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SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES
7-9
FIGURE 7.11-1. Full Bridge Schematic for 100 ohm PRT
7.11 100 OHM PRT IN 4 WIRE FULL
BRIDGE
This example describes obtaining the
temperature from a 100 ohm PRT in a 4 wire
full bridge (Instruction 6). The temperature
being measured is in a constant temperature
bath and is to be used as the input for a control
algorithm. The PRT in this case does not
adhere to the DIN standard (alpha = 0.00385)
used in the temperature calculating Instruction
16. Alpha is defined as ((R
100
/R
0
)-1)/100,
where R
100
and R
0
are the resistances of the
PRT at 100°C and 0°C, respectively. In this
PRT alpha is equal to 0.00392.
The result given by Instruction 6 (X) is 1000
V
s
/V
x
(where V
s
is the measured bridge output
voltage, and V
x
is the excitation voltage) which
is:
X = 1000 (R
s
/(R
s
+R
1
)-R
3
/(R
2
+R
3
))
The resistance of the PRT (R
s
) is calculated
with the Bridge Transform Instruction 59:
R
s
= R
1
X'/(1-X')
Where
X' = X/1000 + R
3
/(R
2
+R
3
)
Thus, to obtain the value R
s
/R
0
, (R
0
= R
s
@
0°C) for the temperature calculating Instruction
16, the multiplier and offset used in Instruction
6 are 0.001 and R
3
/(R
2
+R
3
), respectively. The
multiplier used in Instruction 59 to obtain R
s
/R
0
is R
1
/R
0
(5000/100 = 50).
It is desired to control the temperature bath at
50°C with as little variation as possible. High
resolution is needed so that 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
s
) range which fills the
measurement range selected in Instruction 6.
The full bridge configuration allows the bridge
to be balanced (V
s
= 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 50°C. The 120 ohm fixed
resistor balances the bridge at approximately
51°C. The output voltage is:
V
s
= V
x
[R
s
/(R
s
+R
1
) - R
3
/(R
2
+R
3
)]
= Vx [R
s
/(R
s
+5000) - 0.023438]
The temperature range to be covered is ±50
±10°C. At 40°C R
s
is approximately 115.8
ohms, or:
V
s
= -802.24x10-6 V
x
Even with an excitation voltage (V
x
) equal to 2500
mV, V
s
can be measured on the +2.5 mV scale
(40°C = 115.8 ohms = -2.006 mV, 60°C = 123.6
ohms = 1.714 mV). There is a change of
approximately 2 mV from the output at 40°C to the
output at 51°C, or 181 µV/°C. With a resolution of
0.33 µV on the 2.5 mV range, this means that the
temperature resolution is 0.0018°C.
The 5 ppm per °C temperature coefficient of the
fixed resistors was chosen so that their 0.01%
accuracy tolerance would hold over the desired
temperature range.
The relationship between temperature and PRT
resistance is slightly nonlinear one. Instruction
16 computes this relationship for a DIN
standard PRT where the nominal temperature