Emerson 54e pH/ORP instruction manual Normal Mode, Analog Outputs

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.

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