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I
i
I
sEcrloN
7.
I
l
i
I
shortening
sivitch life,
a
transient may
be
induced in other
wires, packaged with the
rain
gauge leadsf each time
the switch
closes. The
100
ohm re$stor protects the switch from arcing
and the
ass{ciated transient from occurring, and
should
be influded any time leads longer than
100
feet
are used with a switch closure.
PROGRAM
1 P 3
Count pulses
from
rain
gauge
1
lrep
1
Pulse
channel#1
2
Switch closure,
all
pulses counted
4 11
Location
to store count
5
0.254
MultiPlier
6 0
Offset
7.9
1OO
OHM PRT IN 4 WIRE HALF
BRIDGE
Instruction 9
is
the best choice for accuracy
(PRT)
is
from other bridge completion
few
hs of
an ohm resistance.
In
this
example,
it
desired
to measure
a
temperature
in
the
of
-10
to 4OoC. The length
of
the
cable from
21X to
the PRT is 500 feet.
MEASUREMENT PROGRAMMING EXAMPLES
same range eliminates any range translation
error
that might arise from the
0.O1"/"
tolerance
of
the range translation resistors
in
the 21X.
lf the voltage
drop across the PRT
(V2)
is kept
on
the 50mV range, self heating of the PRT
should be less
than 0.001oC in still
air.
The
resolution
of
the measurement is increased as
the excitation voltage (V*) is increased as long
as
the lnput Range is
not
exceeded. The
voltage drop across the PRT is
equalto V*
multiplied by
the ratio of R. to the total
resistance, and is greatest
when
R,
is
greatest
(Rs=115.54 ohms at
40oC). To find the
maximum excitation voltage that
can be used,
we assume
V2
equalto
50mV
and use Ohm's
Law
to solve for the resulting current,
l.
l=
SomV/Rs
=
50mV/115. 54 ohms = 0.433mA
Next solve for V*:
Vx=l(Rr+R.+R)=4.42V
lf
the actual resistances were the nominal
values, the 21X
would
not
overrange with V"
=
4.4V.
To allow for
the tolerances
in
the actual
resistances, it is decided
to set V" equal to 4.2
volts (e.9.,
if
the 10 kohms resistor
is
57o
low,
Rr/(R1+Rr+Rt)=115.54/9715.54, and
V* must
be
4.204V
to
keep
V,
less
than 50mV).
The result of Instruction
9
when the first
differentialmeasurement
(V1)
is not made on
the 5V range
is
equivalent
to Rr/R1. Instruction
16
computes the
temperature
(oC)
for
a
DIN
43760
standard PRT from the ratio of the PRT
resistance
to its resistance at
OoC
(Rs/Rg).
Thus, a multiplier of
Ry'Ro
is
used in lnstruction
9
to obtain the desired intermediate, Rs/R'
(=Rs/B x Ry'Re). lf
R.
and
Re
were each
exactly 100 ohms, the multiplierwould be
1.
However, neither resistance is likely
to be exact.
The
correct multiplier
is
found by connecting the
PRT to the 21X and
entering lnstruction
9
with a
multiplier
of
1.
The PRT
is
then placed in an
ice
bath
(OoC;
Rr=Rs), and
the result of the bridge
measurement is read using
the *6
Mode.
The
reading is
Rr/R1,
which
is
equalto
Ro/Rl
since
R"=Ro. The correct value
of
the multiplier,
Ry'Ro, is
the reciprocal
of
this
reading.
The
initial reading assumed for
this example was
0.9890.
The
correct multiplier is:
Ry'Ro
=
1/0.9890
=
1.01 11.
1
2
3
EX1
Ht2
21X
LO2
Ht
1
LO1
+
I
1OK
OHM
vz(
RF
100
oHM
Rs
lOO
OHM
PRT
FIGURE
7,19-1.
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. Becquse the fixed resistor
(R)
and
the
PRT
(RJ
hdve approximately
the same
resistance,
fhe differential measurement
of
the
voltage
drop across the PRT can be made on
the
same
r{nge
as
the differential measurement
of
the
volta$e drop across
R1.
The use
of
the
7-5
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