Section 7. Installation
BRFull()
. In this example, the PRT is in a constant-temperature bath and the
measurement is to be used as the input for a control algorithm.
As described in table
Resistive-Bridge Circuits with Voltage Excitation
(p. 338),
the
result of
BRFull()
is X,
X = 1000 V
S
/V
X
where,
V
S
= measured bridge-output voltage
V
X
= excitation voltage
or,
X = 1000 (R
S
/(R
S
+R
1
) – R
3
/(R
2
+R
3
)).
With reference to figure
PT100 in Four-Wire Full-Bridge
(p. 244),
the resistance of
the PRT (R
S
) is calculated as:
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
PRTCalc()
, the multiplier and offset used in
BRFull()
are 0.001 and
R
3
/(R
2
+R
3
), respectively. The multiplier (R
f
) used in the bridge transform
algorithm (X = R
f
(X/(X-1)) to obtain R
S
/R
0
is R
1
/R
0
or (5000/100 = 50).
The application requires control of the temperature bath at 50 °C with as little
variation as possible. High resolution is desired so the control algorithm will
respond to very small changes in temperature. The highest resolution is obtained
when the temperature range results in a signal (V
S
) range that fills the
measurement range selected in
BRFull()
. The full-bridge configuration allows
the bridge to be balanced (V
S
= 0 V) 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 Ω at 50 °C. The 120 Ω 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
)]
= V
X
• [R
S
/(R
S
+5000) – 0.023438]
The temperature range to be covered is 50 °C ±10 °C. At 40 °C, R
S
is
approximately 115.8 Ω, or:
V
S
= –802.24E–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 Ω /
–
2.006 mV, 60 °C / 123.6 Ω / 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 0.0009 °C.
243
Summary of Contents for CR1000
Page 2: ......
Page 4: ......
Page 6: ......
Page 32: ......
Page 36: ......
Page 38: ......
Page 40: ......
Page 60: ...Section 4 System Quickstart Figure 16 PC200W View Line Graph 60 ...
Page 96: ......
Page 98: ...98 ...
Page 302: ......
Page 453: ...Section 8 Operation Figure 115 Using the Keyboard Display 453 ...
Page 456: ...Section 8 Operation Figure 118 Real Time Custom 456 ...
Page 457: ...Section 8 Operation 8 8 1 3 Final Memory Tables Figure 119 Final Memory Tables 457 ...
Page 458: ...Section 8 Operation 8 8 2 Run Stop Program Figure 120 Run Stop Program 458 ...
Page 460: ...Section 8 Operation Figure 122 File Edit 460 ...
Page 461: ...Section 8 Operation 8 8 4 PCCard Memory Card Display Figure 123 PCCard CF Card Display 461 ...
Page 478: ......
Page 506: ......
Page 536: ......
Page 636: ......
Page 642: ......
Page 644: ......
Page 676: ......
Page 677: ......