Pressure & Vacuum Gauge Accuracy

[Brewer]

I am attempting to get a good estimate of the flow-rate of my current pump, before swapping my single-speed pump motor for a 2-speed, but I am having trouble with the readings on my gauges. I purchased a Harvard vacuum gauge ( same make as mt filter pressure gauge & polaris test gauge ). Full scale is 30" Hg & the scale divisions are 1/2". The low end of the scale, however only has four major divisions, so I am assuming that it will not indicate a vacuum below 1". The filter gauge, graded in 1 psi divisions has an upper range of 60 psi, however the "peg" is 3 divisions below the 5 psi mark ( 2 psi min. ? ). The test pressure gauge, also graded in 1 psi divs. has an upper range of 45 psi. I my equipment pad was a "normal" configuration, there would be no problem with static head, and all gauges would sit on their pegs with the pump off. I my case, the filter gauge sits 2' below pool-surface level, and the pump gauge ( either pressure or vacuum ) sits 5' below surface level. Pump-off, the filter gauge reads 2-divisions off the peg while the vacuum gauge ( at the pump inlet ) sits on the peg. Pump-on ( clean filter ) the filter gauge reads 24 psi & the vacuum gauge sits on the peg. When the test-pressure gauge is substituted for the vacuum gauge, it shows 3 divisions off the peg static and with the pump on it wiggles off the peg ( 1/2 a division ). The formula used on this forum would indicate a return-head of 55.44'. The calculated static head at the filter ( assuming 2 divisions=2 psi ) would be 4.62', not 3'. The dynamic suction head, assuming 1 psi/div. 1.16', calculated, which I assume would be subtracted from the return head to give a total head of 54.28'. Plotted against the Tristar 3/4 HP curve, the flow would only be about 20 GPM. This seems to be way to low , as the return flow going through the 1" normal return & the 3/4" polaris feed-line is enough to cause the polaris to crawl along the bottom of the shallow end of the pool.
Comments and suggestions are appreciated.

 

[Bama Rambler]

With the filter gauge sitting 2' below the water surface it sees .87 psig when the pump is off.
With the vacuum gauge sitting 5' below the surface it sees 2.2 psig (or +4.4" Hg) when the pump is off.

If your suction plumbing is fairly efficient you may not have any suction head (negative pressure) when the pump is running. (and from the pressure reading you get with "test gauge" it sounds like you don't).

Using just the filter pressure as the total head measurement you have about 55' of head (close to what you got). I don't know what curve you used but the one I found for Tristar pumps has that pump putting out about 70 gpm at 55' of head.

 

[Brewer]

You are right about the curves. The one I had looked at previously, was for the 3/4hp Max-rate pump, not the 3207EE Full-rate pump. At 80 GPM I am probably overloading (slightly) my current DE3600 filter ( Design flow-rate 72 GPM ). Because gauging the suction is such a problem, I will probably install an inline flow-meter into the plumbing if I swap the Tristar for a 2-speed motor, or pop for the Ecostar. Unfortunately, the Edison rebate for the Ecostar runs out in 21 days ( with the rebate it will cost about twice that of a 1 hp 2-speed motor ). the E-star has the advantage of being a virtual "drop-in" package on the T-star mount ( same 2" unions in & out ).

 

[jblizzle]

If you have a Jandy check valve in your plumbing, this is an easy way to add a flow meter:
http://tftestkits.net/Water-Flow-Meters-c12/

 

[Brewer]

Only check valve in the system is on the return side of the heater, with a 90* elbow immediately after. As I have a good 3-1/2' length of horizontal PVC pipe between the filter & the heater, I can install a Blue-White pitot-type meter there, where it can easily be read, and compared with the values of the p-gauges on the pump & the filter.