explain regular daily pressure variation?

[robstar]

Hey folks! So, i don't have a problem, just an interesting observation that i can't explain.

I run my pump 5x 1-hr intervals/day and i have a digital pressure gauge in addition to the standard analog one.

What i've noticed is that there's a fairly consistent baseline pressure variation with a period of about a day. The amplitude of the variation is about 1.5psi, so it's not particularly noticeable on the analog gauge. Also interesting is that the variation is there regardless of whether the pump is on or off -- that is, you can see that the pump is *adding* about 11psi when it's on, but the curve continues fairly smoothly despite the offset.



I can understand why the "off" pressure is negative (the water in the filter is suspended about 4ft above the level of the pool by the vacuum), but does anyone have an idea about what would cause the variation? The minimum is around 3-4pm. The pool gets sunlight, but the pump and filter are inside the garage. Maybe i'll start tracking temperature (water, outdoor air, and garage air) to see if that correlates. Maybe i should track barometric pressure as well?

In-ground pool. Cartridge filter. Just one pump. Main drain/skimmer balance comes from a skimmer float valve.

Light gray is the the full range of data, but it's noisy. Dark gray is a 5-min average, which is much smoother.

 

[mas985]

If the gauge is properly vented (to atmosphere), then temperature and air pressure should not have an effect on the reading. But if it isn't vented, you will see that type of variation.

 

[robstar]

oh, you mean that the normal analog gauge shows filter tank pressure relative to atmospheric pressure and that maybe my (probably sealed piezoelectric) gauge is showing an absolute pressure and is just calibrated to show "zero" at 1 bar? Seems legit Does atmospheric pressure really vary by that much during the day, or is it somehow being amplified by the vacuum-suspended water?

 

[mas985]

oh, you mean that the normal analog gauge shows filter tank pressure relative to atmospheric pressure and that maybe my (probably sealed piezoelectric) gauge is showing an absolute pressure and is just calibrated to show "zero" at 1 bar?

Yes, if they simply remove an offset for atmospheric (e.g. 14.7 PSIa), then it will vary with temperature and air pressure. However, if they measure atmospheric pressure too, then they can offset the pressure absolutely. But given that atmospheric pressure changes with elevation, weather and temperature, they should be dynamically compensating for the atmospheric pressure. The manual should describe how it works. If they don't then you will see that type of variation. If they do compensate but the sensor is in a non vented container, it will still vary because the pressure in the container is different than atmospheric.