Flowmeter Placement Questions

[waste]

Keep in mind that you need 14 times the pipe diameter of straight pipe for the flow meter to remain accurate. Most pumps don't have that much pipe on the suction side. Also, on the suction side of the pump you have to deal with debris clogging the flow meter. I would recommend a pressure side install after the filter.

How would it effect the accuracy, would it read higher or lower? What would the effect be on 2" pipe if the flow meter was installed ~ 6" from a 90 or 45?

[mas985]

I haven't seen any data regarding that but the manufacture's specify the length for a reason so I would assume it is important. If the meter is too close to a fitting it could read higher or lower. Close to fittings, the pressure varies considerably in different locations of the pipe. I think the specification are set to ensure that there are no local eddies or pressure differentials. Any closer and there is no guarantee.

Venturi type flow meters are not all that accurate to begin with. They usually have an error which is 10% of the maximum reading so if your flow meter goes up to 100 GPM, that is +- 10 GPM. Unless you spend a lot of money for a paddle wheel flow meter, the accuracy is probably worse than a direct head measurement.

[waste]

Thanks Mark! I've got 6 pools with 8 filters and flow meters and only ONE of them is installed in the correct part of the pipe, 2 of them are installed on > 1' sections of pipe with 90s and/ or 45s on either end. According to the readings I take every day, I don't have enough flow to do the required 4 turnovers/ day - I was hoping that your answer would be that the flow rate would read low. (I imagine that the pools were designed to do the 6 hour turnover rate, but I can't get my hands on the blueprints [GRRR!!!] - the company that built them is an established pool co., but some things I see with our pools makes me dubious about how qualified they are to build a bird bath.)

[mas985]

You could always confirm/calibrate the flow meters with a head measurement. Assuming of course, you have a head curve for the pump.

[BigDave]

Sorry for being dopey Mark but... How would you measure flow using pressure?

[mas985]

Every pump has a head curve which shows head vs GPM. To determine GPM, you need to know the plumbing head loss and to find that you can measure the pump pressure on the return side and the pump suction. Most pumps have drain plugs which work fairly well to measure both pressure and suction with a pressure and suction gauge. Once both measurements are done, you can find head loss with the following equations:

Return Head (ft) = Pump Pressure (PSI) * 2.31

Suction Head (ft) = Pump Suction (in-hg.) * 1.13

Total Head (ft) = Return Head + Suction Head

Once you have Total Head, you can look up the GPM on the pump's head curve.


There are a couple of other methods that are a little easier to do but slightly less accurate. For example, you can use the filter pressure gauge to determine return head but then you need to use a slightly different equation to calculate the return head because the gauge is above the pump:

Return Head (ft) = Filter Pressure (PSI) * 2.31 + 3


I can also estimate the flow rate by knowing your pump model, filter pressure and a description of the suction side plumbing (pipe diameter, length and number of runs).

[BigDave]

Ah, That makes sense. Thanks.
However, What I'm planning to do is put in a variable speed pump and adjust the pump speed to meet the flow requirements of the solar + filter + piping as built and the flow requirements of the filter + piping (w/o solar). This configuration would also (I'm thinking) allow me to meet the needs of a larger AG or an IG in the future. I realize that available VS pumps are WAY overpowered at full speed, but can be backed down enough for my small pool, and operates efficiently at lower speeds.
+/- 10% of full scale of the flow meter would leave me well within the operating range of the filter and solar.
Am I crazy?