Figure 3-21 Control Of Wastewater Ph Using A Time Proportioning (Dat) Control Signal; Figure 3-22 Function Block Diagram Of Figure 3-21 - Honeywell VPR100 User Manual

Programming and Operating Concepts
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 3-21.
CAUSTIC
REAGENT

Figure 3-21 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 3-21 has been drawn in Figure 3-22.
The same diagram method was used to produce Figure 3-20.
250 Ω
+
4 TO 20
1 TO 5
mA VDC
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 24 discrete outputs (DO1 through DO24) are
potentially available depending on the instrument's model number.
64
INSTRUMENT
PV 4.00
SP 7.00
OUT 90.5%
DAT CONTROL
SIGNAL
PUMP
AI1 OV
AI1
-
RANGE LOW = 0
RANGE HIGH = 14
CIRCUIT LOW = 1
CIRCUIT HIGH = 5

Figure 3-22 Function Block Diagram Of Figure 3-21

VPR & VRX – User Manual
LINEAR pH
TRANSMITTER
4 TO 20 mA
4.00
pH
WASTE WATER TREATMENT VESSEL
WITH IMMERSION STYLE pH
AND MIXING IMPELLER
AO1 BC
FB
PV
LP1 OV
LP1
TYPE = STANDARD
SP1 = 7.00
CONNECT
TO PUMP
Õ
IN AO1 DO1
OUTPUT TYPE = DAT
IN LOW LIMIT = 0
IN HIGH LIMIT = 100
IMPULSE TIME = 150
12/00