. o not require any external curacies of ±1 C at +25 C 5 C temperature range.!" # Figure 3. R-8 (SOIC_N) 7 & 4 E*))*2$"9:0 %1;<$<;$#=>?@, #.-)/*01 391$7F$!" # G$391$&F$" *-) G$391$4F$C1/ Figure 4. T-3 (TO-92) #.-)/*01 " *-)!" # )234+5 )23465 )234( Figure 1. PIN CONFIGURATIONS " *-) C1/ 7 & 4 )*3$"9:0 %1;<$<;$#=>?@, + 8 $B$1*$A*11:A) 7 & 4 8 C1/ #.-)/*01 Figure 2. RJ-5 (SOT-23) )*3$"9:0 %1;<$<;$#=>?@, D ( 6 + $B$1*$A*11:A)!" # Figure 3. R-8 (SOIC_N) #.-)/*01 e TMP35/TMP36/TMP37 and 7 & 4 ibration The TMP37 simplify is interfacing intended for to applications over the range of 5 C d to ADCs. 100 C All and three provides devices an are E*))*2$"9:0 ME 121: Introduction output scale to factor Systems of 20 mv/ C. and Control The %1;<$<;$#=>?@, page 7 ation TMP37 from provides 2.7 V to 5.5 a 500 V maxiwell to below 150 C 50 with µa, reduced providing accuracy for all devices when operating 391$7F$!" # G$391$&F$" *-) G$391$4F$C1/ mv output at 25 C. Operation extends 0.1 C from in a still 5 V air. supply. In addition, a Figure 4. T-3 (TO-92) to cut the supply current to less The TMP35/TMP36/TMP37 are available in low cost 3-lead " *-) 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.

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.

9 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: The T = F (R) relationship for thermistors is nonlinear. The temperature coefficient k is the slope in the curve over a narrow range of temperatures and resistances. ME 121: Introduction to Systems and Control page 8

10 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 indicate the change in resistance as a change in voltage. ME 121: Introduction to Systems and Control page 9

11 Advantages Thermistors (3) Output is directly related to absolute temperature no reference junction needed. Relatively easy to measure resistance or convert resistance to voltage with a voltage divider. High precision thermistors have interchangeable tolerances of ±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 Precision thermistors are 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 due to slow(er) response and self-heating ME 121: Introduction to Systems and Control page 10

12 Thermistors (4) Calibration uses the Steinhart-Hart equation Data Curve Fit T = 1 c 1 + c 2 ln R + c 3 (ln R) 3 Nominal resistance is controllable by manufacturing. Typical resistances at 21 C: kω, 20 kω, kω T ( C) Resistance (kω) ME 121: Introduction to Systems and Control page 11

13 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 R V R = V I For a typical ohmmeter, the current source and voltage measurement are inside the device. The leads connect the internal current source to the external resistance element being measured. I V ohmmeter leads R ME 121: Introduction to Systems and Control page 12

14 Direct Resistance Measurement of Thermistors (1) Two-wire resistance measurement: R T = V I. Ohmmeter V Thermistor R T Resistance in the lead wires can contribute to inaccuracy in the temperature measurement. ME 121: Introduction to Systems and Control page 13

15 Direct Resistance Measurement of Thermistors (2) Four-wire resistance measurement eliminates the lead resistance 1 Ohmmeter R lead V R lead Thermistor RT R lead R lead 1 Sketch adapted from Hints for Making Better Digital Multimeter Measurements, Agilent Technologies Corporation, ME 121: Introduction to Systems and Control page 14

16 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. 5V thermistor 10 kω Analog input ME 121: Introduction to Systems and Control page 15

17 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) { int i, 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: Introduction to Systems and Control page 16

Temperature Measurement with Thermistors

Temperature Measurement with Thermistors Gerald Recktenwald Portland State University Department of Mechanical Engineering gerry@me.pdx.edu February 26, 2013 EAS 199B: Engineering Problem Solving Temperature