wtw CL 298, Руководство по эксплуатации, Страница: 43 / 76

pH 298, LF 298, Oxi 298, Cl 298
Outputs
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ba77196e03 07/2018
Output with two
relays
figure 5-3 Output with two relays
Proportional
output with break
point
Control with a simple proportional range is not suitable for all control types.
The controller integrated in the measuring transmitter can be adapted to meet
the special requirements of the process. The general static characteristic curve
1 Start value
Measured value with minimum pulse width or switching frequency
2 End value
Measured value with maximum pulse width or switching frequency
Proportional bands
Relay 1 and 2
Measured value
Relay 1
Relay 2
1
2
2
1
10
100
90
0
Switching frequency f or
Pulse
v width
Processes to control the pH value are nonlinear. Frequently, the
transmission constant in the range of the target value is consider-
ably greater than at the limits of the control range. The use of the
controller with fixed setting values will result either in an instability of
the control cycle near the target value or in extremely long settling
times (for batch processes) or high deviations (for continuous
processes with greater disturbance variations).