If your PSI pressure gauge were measuring correctly, then it should be measuring the pressure at the input to the filter so essentially should be measuring the output (pressure) head though you need to convert from PSI to feet of head by multiplying by 2.3 (these are just two ways of measuring pressure). That would just leave the input (suction) pressure that you would need to estimate. So the fact that you found nearly zero PSI on your pressure gauge seems a bit strange. At 800 RPM, you should be seeing around 4-5 feet of head or 1.7 - 2.2 PSI and around 20-30 GPM flow rate assuming your system curve is hitting somewhere near the efficiency point for this pump. Perhaps the 2 PSI looks like nearly zero on your gauge. As waterbear points out, an accurate vacuum gauge and pressure gauge will give you the precise numbers you want, but notice how relatively flat the pump curves are, especially at the lower RPM. It will be hard to know exactly what flow rate you have, even knowing RPM and Feet of Head. The formula I gave may be accurate, but your error in measuring low PSI or pressures will make the GPM estimate vary quite a lot.

If you measure some higher RPM and PSI points that are easier to measure and estimate the suction head, then you can calculate the GPM with the formula I gave and these multiple points (at different RPM) give you a system curve. You can extrapolate that curve and probably get a better estimate at what goes on at the lower RPM. As a rough estimate for the shape of most system curves, a doubling of GPM requires the Feet of Head to be 3.5 times higher. That is, the relationship is roughly

Head = constant * ( GPM^1.8 )

and at lower GPM of 30 and below the exponential factor is closer to 1.75 where this relationship is for the pressure loss found in pipes. The curve for the filter may be a bit different, but not usually by much since the physics is similar so long as the flow is turbulent which it generally is except for extremely low flow rates. The "constant" is a function of your pipe size and length as well as filter characteristics, but as I mentioned above you can just measure a few points at higher RPM to get a decent idea of the shape of the system curve and then see where it intersects with the pump curves. Or in formulas, you will have two simultaneous equations to solve at your known RPM.

The bottom line, however, is that it sounds like what you are really trying to do is accurately estimate the flow rate so that you can optimize for one turnover of water per day. As you can see, that is harder to do with the IntelliFlow 4x160 since you only know RPM from the pump and not GPM while the full IntelliFlo with its flow meter gives you GPM directly.

[EDIT]

I'm going to use peterl365's numbers as an example to hopefully make things more clear. Remember that Feet of Head = 2.3 * PSI. At 2100 RPM the pressure head is, say, 9 PSI (say 21 feet of head). At 2800 RPM the pressure head is, say, 17 PSI (say, 39 feet of head). Let's first ignore the suction head and then estimate that later. From my formula for the Intelliflo curves we can solve for GPM as follows.

Head = (RPM/350)^2 - (GPM^2)/470

so GPM = sqrt( 470 * ( (RPM/350)^2 - Head ) )

At 2100 RPM, GPM = sqrt(470*((2100/350)^2 - 21)) = 84 GPM

At 2800 RPM, GPM = sqrt(470*((2800/350)^2 - 39)) = 108 GPM

Now let's estimate the suction head and I'll assume two separate 1.5" pipe lines for the skimmer and floor drains. There are many tables and formulas one can use and I am assuming Schedule 40 pipe. In that case, the following approximate formulas may be used:

For 1.5" (nominal; 1.610 inner diameter; 1.900 outer diameter) pipe: Head Loss per 100 feet = 0.011 * ( GPM^1.8 )

For 2" (nominal; 2.067 inner diameter; 2.375 outer diameter)) pipe: Head Loss per 100 feet = 0.0034 * ( GPM^1.8 )

So at 84/2 = 42 GPM, that's a suction head loss of around 9 feet per 100 feet and I assume 100 feet from the pump to the skimmer and floor drains. At 108/2 = 54 GPM, that's a suction head loss of around 15 feet of head per 100 feet. So including the suction head into our formulas above I get:

At 2100 RPM, GPM = sqrt(470*((2100/350)^2 - 21 - 9)) = 53 GPM

At 2800 RPM, GPM = sqrt(470*((2800/350)^2 - 39 - 15)) = 69 GPM

Doing another iteration, at 53/2 = 26.5 GPM, that's a suction head loss of around 4 feet per 100 feet. At 69/2 = 34.5 GPM, that's a suction head loss of around 7 feet per 100 feet. So,

At 2100 RPM, GPM = sqrt(470*((2100/350)^2 - 21 - 4)) = 72 GPM

At 2800 RPM, GPM = sqrt(470*((2800/350)^2 - 39 - 7)) = 92 GPM

I'll do one more iteration even though I made some approximate assumptions. At 72/2 = 36 GPM, that's a suction head of around 7 feet per 100 feet. At 92/2 = 46 GPM, that's a suction head of around 10 feet per 100 feet. So,

At 2100 RPM, GPM = sqrt(470*((2100/350)^2 - 21 - 7)) = 61 GPM

At 2800 RPM, GPM = sqrt(470*((2800/350)^2 - 39 - 10)) = 84 GPM

Now let's use the other formula to see if these two points are consistent with a system curve.

Head = constant * ( GPM^1.8 )

so constant = Head / ( GPM^1.8 )

At 2100 RPM, constant = (21+7) / ( 61^1.8 ) = 0.0171

At 2800 RPM, constant = (39+10) / ( 84^1.8 ) = 0.0168

so at least we have some consistency which is good. So let's say the "constant" is 0.017 and can now use the formula to calculate GPM as follows:

1) Head = (RPM/350)^2 - (GPM^2)/470

2) Head = 0.017 * ( GPM^1.8 )

0.017 * ( GPM^1.8 ) + (GPM^2)/470 = (RPM/350)^2

You can't easily solve for GPM in the above equation, but can iteratively estimate it for a given RPM. At an RPM of 750, the above formula gives a GPM of

0.017 * ( GPM^1.8 ) + (GPM^2)/470 = 4.59

so a GPM of 20 makes the above formula work. The Head is

Head = (750/350)^2 - (20^2)/470 = 3.74 feet = 1.6 PSI so barely measurable on a standard PSI pressure gauge on a filter.

By the way, the IntelliFlo chart that shows 750 RPM is wrong and is really 950 RPM. Pentair has other charts (see PDF page 53, manual page 47 of

this link) for the full IntelliFlo (which is the same pump, but with extra electronics and a flow meter) that show 690 RPM and make clear that the 750 curve is incorrect and the pump formulas should be fairly consistent at all RPM since they are based on impeller shape more than anything else.

The thing to remember is that lower speeds have diminishing returns since the electrical efficiency drops at low RPM. It appears that the IntelliFlo pumps have a fixed electrical loss of around 80 Watts or so. Running the pump at very low RPM would theoretically have less frictional losses, but the fixed electrical losses make this less desirable. It would take another set of calculations to calculate the "sweet spot" which may very well be at flow rates below what it would take to achieve one turnover in 24 hours in which case the turnover requirement would take precedence.

I would estimate the IntelliFlo electrical power at or near the efficiency point at being roughly the following:

Input (electrical) Watts = (Output Watts) * 2 + 80 = (GPM * (Head In Feet) * 0.188165) * 2 + 80

[END-EDIT]

Richard