Apogee Instruments SI Series Owner's Manual - Page 13
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Temperature Measurement with Internal Thermistor
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.
The internal thermistor provides a temperature reference for calculation of target temperature. Resistance of the
thermistor changes with temperature. Thermistor resistance (R
measurement, requiring an excitation voltage input (V
R
24900
T
V
where 24900 is the resistance of the bridge resistor in Ω. From resistance, temperature (T
with the Steinhart-Hart equation and thermistor specific coefficients:
T
K
A
B
ln(
R
where A = 1.129241 x 10
If desired, measured temperature in Kelvin can be converted to Celsius (T
T
T
273
.
15
C
K
NOTE: The wiring diagram below is based off the new wiring
colors for serial numbers 7283 and above.
V
EX
1
(1)
OUT
1
(2)
3
)
C
(ln(
R
))
T
T
-3
-4
, B = 2.341077 x 10
, and C = 8.775468 x 10
.
(3)
, in Ω) is measured with a half-bridge
T
) and a measurement of output voltage (V
EX
-8
(Steinhart-Hart coefficients).
):
C
):
OUT
, in Kelvin) is calculated
K