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SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-7
FIGURE 7.9-1. Wiring Diagram for PRT in 4 Wire Half Bridge
The result of Instruction 9 when the first
differential measurement (V
1
) is not made on
the 2.5 V range is equivalent to R
s
/R
f
.
Instruction 16 computes the temperature (°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 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 CR10 and entering
Instruction 9 with a multiplier of 1. The PRT is
then placed in an ice bath (@ 0°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
o
/R
f
since R
s
=R
o
. 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 ohm 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 specified temperature
range. Because the measurement is
ratiometric (R
s
/R
f
), the properties of the 10
kohm resistor do not affect the result.
PROGRAM
01: P9 Full BR w/Compensation
01: 1 Rep
02: 23 25 mV 60 Hz rejection EX
Range
03: 23 25 mV 60 Hz rejection BR
Range
04: 1 IN 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: P16 Temperature RTD
01: 1 Rep
02: 1 R/Ro Loc Rs/Ro
03: 2 Loc [:TEMP C ]
04: 1 Mult
05: 0 Offset