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SECTION
7.
MEASUREMENT PROGRAMMING EXAMPLES
7.11
100
PRT IN 4 WIRE FULL
BRIDGE
This exa
describes obtaining
the
from
a
100
ohm PRT in a
4
wire
fullbridge
6).
The
temperature
being
is in
a
constant
temperature
bath and
is
algorithm.
be used as the input for a control
PRT
in
this case
does not
adhere
to
DIN
standard (alpha = 0.00385)
used
in
the
calculating
I
nstruction
16.
Alpha
defined
as
(R1e6/Ro-1)/100,
where
are
the
resistances
of
the PRT at
Rloo and
100oC
and
alpha
is eq
),
respectively. ln this
PRT,
to 0.00392.
5K
OHM 0.01%
PROGRAM
3
Wire Half Bridge
Rep
50 mV slow Range
lN Chan
Excite
all reps w/EXchan
1
mV Excitation
Loc
[:Rs/R0
]
93
Mult
Otfset
Temperature RTD
Rep
R/Ro Loc Rs/R0
Loc [:TEMP degO]
Mult
Offset
R"
=
Rt
X'/(1-X')
Where
X'=
)U1000
+
R3/(R2+R3)
Thus, to obtain the
value R"/Rs,
(Ro
=
R.
@
OoC)
for the temperature
calculating Instruction
16,
the multiplier and offset used in lnstruction
6
are 0.001
and
R3/(R2+R3),
respectively. The
multiplier used
in Instruction 59 to obtain Rr/R6
is Rt/Rs (5000/100 =
50).
It
is
desired to
controlthe temperature
bath at
50oC
with as little variation as
possible.
High
resolution
is
desired so the
control algorithm will
be able
to
respond
to minute changes
in
temperature. The highest resolution
is
obtained
when the
temperature range results
in
an output
voltage
(V.)
range which
fills the measurement
range selected
in Instruction
6.
The
full bridge
configuration allows
the bridge to be balanced
(V. = 0V)
at
or near the control temperature.
Thus,
the output voltage
can
go both positive
and negative as
the bath temperature changes,
allowing the
full use
of
the measurement range.
The resistance
of
the PRT is approximately
119.7 ohms at
50oC. The 120 ohm fixed
resistor balances
the bridge at approximately
51oC. The output voltage
is:
V.
=
V" [R"/(R.+R1) -
R3/(R2+R3)]
=
V* [Rs/(Rs+5000)
-
0.023438]
The
temperature range to be covered
is
50
t10oC.
At 40oC
R.
is
approximately 115.8
ohms, or:
Ys=
-802.24x10-6 Vx
even
with an excitation voltage (V")
equalto
5000mV,
V, can
be measured on
the
t5mV
scale
(40oC
=
1
15.8
Cl
=
-4.01
mV,
60oC
=
123.6
C2
=
3.428mV). There
is
a change
of
approximately 4mV from the output at
40oC
to
the output at 51oC, or
364pV/oC. With a
resolution of 0.33pV on
the SmV range, this
means that the
temperature resolution
is
0.00090c.
The 5 ppm per
oC
temperature coefficient
of
the
fixed resistors was chosen so that their 0.01%
accuracy tolerance
would hold over the desired
temperature range.
RJ
120
oHM O.0lZ
OF ALL
FIXED
RESISTORS
<
5
PPM/'C
FIGURE
?.11-1. Full Bridge Schematic
for
100
ohm
PRT
The result
([)
given
by lnstruction 6 is 1000
V.A/*
(wheile
V,
is
the measured bridge output
voltage,
an{ V,
is
the excitation voltage)which
is:
X
=
1000
(R"/(R"+81)-R./(R2+R.))
The resistahce of the PRT
(Rs) is
calculated
with
the Briflge Transform Instruction
59:
7-7
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