Why does my pool filter have a gauge?

Written by Javier Payan

Ever wonder why there is a gauge on your pool filter?

Your pool filter has a pressure gauge on the top of the upper tank that marks the filter pressure in Pounds per Square Inch (PSI). When your pool filter is new or freshly cleaned, the PSI will register the filter as “clean working pressure”. It is important to note that reading so you know what your pool filter’s pressure should be when the filter is clean. As the filter gets dirty the PSI reading will gradually rise. When the filter pressure rises 3-5 PSI higher than the clean working pressure, it is time to clean the filter.

Residential pool filers usually only have one gauge in the pool system, while commercial pools are required to have two gauges as well as a flow meter to comply with the state and county health codes. In a commercial set up the gauge on top of the filter is considered the (influent gauge) which reads the PSI inside the filter. The other gauge, which gets hard plumbed into the plumbing comes off directly out of the filter. This gauge should be mounted at the same elevation as the filter gauge. This second gauge is called the (effluent gauge) and measures the PSI coming out of the filter.

When the filter is clean, the two gauges should be reading about the same reading within 2 PSI. As the filter gets dirty, the influent gauge will rise and the effluent gauge will drop. This is to aid the County Health Inspector (required for commercial pools) to know if a filter is running clean. The inspector is will know that if the PSI difference between the influent and effluent gauge has a difference of more than 7 PSI, the filter needs to be back flushed or cleaned.

Tip: If you have a variable speed pump, the PSI changes as the speed changes, therefore pick a high speed on the pump and note the PSI on your filter as the “clean working pressure” at that speed. Note the speed you are using to be consistent and accurate.

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