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SECTION
7.
MEASUREMENT PROGRAMMING EXAMPLES
The
fixed
100
ohm resistor must
be
thermally
stable.
lts precision is not important because
the
exact resistance is incorporated, along with
that
of
the PRT, into the calibrated multiplier.
The 10
ppm/oO
temperature coefficient
of
the
fixed resistor will limit the error due to its change
in resistance with temperature
to less than
0.1soO over
the -10 to
40oC
temperature range.
Because
the measurement is ratiometric
(Rs/Rf),
the properties of the
1O
kohm resistor
do not
affect
the result.
As
in
the example in Section 7.9,
the excitation
voltage
is
calculated
to
be
the maximum
possible, yet allow
the +50mV measurement
range. The
10 kohm resistor has
a
tolerance
of
t1%;thus,
the lowest resistance to expect from
it is
9.9
kohms.
We calculate
the maximum
excitation
voltage
(V*)
to keep the voltage drop
across
the PRT less than 50mV:
0.050V
> Vx
115.54/(9900+115.54); V"
.
4.
The excitation
voltage used
is
4.3V.
The multiplier used in Instruction 7
is
in
the same manner as in Section
7.9.
In
this
example,
the multiplier (R/Ro)
is
assumed
to
100.93.
The
3
wire half bridge compensates for lead wi
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
2o/o,
but
is more likely to be on the
order
of 1"/". The resistance of R. calculated
with Instruction 7,
is
actually
R,
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
the resistance measured for the PRT
(Ro =
100
ohms)
in
the ice bath would be 100.17
ohms, and
the resistance at
40oC
would
be
115.71. The measured ratio R./Ro 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 0.1oC lower
than the actual temperature
of
the
PRT.
This
source of error
does not exist
in
the example
in
Section 7.9, where
a 4
wire half bridge is used
measure PRT resistance.
The advantages
of
the
3
wire half bridge are
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 ditferential channels.
01:
01:
02:
03:
o4:
05:
06:
07:
08:
09:
02:
P16
01:
1
02:
1
03:
2
04:
1
05:
0
P9
1
3
3
1
1
4200
1
PROGRAM
Full
BR
w/Compensation
Rep
50 mV slow EX Range
50 mV slow BR Range
lN Chan
Excite
all reps w/EXchan
1
mV Excitation
Loc
[:Rs/R0
I
1.0111
Mult
0
Offset
Temperature
RTD
Rep
R/Ro
Loc
Rs/RO
Loc [:TEMP degC]
Mult
Offset
7.10
1OO
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
7.10-1.
EX]
21X
Ht
1
LOI
+
Rp
10K
OHM
1%
TEIVPERATURE COEFFICIENT
<
25
5OO,LEAD
LENGTH,
,, O*'(
6
PPM/'c
RS
100
oHM
PRT
FIGURE
7.10-1.
3
Wire Half Bridge Used
to
Measure 100
ohm
PRT
7-6
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