Campbell CR850, Operator'S Manual

Page: 259 / 566

Section 7. Installation
259
Figure PT100 in Four-Wire Half-Bridge
(p. 260)
shows the circuit used to measure a
100- Ω PRT. The 10-k Ω 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 CR800.
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 CR800 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-k Ω 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).
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 ohms, the multiplier would be 1.
However, neither resistance is likely to be exact. The correct multiplier is found
by connecting the PRT to the CR800 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.
Choosing Rf
The fixed 100- Ω resistor must be thermally stable. Its precision is not important
because the exact resistance is incorporated, along with that of the PRT, into the
calibrated multiplier. The 10 ppm/°C temperature coefficient of the fixed resistor
will limit the error due to its change in resistance with temperature to less than
0.15°C over the -10 to 40°C temperature range. Because the measurement is
ratiometric (R
S
/R
f
), the properties of the 10-k Ω resistor do not affect the result.