Campbell Scientific CR7 Measurement and Control System - Manual (Page 75)

SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-7
FIGURE 7.10-1. Wiring Diagram for
Raingage with Long Leads
In a long cable, there is appreciable
capacitance between the lines which is
discharged across the switch when it closes. In
addition to shortening switch life, a transient
may be induced in other wires, packaged with
the rain gage leads, each time the switch
closes. The 100 ohm resistor protects the
switch from arcing and the associated transient
from occurring, and should be included any time
leads longer than 100 ft. are used with a switch
closure.
PROGRAM
01: P3 Pulse
01: 1 Rep
02: 2 IN Card
03: 1 Pulse Input Chan
04: 2 Switch closure
05: 11 Loc [:RAIN mm ]
06: 0.254 Mult
07: 0 Offset
7.11 100 OHM PRT IN 4 WIRE HALF
BRIDGE
Instruction 9 is the best choice for accuracy
where the Platinum Resistance Thermometer
(PRT) is separated from other bridge
completion resistors by a lead length having
more than a few thousandths of an ohm
resistance. In this example, it is desired to
measure a temperature in the range of -10 to
40 oC. The length of the cable from the CR7 to
the PRT is 500 feet.
FIGURE 7.11-1. Wiring Diagram for PRT in 4
Wire Half-Bridge
Figure 7.11-1 diagrams the circuit used to
measure the PRT. The 10 kohm resistor allows
the use of a high excitation voltage and low
voltage ranges on the measurements. This
insures that noise in the excitation does not
have an effect on signal noise. Because the
fixed resistor (Rf) and the PRT (Rs) have
approximately the same resistance, the
differential measurement of the voltage drop
across the PRT can be made on the same
range as the differential measurement of the
voltage drop across Rf. The use of the same
range eliminates any range translation error that
might arise from the 0.01% tolerance of the
range translation resistors in the CR7.
If the voltage drop across the PRT (V2) is kept
on the 50 mV range, self heating of the PRT
should be less than 0.001 oC in still air. The
resolution of the measurement is increased as
the excitation voltage (Vx) is increased. The
voltage drop across the PRT is equal to Vx
multiplied by the ratio of Rs to the total
resistance, and is greatest when Rs is greatest
(Rs=115.54 ohms at 40 oC). To find the
maximum excitation voltage that can be used,
we assume V2 equal to 50 mV and use Ohm's
Law to solve for the resulting current, I.
I = 50mV/Rs = 50mV/115. 54 Ohms = 0.433mA
Next solve for Vx:
Vx = I(R1+Rs+Rf) = 4.42V
If the actual resistances were the nominal
values, the CR7 would not overrange with Vx =
4.4 V. To allow for the tolerances in the actual
resistances it is decided to set Vx equal to 4.2
volts (e.g., if the 10 kohms resistor is 5% low,
Rs/(R1+Rs+Rf)=115.54/9715.54 and Vx must
be 4.204V to keep Vs less than 50 mV).
The result of Instruction 9 when the first
differential measurement (V1) is not made on
the 5V range is equivalent to Rs/Rf. Instruction
16 computes the temperature (oC) for a DIN
43760 standard PRT from the ratio of the PRT
resistance to its resistance at 0 oC (Rs/R0).
Thus, a multiplier of Rf/R0 is used in Instruction
9 to obtain the desired intermediate, Rs/R0 (=
Rs/Rf x Rf/R0). If Rs and R0 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 CR7 and entering Instruction 9 with