pH Correction
The action of a pH control system will be to either add an acid reagent to lower the pH of a solution to a desired value, or add an alkali reagent to raise the pH of a solution to a desired value. In some instances such as waste water neutralisation it may be necessary to do both to meet consent levels.

Most pH control systems utilise on/off, or time proportional and pulse proportional control. The instrument has a set point which activates a relay when the pH reading is beyond a set level. This switches either a valve or a dosing pump on to add the reagent.

Sufficient time must be allowed for the added reagent to mix and react with the sample. In very general terms a resident time in a mixing tank of 3 – 5 minutes is a reasonable minimum. (Resident time can be calculated by dividing the volume of the tank by the flow rate through it).Thorough mixing prior to the measurement point is essential, but long lag times create overshoot. Ideally the lag time between the dosing point and measurement point should be between 5 and 30 seconds.On/off control devices can only be successfully used when dosing into systems which have sufficiently large mixing tanks to smooth out the changes in pH which will occur between dosing and not dosing.If inline dosing is proposed then a proportional controller with a modulating control valve, or dosing pump with an automatic stroke positioner is essential.

pH instruments offer two types of output which can be used to control a valve or a dosing pump. This will either be a relay switch output or a linear 4-20mA current output signal.

The relay output can be set to energise on either rising or falling pH values (high or low operation). This allows it to control the addition of acid or alkali reagents. The relay will have a dead band (hysteresis) which is used to prevent the relay switching on and off for very small changes in pH. In order to provide a more precise form of On/Off control the relay is switched on and off as the desired set point is approached. This slows down the addition of the reagent and helps to prevent overshoot. In time proportioning mode the pulse on and off times vary in proportion to the deviation from set point, whereas in the pulse proportional mode the pulse rate varies.An analogue output signal of 4 – 20 mA can be utilised to adjust the opening of a control valve or the dose rate of a pump.

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