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Programming and Operating Concepts
Video Recorder – User Manual 65
Time Proportioning Relay Driven Pump
A second control scheme is to use a relay to produce a time proportioning or Duration Adjusting Type
(DAT) control signal. Such an application is depicted in Figure 3-17.
PV 4.00
SP 7.00
OUT 90.5%
INSTRUMENT
LINEAR pH
TRANSMITTER
4.00
pH
CAUSTIC
REAGENT
4 TO 20 mA
DAT CONTROL
SIGNAL
PUMP
WASTE WATER TREATMENT VESSEL
WITH IMMERSION STYLE pH ELECTRODE
AND MIXING IMPELLER
Figure 3-17 Control Of Wastewater pH Using A Time Proportioning (DAT) Control
Signal
This application requires a basic time proportioning control loop to monitor and control the pH of the
wastewater to a local set point of 7 pH units. That is, the loop will “neutralize” the wastewater so that it
can be safely released to the environment. The wastewater pH, which is assumed to be primarily acidic,
will be controlled by introducing a caustic reagent to the contents of the treatment vessel. This will be
done through use of a time proportioning relay signal that will pulse a pump connected to a caustic
reagent source.
A function block diagram representing the control scheme of Figure 3-17 has been drawn in
Figure 3-18. The same diagram method was used to produce Figure 3-16.
%
CONNECT
TO PUMP
AI1
AI1 OV
RANGE LOW = 0
RANGE HIGH = 14
CIRCUIT LOW = 1
CIRCUIT HIGH = 5
LP1
PV
LP1 OV
TYPE = STANDARD
SP1 = 7.00
AO1
IN
OUTPUT TYPE = DAT
IN LOW LIMIT = 0
IN HIGH LIMIT = 100
IMPULSE TIME = 150
FB
AO1 BC
DO1
4 TO 20
mA
250 Ω
+
1 TO 5
VDC
-
Figure 3-18 Function Block Diagram Of Figure 3-17
This drawing is similar to the temperature control application. The analog input, control loop, and analog
output function blocks (AI1, LP1, and AO1) have been used similarly. The discrete output function block
was added, drawn as a circle at AO1’s apex and named “DO1.” Recall that any analog input, control
loop, analog output, or discrete output available may be used. Up to 36 discrete outputs (DO1 through
DO36) are potentially available depending on the instrument’s model number.