Transcript Thermistors (1) - Portland State University

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 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
AD590, AD22103, TMP35, TMP36, TMP37
DS1621, DS18B20
Max675, REF-01, LM45
LM35, LM335, LM75, LM78
page 2
IC Temperature Sensors (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 www.digikey.com.
$1.42 each (Qty 1) in Feb 2013
See http://learn.adafruit.com/
tmp36-temperature-sensor/
overview for instructions on how to use
the TMP36.
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 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 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

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 Solving
page 6
Thermistors (4)
Calibration uses the Steinhart-Hart equation
40
1
30
°
c1 + c2 ln R + c3 ( 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 resistance1
Ohmmeter
Rlead
V
Rlead
Thermistor
RT
Rlead
Rlead
1Sketch 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 LM334 or TMP36.
Instead, we will use our familiar strategy of measuring
resistance with a voltage divider.
ME 121: Engineering Problem Solving
5V
thermistor
Analog input
10 kΩ
page 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) {
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: Engineering Problem Solving
page 12