Flowmeter Placement Questions

[BigDave]

Three quick questions:
Can I install a pitot tube type flowmeter in the straight section of pipe leading to the pump inlet?
Will it work on the suction side?
Does the inlet pipe have to be the full size of the pump inlet?

[Manslick]

I say yes to all three.

What type are you using?

Like this? http://www.keyinstruments.com/images...ucts/pr/PR.pdf

[BigDave]

Yep like that, Ben had pointed me to this http://www.bluwhite.com/Products/Var...roductpage.asp.
Thanks!

[mas985]

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.

[BigDave]

Good point, junk in the water.
Thanks

[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?