Operator`s manual
SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-5
shortening switch life, a transient may be
induced in other wires, packaged with the rain
gauge 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 feet are used with a switch closure.
PROGRAM
1 P 3 Count pulses from rain
gauge
11 1 rep
2 1 Pulse channel #1
3 2 Switch closure, all
pulses counted
4 11 Location to store count
5 0.254 Multiplier
6 0 Offset
7.9 100 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
o
C. The length of the
cable from the 21X to the PRT is 500 feet.
FIGURE 7.9-1. Wiring Diagram for PRT in 4
Wire Half Bridge
Figure 7.9-1 diagrams the circuit used to
measure the PRT. The 10 kohm resistor allows
the use of a high excitation voltage and a low
input range. This insures that noise in the
excitation does not have an effect on signal
noise. Because the fixed resistor (R
f
) and the
PRT (R
s
) 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 R
f
. 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 21X.
If the voltage drop across the PRT (V
2
) is kept
on the 50mV range, self heating of the PRT
should be less than 0.001
o
C in still air. The
resolution of the measurement is increased as
the excitation voltage (V
x
) is increased as long
as the Input Range is not exceeded. The
voltage drop across the PRT is equal to V
x
multiplied by the ratio of R
s
to the total
resistance, and is greatest when R
s
is greatest
(R
s
=115.54 ohms at 40
o
C). To find the
maximum excitation voltage that can be used,
we assume V
2
equal to 50mV and use Ohm's
Law to solve for the resulting current, I.
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 21X 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 21X 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.