CR10X User Guide
7-10
R
s
/
(
R
1
+R
s
+R
f
)=115.54/9715.54, and V
x
must be 2.102V to keep V
s
less than
25mV).
The result of Instruction 9 when the first differential measurement (V
1
) is not
made on the 2.5V range is equivalent to R
s
/R
f
. Instruction 16 computes the tem-
perature (
o
C) for a DIN 43760 standard PRT from the ratio of the PRT resistance
at the temperature being measured to its resistance at 0°C (R
s
/R
0
). Thus, a
multiplier of R
f
/R
0
is used in Instruction 9 to obtain the desired intermediate,
R
s
/R
0
(=R
s
/R
f
x R
f
/R
o
). 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 multi-
plier is found by connecting the PRT to the CR10X and entering Instruction 9
with a multiplier of 1. The PRT is then placed in an ice bath (at 0
o
C; R
s
=R
0
), and
the result of the bridge measurement is read using the *6 Mode. The reading is
R
s
/R
f
, which is equal to R
0
/R
f
since R
s
=R
0
. 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.
The fixed 100
Ω
resistor must be thermally stable. Its precision is not important if
the sensor is to be calibrated because the exact resistance can be incorporated,
along with that of the PRT, into the calibrated multiplier. The 10ppm/
o
C temp-
erature coefficient of the fixed resistor will limit the error due to its change in
resistance with temperature to less than 0.15
o
C over the specified temperature
range. Because the measurement is ratiometric (R
s
/R
f
), the properties of the
10k
Ω
resistor do not affect the result.
A terminal input module (Model 4WPB100) can be used to complete the circuit
shown in Figure 7-7.
Do not
simply copy this program without reading the
description associated with it. The multiplier for your
application will probably be different.
Please note that if you purchase the PT100 element to a
specified, known tolerance and the 100
Ω
resistor has an
accuracy of 0.01% or better, it is not necessary to perform the
zero calibration. A multiplier of 1.0 can be assumed.
Program
01:Full Bridge w/mv Excit (P9)
01:
1
Rep
02:
33
25mV 50Hz rejection EX Range
03:
33
25mV 50Hz rejection BR Range
04:
1
DIFF Chan
05:
1
Excite all reps w/EXchan 1
06:
2100
mV Excitation
07:
1
Loc [:Rs/Ro ]
08:
1.0111
Mult (Rf/Ro)
09:
0
Offset
02:Temperature RTD (P16)
01:
1
Rep
02:
1
R/Ro Loc Rs/Ro
03:
2
Loc [:TEMP C ]
04:
1
Mult
05:
0
Offset
CAUTION