How Does Proportional Feed Work for Maintaining pH?

In Blog by David Jerkins1 Comment

Most chemistry controllers offer a function typically called “proportional feed” or “time modulation”. This function is important to carefully managing water chemistry, particularly when it comes to muriatic acid, which at full strength will drop pH rapidly in small bodies of water. When purchasing a controller, this function should be sought after as it offers the best flexibility in managing chemistry.

To explain how proportional feed works, we will use a simple scenario:

  • Controller is set up to run a metering pump to pump muriatic acid into a pool.
  • This controller has a feed period configured at 60 seconds. This does not mean that the pump will run for 60 seconds every time as we will see.
  • The TurnON setpoint is 7.6 and the TurnOFF setpoint is 7.4. This means that the target pH is 7.4.

Here’s where the proportional part of this comes in. Let’s say the pH probe is currently reading 7.8 pH. This is high. It is above our TurnON setpoint of 7.6, so the acid pump comes on of course. It will usually start the feed at full 60 seconds. Why? Because at 7.6 or higher, that is 100% (or more) of our TurnON Setpoint, so it runs at max.

Under normal operations, the pH should never be as high as that scenario. Let’s instead say our pH is at 7.41. The pump might only run for a single second. It’s running proportional to the setpoints we’ve set. The pump might run for 1% of the feed period, or 2%, depending on several factors. These include the primaries such as feed period and setpoints but also on how long the pump has been running in the last hour, the variance in the chemistry in the last hour, and others.

Proportional feed allows the chemistry to stay as near to the setpoint as possible while minimizing risk of overfeeding or bouncing values. It eases into it.

It’s often not desirable to set it to the minimum feed time. If you set it to the minimum, the proportions will all be short. A longer feed period will allow for more gentle sloping and will usually mean that it will never reach that TurnON setpoint that’s configured, always gently keeping it close to the TurnOFF setpoint instead. For example, if the feed period is set to 10 seconds and you need to run for 1% how is the pump going to run for 0.1 second? Most will have a minimum feed time of 1 second. A longer max feed period will mean it runs at a more reasonable proportion.

This is of course not limited to acid and pH. Any chemical being injected into the system can be handled with proportional feed. Another common chemical that should be handled this way is liquid chlorine.

Note: Never use proportional feed with chlorine generators. While metering pumps can be turned on and off many many times, salt systems are a bit like engines in that they should not be started and stopped often. The output should be a simple on/off for chlorine generators.

A final word on muriatic acid and metering pumps: If it’s a very small body of water (typically less than 1000 gallons), then proportional feed and very short feed periods may still not be enough to avoid overfeeding. In those cases, the acid should be diluted, and variable rate metering pumps used to better control the pH.

By using proportional feed, more consistent water quality can be attained. If you have any questions or would like to know how to check your configuration on your TMI RSC controller, please contact us at tech@tmiaquatics.com.

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