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How do I implement the following:

I need to control the pH of water by adding sulphuric acid. The incoming water flow rate is measured. The pH of the solution after mixing water and acid is measured.

I have two inputs:
1. Water flow rate
2. Post mixer pH

I have two outputs
1. chemical dosing pump speed.
2. chemical dosing pump stroke.

Do I use two seperate PID loops, 1 for flow and 1 for pH?
Why do I need to adjust pump speed and stroke?

Well, you're in luck. This is easy.

If you have a chemical metering pump that has an input for a flow signal, there's one loop.

If you have a high end chemical metering pump that has an input for the pH sensor, there's another.

This is one of the simplest loops imaginable. Companies like LMI, Pulsafeeder, ProMinent, Iwaki, and a host of others make metering pumps with built in loop controllers.

There are also inexpensive controllers designed specifically for this function.

What you have is what is called a compound loop. The primary loop is the flow proportional loop. It is a "feed forward" loop. This drives the speed
of the metering pump. The secondary loop, the trim loop, which is a "feed back" loop, is the pH meter, and its output drives the stroke length
adjustment.

So no, you don't use two separate loops, exactly, but you do. They aren't separate because they interact. And mostly, you don't use the Integral term at all, and often not the Derivative term. Most of these devices are proportional controllers only.

You can even find some that are simple deadband controllers, and some with hysteresis.

All you got to worry about is proper and rapid mixing and as short as possible physical loop lag time.

Walt Boyes
Editor in Chief
Control and Controlglobal.com
www.controlglobal.com
Mailto:wboyes@putman.net
Read my blog SoundOFF!! At www.controlglobal.com/soundoff

I would set the pump speed/stroke as a ratio of the water flow. The ratio setpoint would be the output of the pH controller, i.e. if the pH was below setpoint the pH controller would increase the ratio of acid to water.

As for the pump I would operate it at variable speed fixed stroke. The stroke set at just enough so the pump is able to control the pH at maximum flow.

The only reason to change the pump stroke is if the pH controller runs out of range, e.g. if the controller output > 90 % increase the pump stroke.
I like to have a metering pump running as fast as possible (high speed/low volume) otherwise the pH will swing too much with each stroke of the pump.

Roy