Apogee Instruments SF-110 Руководство пользователя - Страница 13

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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 in the simulated leaf has a resistive element where resistance changes with temperature.
Thermistor resistance (R
T
(V
) and a measurement of output voltage (V
EX
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
measured temperature in Kelvin can be converted to Celsius (T
=
−
T
T
273
.
15
C
K
, in Ω) is measured with a half-bridge measurement, requiring an excitation voltage input
):
OUT
(1)
1
(2)
+
3
)
C
(ln(
R
))
T
T
-3
-4
, B = 2.341077 x 10
, and C = 8.775468 x 10
(3)
-8
(Steinhart-Hart coefficients). If desired,
):
C
, in Kelvin) is calculated
K