Filter
Top Products
48
MODEL 54e pH/ORP SECTION 6.0
THEORY OF OPERATION
TPC offers precise control by forcing the pump to feed
chemical for shorter periods of time as you approach
the desired setpoint. If the process faces a large upset,
TPC mode forces the pump to feed chemical for longer
periods of time as the process deviates further from the
setpoint. This action continues until the pump is feed-
ing all the time, providing a speedy recovery from large
up-sets.
The controller can be programmed to be direct or re-
verse acting, depending on the pH (or temperature)
value selected for URV. For example, if the controller is
direct acting based on pH, such as in caustic chemical
addition control, the pH will rise as chemical is added,
so the URV value will be below the LRV (i.e. below
zero). As the pH rises toward the control point value,
the pump will be on for gradually less time. Conversely,
if the controller is reverse-acting based on pH, such as
in acid addition for control, the pH will drop as acid is
added, and the URV value will be positive. The pH will
fall toward the control point value, and the pump will be
on for gradually less time.
Complete TPC configuration is explained and typical
settings for these parameters are listed in Section 5.0.
After startup, the operator needs to adjust only the 0%
On to maintain the desired chemical concentration.
6.6 NORMAL MODE
Normal mode is on-and-off control based on an alarm
setpoint. To prevent nuisance alarms, a hysteresis
(deadband) setting, and/or a time delay can be pro-
grammed during configuration. You can configure each
alarm to trigger above the setpoint as a high alarm or
below the setpoint as a low alarm. The operator need
only raise or lower the alarm setpoint as necessary.
6.7 ANALOG OUTPUTS
The Model 54e pH/ORP controller includes a second
analog output. An analog output produces an electri-
cal current signal which varies in linear proportion to a
value measured by the controller. You can configure
the controller to produce a 4-20 mA or 0-20 mA cur-rent
output proportional to pH or temperature. See Section
5.6 for programming details.
The analog output must be "scaled" so that 4 (or 0) mA
corresponds to the low end of the scale and 20 mA cor-
responds to the high end. The operator can scale the
output as in the following example:
The Model 54e pH/ORP is connected to a strip chart
recorder with a 0 to 100% scale. The average value of
the bath is 10.0 pH, plus or minus 0.5 pH. The opera-
tor wants to match this value with the 50% mark on the
recorder. To do so, the operator selects 9.5 pH as the
4 mA value and 10.5 pH as the 20 mA value by enter-
ing them as in Section 5.3. The chart on the recorder
will display 0% when the pH is 9.5 or below and 100%
when the pH equals 10.5 or greater. (this is for a 4-20
mA recorder).
The Model 54e pH/ORP is also capable of PID control
where the analog output will be proportional to the dif-
ference between the setpoint and the measured vari-
able, either pH or temperature. This control mode is
used to modulate a pump or valve, rather than to turn
a device off and on.