4WPB100, 4WPB1K PRT Bridge Terminal Input Modules
8
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 4 wire half bridge instruction to obtain the desired intermediate, R
s
/R
0
= (R
s
/R
f
x R
f
/R
o
). If R
f
and R
0
are equal, the multiplier is 1.
The fixed resistor must be thermally stable. The 4 ppm/
°
C temperature
coefficient would result in a maximum error of 0.05
°
C at 60
°
C. The 8
ppm/
°
C temperature coefficient would result in a maximum error of 0.33
°
C at
125
°
C. Because the measurement is ratiometric (R
s
/R
f
) and does not rely on
the absolute values of either R
s
or R
f
, the properties of the 10 kOhm resistor do
not affect the result.
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
V
2
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
1
+R
s
+R
f
) = 2.21 V
If the actual resistances were the nominal values, the 25 mV range would not
be exceeded with V
x
= 2.2 V. To allow for the tolerances in the actual
resistances, it is decided to set V
x
equal to 2.1 volts (e.g., if the 10 kOhm
resistor is 5% low, then R
s
/(R
1
+R
s
+R
f
)=115.54/9715.54, and V
x
must be 2.102
V to keep V
s
less than 25 mV).
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.
The result of the 4 wire half bridge is:
V
V
I R
I R
R
R
s
f
s
f
2
1
=
⋅
⋅
=
