Apogee Instruments ST-300 Owner's Manual - Page 10

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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
is measured with a half-bridge measurement, requiring a known excitation voltage input (V
of output voltage (V
):
OUT
where 24900 is the resistance of the bridge resistor in Ω. From resistance, temperature (T
with the Steinhart-Hart equation and thermistor specific coefficients:
-3
where A = 1.129241 x 10
If desired, measured temperature in Kelvin can be converted to Celsius (T
=
−
T
T
273
.
15
C
K
(1)
(2)
-4
, B = 2.341077 x 10
, and C = 8.775468 x 10
.
(3)
NOTE: The wiring diagram above is based off the new
wiring colors for serial numbers 2725 or 1352 and above.
) and a measurement
EX
, in Kelvin) is calculated
K
-8
(Steinhart-Hart coefficients).
):
C
, in Ω)
T