Campbell Scientific CR10X Measurement and Control System User Manual

Page 92

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

7-8

FIGURE 7.9-1. Wiring Diagram for PRT in 4 Wire Half Bridge

The result of Instruction 9 when the first
differential measurement (V

) is not made on

the 2.5 V range is equivalent to R

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

Thus, a multiplier of R

is used in Instruction

9 to obtain the desired intermediate, R

(=R

x R

). If R

and R

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 CR10X and entering Instruction 9
with a multiplier of 1. The PRT is then placed in
an ice bath (@ 0

°C; R

), and the result of

the bridge measurement is read using the

∗6

Mode. The reading is R

, which is equal to

since R

. The correct value of the

multiplier, R

, is the reciprocal of this

reading. The initial reading assumed for this
example was 0.9890. The correct multiplier is:
R

= 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

), the properties of the 10 kohm resistor

do not affect the result.

A terminal input module (Model 4WPB100) can
be used to complete the circuit shown in Figure
7.9-1.

PROGRAM

01:

Full Bridge w/mv Excit (P9)
1:

Reps

±25 mV 60 Hz Rejection
Ex Range

±25 mV 60 Hz Rejection
Br Range

DIFF Channel

Excite all reps w/Exchan 1

6: 2100

mV Excitation

Loc [ Rs_Ro ]

1.0111

Mult

Offset

02:

Temperature RTD (P16)
1:

Reps

R/Ro Loc [ Rs_Ro ]

Loc [ Temp_C ]

Mult

Offset