length having more than a few thousandths of an ohm resistance. In this example,
the measurement range is –10° to 40 °C. The length of the cable from the
CR1000 and the bridge resistors to the PRT is 500 feet.
Figure PT100 in Four-Wire Half-Bridge (p. 240) shows the circuit used to measure a
100 Ω PRT. The 10 kΩ resistor allows the use of a high excitation voltage and a
low input range. This ensures that noise in the excitation does not have an effect
on signal noise. Because the fixed resistor (R
f
) and the PRT (R
S
) have
approximately the same resistance, the differential measurement of the voltage
drop across the PRT can be made on the same range as the differential
measurement of the voltage drop across R
f
. The use of the same range eliminates
range translation errors that can arise from the 0.01% tolerance of the range
translation resistors internal to the CR1000.
7.9.15.2.1 Calculating the Excitation Voltage
The voltage drop across the PRT is equal to V
X
multiplied by the ratio of R
S
to the
total resistance, and is greatest when R
S
is greatest (R
S
= 115.54 Ω at 40 °C). To
find the maximum excitation voltage that can be used on the ±25 mV input range,
assume V
2
is 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
Next solve for V
X
:
V
X
= I*(R
1
+ R
S
+ R
f
) = 2.21 V
If the actual resistances were the nominal values, the CR1000 would not over
range with V
X
= 2.2 V. However, to allow for the tolerance in actual resistors, set
V
X
equal to 2.1 V (e.g., if the 10 kΩ resistor is 5% low, i.e.,
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).
7.9.15.2.2 Calculating the BrHalf4W() Multiplier
The result of BrHalf4W() is equivalent to R
S
/R
f
.
X = R
S
/R
f
PRTCalc() computes the temperature (°C) for a DIN 43760 standard PRT from
the ratio of the PRT resistance to its resistance at 0 °C (R
S
/R
0
). Thus, a multiplier
of R
f
/R
0
is used in BrHalf4W() to obtain the desired intermediate, R
S
/R
0
(=R
S
/R
f
• R
f
/R
0
). If R
S
and R
0
were each exactly 100 Ω, the multiplier would be 1.
However, neither resistance is likely to be exact. The correct multiplier is found
by connecting the PRT to the CR1000 and entering BrHalf4W() with a multiplier
of 1. The PRT is then placed in an ice bath (0 °C), and the result of the bridge
measurement is read. The reading is R
S
/R
f
, which is equal to R
0
/R
f
since R
S
=R
0
at 0 °C. The correct value of the multiplier, R
f
/R
0
, is the reciprocal of this
reading. The initial reading assumed for this example was 0.9890. The correct
multiplier is: R
f
/R
0
= 1/0.9890 = 1.0111.
239
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