Campbell Scientific 3WHB10K 3-Wire Half-Bridge Terminal Input Module - Manual (Page 10)

3WHB10K 3-Wire Half Bridge Terminal Input Module
6
The result of the 3-wire half bridge instruction is equivalent to the ratio of the
PRT resistance, R
s
to the resistance of the 10 k fixed resistor, R
f
.
R
R
s
f
The RTD Instruction (16) computes the temperature ( °C) for a DIN 43760
standard PRT from the ratio of the PRT resistance at the temperature being
measured (R
s
) to its resistance at 0 °C (R
0
). Thus, a multiplier of R
f
/R
0
is used
with the 3-wire half bridge instruction to obtain the desired intermediate, R
s
/R
0
= (R
s
/R
f
x R
f
/R
o
). When R
f
= 10,000 and R
0
= 100, the multiplier is 100; when
R
0
is 1000 the multiplier is 10.
The fixed resistor must be thermally stable. Over the -55 ° to 85°C extended
temperature range for the datalogger, the ±4 ppm/ °C temperature coefficient
would result in a maximum error of ±0.04 °C at 60°C. The
±8 ppm/ °C temperature coefficient would result in a maximum error of
±0.13 °C at -55°C.
5.1 Excitation Voltage
The best resolution is obtained when the excitation voltage is large enough to
cause the signal voltage to fill the measurement voltage range. The voltage
drop across the PRT is equal to the current, I, multiplied by the resistance of
the PRT, R
s
, and is greatest when R
s
is greatest. For example, if it is desired to
measure a temperature in the range of -10 to 40 °C, the maximum voltage drop
will be at 40 °C when R
s
=115.54 Ohms. To find the maximum excitation
voltage that can be used when the measurement range is ±25 mV, we assume
V2 equal to 25 mV and use Ohm's Law to solve for the resulting current, I.
I = 25 mV/R
s
= 25 mV/115.54 Ohms
= 0.216 mA
V
x
is equal to I multiplied by the total resistance:
V
x
= I(R
s
+R
f
) = 2.18 V
If the actual resistances were the nominal values, the 25 mV range would not
be exceeded with V
x
= 2.18 V. To allow for the tolerances in the actual
resistances and to leave a little room for higher temperatures, set V
x
equal to
2.1 volts.
5.2 Calibrating a PRT
The greatest source of error in a PRT is likely to be that the resistance at 0 °C
deviates from the nominal value. Calibrating the PRT in an ice bath can
correct this offset and any offset in the fixed resistor in the Terminal Input
Module.
With the PRT at 0 °C, R
s
=R
0
. Thus, the above result becomes R
0
/R
f
, the
reciprocal of the multiplier required to calculate temperature, R
f
/R
0
. By
making a measurement with the PRT in an ice bath, errors in both R
s
and R
0
.
can be accounted for.