Just want to share my trouble shooting sequence.

A few days back, I just replaced my Hayward Cartridge filter pressure gauge. This filter is downstream of my Hayward sand.
I buy liquid filled one. The Hayward Cartidge one is a suspect because I can see some sort of fine white deposit on the needle and it has condensation sometimes when my pool boy work "wet" around it. When I tap it with my finger and it can jump 1 psi, its time to change.

So now I assumed I have a more reliable pressure gauge downstream of my sand on the cartridge filter, to experiment.

I been suspecting the Hayward sand filter pressure gauge sort of in accurate lately when at anything above 10 psi. It kept registering 12 psi as being highest when my cartidge is already hitting 15 psi according to the log book. Doesn't make sense. The sand press gauge supposedly reads my cartridge filter pressure + the sand itself. So no way a cartridge press gauge can read higher than the sand one.

So I conducted a test today by slowly closing partially the inlet valve to the pool to create higher pressure reading.
The sand reads 13 psi when the cartridge reads 15 psi...ha ha ha. Before I play with the valve partial closing test, when sand reads 10, cartridge reads 10. This is not possible, the cartridge filter itself will contribute at least 3-5 psi of restriction when in clean state.

I just looked back at my log book May 2007 and it seems I have a sticky press gauge on the sand filter for a year already..... Man, I should have noticed a long time ago. Again thanks TFP because I pay attention more now...YOU GUYS ROCK !!!!

Sand press been always 10 psi regardless of cartridge being 5 to 10 psi.
The moment cartidge hits 11 psi, then the sand starts to creep up. When pump is off, it reads zero.

Will buy new pressure gauge tomorrow.
I can never troubleshoot my poor suction side unless I get the output side correct first...

This kind of troubled gauge is a headache. If I were not to have a second press gauge downstream, I will never knew the sand press gauge was "un-healthy".

I should have noticed this when I did the pipe resistance calculation.... , dumb me. My sand filter is not registering enough PSI / resitance in the first place, considering I have a 100 feet pipe run and at 1.5" which is too small by reccomendation. I can't remember accurately what was the sand filter pressure clean before I install the cartridge filter downstream ( March 2007 ) ...aarrrrggggg.

Thanks all.