Losing pressure between pump and diaphragm tank

[PK]

I've just replaced my old diaphragm tank (ruptured bladder), which has given me a chance to observe something I was unaware of. My pressure switch is set to kick on at 40psi and kick off at 58 psi (according to the gauge at the diaphragm manifold). Tank air pressure when the tank is empty is 38psi.

When the pump runs, it successfully drives the pressure up to 58psi and then cuts off. As soon as the pump switch off, however, the gauge shows the pressure slowly beginning to drop until it reaches about 55 psi after a minute or two. Once it reaches 55 psi, it seems to hold pretty steady. I haven't been able to leave it overnight yet, but so far it has held 55 psi for an hour or two with no noticeable loss.

There's a ball shutoff valve immediately downstream from the pressure tank. The pressure drop I've described above occurs even when the shutoff valve is closed, so it seems like the issue must lie somewhere between the well and the pressure tank. I suppose it could be an issue with the gauge itself, although I can't figure out how it could behave like that.

Is this normal... and if not, any thoughts on what might be causing it?

PK

[sammyhydro11]

PK,
there is a valve on your system thet keeps water from rushing back down the well. Depending on what type of system you have it could be a foot valve or a check valve. It sounds like that valve is not closing fast enough and is allowing some of the water back down the well. Do you have a submersible pump or a jet pump? With a submersible pump you would only see the tank in the basement with a pressure switch mounted on the front of it. If you have a jet pump its most likely in your basement. A deep well jet pump will have 2 pipes going into the front and a shallow well jet pump will have 1.

SAM

[Pumpman]

While Sammy may be right, it isn't uncommon for a pressure gauge to drop a couple of psi after the pressure waves subside and things settle down. If you decide to replace it, invest in a good liquid filled gauge. It'll last for years, and will set you back about $20.
Ron

[Bob1000]

Is there a liquid filled manometer? what kind of liquid is that? what make you suggest?

[speedbump]

I'm with Ron, I see this all the time. It's a normal thing.

bob...

[Pumpman]

While a manometer is more for measuring pressure in gases and vapors, they, along with PSIG and PSIA gauges, do indeed, come liquid filled. I believe they're filled with glycerine, which makes them more shock proof and less sensitive to vibrations.
Ron

[PK]

Thanks for the helpful responses. If the gauge is just settling down for the first minute or two, should I raise my pump cutoff switch to let the pump run until the gauge shows 63 psi instead of 58 psi? It seems like that would result in a pressure of 60 psi after the gauge settles.

On my 35 gallon tank, the extra 5 psi would give me about 24% more water in the tank each cycle. On the other hand, I don't want to stretch or rupture the diaphragm if there's any risk.

PK

[sammyhydro11]

PK,
was the gauge replaced?

SAM

[speedbump]

What brand and model of tank?

[Bob NH]

Is there a liquid filled manometer? what kind of liquid is that? what make you suggest?

A manometer measures pressure based on the height of a liquid column. If you wanted to measure tank pressure with a manometer containing glycerin, you would need a column more than 100 ft high.

If you had one filled with mercury, it would need to be 7 to 8 ft high at 40 psi. For 60 psi you would need more than 10 ft.

Not many peope want a 10 ft mercury manometer connected to their water system.