NOTE: The wiring diagram above is based off the new
wiring colors for serial numbers 2725 or 1352 and above.
Measurement devices (e.g., datalogger, controller) do not measure resistance directly, but determine resistance
from a half-bridge measurement, where an excitation voltage is input across the thermistor and an output voltage
is measured across the bridge resistor.
An excitation voltage of 2.5 V DC is recommended to minimize self-heating and current drain, while still
maintaining adequate measurement sensitivity (mV output from thermistor per C). However, other excitation
voltages can be used. Decreasing the excitation voltage will decrease self-heating and current drain, but will also
decrease thermistor measurement sensitivity. Increasing the excitation voltage will increase thermistor
measurement sensitivity, but will also increase self-heating and current drain.
Conversion of Thermistor Resistance to Temperature
The thermistor is a resistive element, where resistance changes with temperature. Thermistor resistance (R
T
, in
Ω
)
is measured with a half-bridge measurement, requiring a known excitation voltage input (V
EX
) and a measurement
of output voltage (V
OUT
):
−
=
1
V
V
24900
R
OUT
EX
T
(1)
where 24900 is the resistance of the bridge resistor in
Ω
. From resistance, temperature (T
K
, in Kelvin) is calculated
with the Steinhart-Hart equation and thermistor specific coefficients:
3
T
T
K
))
R
(ln(
C
)
R
ln(
B
A
1
T
+
+
=
(2)
where A = 1.129241 x 10
-3
, B = 2.341077 x 10
-4
, and C = 8.775468 x 10
-8
(Steinhart-Hart coefficients).
If desired, measured temperature in Kelvin can be converted to Celsius (T
C
):
15
.
273
T
T
K
C
−
=
.
(3)