We inherited a 1983 inground 24,000 gal pool/spa last year. The previous owner had changed out the original DE filter with a very nice, very large 4 cartridge Hayward filter about 8 or 9 years ago. The original Hayward 2" pressure gauge was still on this filter and the plastic lense had clouded something fierce. It took a lot of work to make out the pressure readings.
Anyway, this gauge recently read 28.5 psi with no solar and 33-34 psi with solar on. Readings were always consistent in that there were no day-to-day fluctuations, and so could be relied upon, for example, to signal when the cartridge filter needed cleaning.
I replaced this Hayward 2" pressure gauge with a 2.5" pressure gauge from the local hardware store yesterday for about $11.
The "new" readings are now 24 psi with no solar and 28.5 psi with solar. My cartridge filter was last cleaned in November, so it is getting time to clean it once more. (The filter is so large that I can get away with only cleaning it once or twice per year). It will be interesting to see what the real psi readings will be after the filter is cleaned.
So my old gauge was "reliable", but it was certainly not "accurate". I wish I had done this little project last summer in that 1) it would have been nice not to have struggled to see the gauge readings for the last year, and 2) I would be used to the right psi readings under the various operating configurations instead of the inaccurate psi readings.
I would heartily recommend to everyone to junk the pressure gauge that comes with your filter and replace it with a "real" pressure gauge. Plus the larger face of the new gauge is very nice compared to the old gauge.