Campbell Scientific CR23X Micrologger - Manual (Page 95)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-7
The multiplier used in Instruction 7 is determined
in the same manner as in Section 7.8. In this
example, the multiplier (R
f
/R
0
) is assumed to be
100.93.
The 3 wire half bridge compensates for lead wire
resistance by assuming that the resistance of
wire A is the same as the resistance of wire B.
The maximum difference expected in wire
resistance is 2%, but is more likely to be on the
order of 1%. The resistance of R
s
calculated
with Instruction 7, is actually R
s
plus the
difference in resistance of wires A and B. The
average resistance of 22 AWG wire is 16.5
ohms per 1000 feet, which would give each 500
foot lead wire a nominal resistance of 8.3 ohms.
Two percent of 8.3 ohms is 0.17 ohms.
Assuming that the greater resistance is in wire B,
the resistance measured for the PRT (R
0
=
100 ohms) in the ice bath would be 100.17
ohms, and the resistance at 40
o C would be
115.71. The measured ratio R
s
/R
0
is 1.1551;
the actual ratio is 115.54/100 = 1.1554. The
temperature computed by Instruction 16 from
the measured ratio would be about 0.1
o C lower
than the actual temperature of the PRT. This
source of error does not exist in the example in
Section 7.8, where a 4 wire half bridge is used to
measure PRT resistance.
The advantages of the 3 wire half bridge are that
it only requires 3 lead wires going to the sensor
and takes 2 single- ended input channels,
whereas the 4 wire half bridge requires 4 wires
and 2 differential channels.
A terminal input module (Model 3WHB10K) can
be used to complete the circuit in Figure 7.9-1. It
uses a
±
0.01%
±
8 ppm precision resistor.
PROGRAM
1: 3W Half Bridge (P7)
1: 1 Reps
2: 22 50 mV, 60 Hz Reject, Slow
Range
3: 1 SE Channel
4: 1 Excite all reps w/Exchan 1
5: 4300 mV Excitation
6: 1 Loc [ Rs_Ro ]
7: 100.93 Mult
8: 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
7.10 100 OHM PRT IN 4 WIRE FULL
BRIDGE
This example describes obtaining the
temperature from a 100 ohm PRT in a 4 wire
full bridge (Instruction 6). The temperature
being measured is in a constant temperature
bath and is to be used as the input for a control
algorithm. The PRT in this case does not
adhere to the DIN standard (alpha = 0.00385)
used in the temperature calculating Instruction
16. Alpha is defined as (R
100
/R
0
-1)/100, where
R
100
and R
0
are the resistances of the PRT at
100 o C and 0 o C, respectively. In this PRT,
alpha is equal to 0.00392.
CR23X
FIGURE 7.10-1. Full Bridge Schematic for
100 ohm PRT
The result (X) given by Instruction 6 is 1000
V
s
/V
x
(where V
s
is the measured bridge output
voltage, and V
x
is the excitation voltage) which
is:
X = 1000 (R
s
/(R
s
+R
1
)-R
3
/(R
2
+R
3
))
The resistance of the PRT (R
s
) is calculated
with the Bridge Transform Instruction 59:
R
s
= R
1
X'/(1-X')
Where
X' = X/1000 + R
3
/(R
2
+R
3
)
Thus, to obtain the value R
s
/R
0
, (R
0
= R
s
@
0 o C) for the temperature calculating Instruction
16, the multiplier and offset used in Instruction 6
are 0.001 and R
3
/(R
2
+R
3
), respectively. The
multiplier used in Instruction 59 to obtain R
s
/R
0
is R
1
/R
0
(5000/100 = 50).