A pressure gauge is attached to the top of your filter. The reading is in pounds per square inch (PSI). Think about it. What this small, round dial is telling you is that the pressure inside the filter is (as an example) 30psi. Imagine a square inch, not very large, is it? Packed inside that little square inch is 30 pounds of energy! Now, think about if you have a 300 square foot filter and every square inch of that is 30 pounds. That’s a lot of power. Do not take it lightly.
Here’s how it works. Every system has a different starting pressure depending on the horsepower of the pump, the size of the filter, and how far or close the equipment is to the pool.(technically called feet of head, but we won’t get into that). When you install a new filter or clean the existing cartridges, you get the starting point of where your pressure is. (This is assuming you don’t have blockages, but I’ll discuss that in a minute). Let’s say the gauge starts at 20psi. I like to mark the number with a Sharpie. From this point of reference many future situations can be determined:
1) A filter needs to be cleaned when , preferably before, it reaches 10 pounds higher than the starting point. If the starting point is 20psi, the filter must be cleaned before it reaches 30psi. Personally, I don’t like to wait that long. Many people are under the misconception that if their filter says “up to 50psi capacity” they can let it go to 50psi. All that is saying is that the filter body can take up to 50psi before it explodes. Prior to that, you will have damaged the cartridges and/or manifold inside the filter. Rule of thumb, clean the cartridges before the pressure gauge reaches more than 10psi past the starting point.*
2) If your starting point reads 20psi and you go out one morning and notice the pressure is down to 10psi, what do you think is happening? Anytime you get a reading below the starting point it indicates a blockage on the suction side of the system. Check your pump basket, check your impeller (usually the culprit), and be sure the weir gate at the skimmer is not stuck in the up position, preventing water from flowing in at a continual rate. (When your pump seems like it wants to prime, then stops, then primes again, then stops, most likely it is a stuck weir gate or a lack of water in the pool). Another situation could be the ‘O’ rings in the Jandy valves or a leak at the pump outlet. All these things must be taken under consideration when determining why there is a drop in psi.
3) It could just be a bad or clogged pressure gauge. Not likely unless it registers at 0, but it could be a possibility.