Campbell Scientific CR23X Micrologger - Manual (Page 94)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-6
I = 50mV/Rs = 50mV/115. 54 ohms = 0.433mA
Next solve for V
x
:
V
x
= I(R
1
+R
s
+R
f
) = 4.42V
If the actual resistances were the nominal
values, the CR23X would not overrange with V
x
= 4.4V. To allow for the tolerances in the actual
resistances, it is decided to set V
x
equal to 4.2
volts (e.g., if the 10 kohms resistor is 5% low,
R
s
/(R
1
+R
s
+R
f
)=115.54/9715.54, and V
x
must be
4.204V to keep V
s
less than 50mV).
The result of Instruction 9 when the first
differential measurement (V
1
) is not made on
the 5V range is equivalent to R
s
/R
f
. Instruction
16 computes the temperature ( o C) for a DIN
43760 standard PRT from the ratio of the PRT
resistance to its resistance at 0
o 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 CR23X and entering Instruction 9
with a multiplier of 1. The PRT is then placed in
an ice bath (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
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/
o C temperature 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 -10 to 40 o C temperature range.
Because the measurement is ratiometric (R
s
/R
f
),
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
7.8-1.
PROGRAM
1: Full Bridge w/mv Excit (P9)
1: 1 Reps
2: 22 50 mV, 60 Hz Reject, Slow,
Ex Range
3: 22 50 mV, 60 Hz Reject, Slow,
Br Range
4: 1 DIFF Channel
5: 1 Excite all reps w/Exchan 1
6: 4400 mV Excitation
7: 1 Loc [ Rs_Ro ]
8: 1.0111 Mult
9: 0.0 Offset
2: Temperature RTD (P16)
1: 1 Reps
2: 1 R/R0 Loc [ Rs_Ro ]
3: 2 Loc [ TEMP_degC ]
4: 1.0 Mult
5: 0.0 Offset
7.9 100 OHM PRT IN 3 WIRE HALF
BRIDGE
The temperature measurement requirements in
this example are the same as in Section 7.9. In
this case, a three wire half bridge, Instruction 7,
is used to measure the resistance of the PRT.
The diagram of the PRT circuit is shown in
Figure 7.9-1.
CR23X
FIGURE 7.9-1. 3 Wire Half Bridge Used to
Measure 100 ohm PRT
As in the example in Section 7.8, the excitation
voltage is calculated to be the maximum
possible, yet allow the +50mV measurement
range. The 10 kohm resistor has a tolerance of
± 1%; thus, the lowest resistance to expect from
it is 9.9 kohms. We calculate the maximum
excitation voltage (V
x
) to keep the voltage drop
across the PRT less than 50mV:
0.050V > V
x
115.54/(9900+115.54); V
x
< 4.33V
The excitation voltage used is 4.3V.