Why does the controller overshoot the set point?
1. Pump is too large
Calculate the flow rate required. The metering pump should be set for no more than 1.5 times the expected depletion rate of the chemical. Either adjust the flow rate of the pump down to this level, or replace the pump with one that better matches the required flow rate.

2. Pump is siphoning
Install anti-siphon valve on metering pump, or on metering pump discharge line. Avoid "down hill" tubing runs. A metering pump injecting into open atmosphere can pump more than its rated capacity if an injection check valve is not installed.

3. Poor mixing/large lag time
Improve mixing, or move the location of the sensor and/or replenishment so that chemical additions reach the sensor faster. If using a flow-through sensor, make sure that the flow rate through the sampling system is sufficient, (less than 1 minute to detect additions). If the lag time can’t be improved, then in pH/ORP applications, a proportional controller should be used. For other applications, it may be possible to use the optional 4-20 mA output for proportional control.

4. Fouled sensor
If the response time of the sensor to a step change in concentration is slow, then the sensor may need to be cleaned, calibrated or replaced. Follow the sensor troubleshooting instructions for that particular sensor.

5. Programmed incorrectly
For on/off controllers, make sure the value of the dead band menu is not too large. For proportional controllers, increase the value of the proportional band menu.

6. Relay stuck closed
Use the "Hand-Off-Auto" menu to control the relay. If an ohmmeter doesn’t show the contact opening and closing, then replace the power supply board or have the unit serviced.