Download Measuring Temperature Using Thermistors

Measure Temperature using Thermistor
© September 2005 EmAnt Pte Ltd
www.emant.com
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Measure Temperature using Thermistor
Thermistors are widely used because of their sensitivity, small size, ruggedness and low cost.
Thermistors have an electrical resistance that varies non-linearly with temperature. The R-T
characteristics of most thermistors can be described by the Steinhart-Hart equation:
1/T = A + B*(Ln R) + C*(Ln R)3
T is the absolute temperature (in Kelvin) and A, B, and C are constants which can be determined by
measuring three sets of resistance and temperature values during calibration.
Most thermistors have a negative temperature coefficient (NTC), their resistance decreases with
increasing temperature. Thermistors are specified according to its nominal resistance at 25 oC and
commonly available thermistors range from 250 ohms to 100 kohms
The thermistor that we are using has the following characteristics
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Nominal resistance @ 25 oC: 10 kohms
negative temperature coefficient (NTC)
Steinhart-Hart equation parameters:
o A= 0.001129148
o B= 0.000234125
o C= 8.76741E-8
As the DAQ module Analog Input measures only
voltage, we will need to provide a current source to
convert the resistance to voltage. The EMANT300
has an 8 bit current DAC (digital to analog
converter). As the DAC has 8 bits resolution, we
can drive the resistance from 0 to 1mA in 255 steps
with increments of about 39uA.
To measure temperature using the thermistor with
the EMANT300 DAQ Training Kit,
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Connect the thermistor to the Light Application
Adaptor screw terminals labeled IDAC and
AGND
Connect a wire from IDAC to AIN3
Connect a wire from AGND to AIN2
© September 2005 EmAnt Pte Ltd
www.emant.com
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The LabVIEW VI that reads the voltage across the thermistor, converts the voltage to resistance and
then temperature is called EMANT300 Example Thermistor.VI
Front Panel
Diagram
In the formula node, the first line calculates the Thermistor resistance. The next line converts
from resistance to temperature in Kelvin and finally converts the temperature from Kelvin to
Centigrade.
LabVIEW is a trademark of National Instruments
© September 2005 EmAnt Pte Ltd
www.emant.com
3