Adjustments for flow meters 1

[PEDARRIN2]

thread124-194412

I read through this thread and am having difficulty understanding where the square root function enters in.

I am looking at a Variable Area Flowmeter, specifically Swagelok VAF-M3

Neglecting density and temperatures ratios, (comparing nitrogen and air at ambient conditions), shouldn't P1 x V1 = P2 x V2 where the V's stand for volumetric flow rates at different pressures such as standard (P1) vs system pressure (P2). I don't think I need to consider compressibility factors and I do not think this varies much from the ideal gas law.

Given a 90 psig (104.7 psia) piping system and a flow meter calibrated at standard condition 0 psig(14.7 psia), using the sqrt[P2/P1] gives a different correction factor (2.669) than P2/P1 (7.122).

What am I missing?

 

[GHartmann]

The square root factor comes into play for meters measuring differential pressure to obtain flow.

A variable area meter may or may not be based on differential pressure.

See attached brochure for one that is based on dP.

There are other meters that would be called "rotameters" that will be true "volumetric" flow meters.

And also follow the compensation rules cited:

http://www.tokyokeiso.co.jp/english/technology/tech_flow/f-0007/index.html

 

[PEDARRIN2]

The information from the Swagelok installation isnstructions indicates using the a field adjustment factor using square roots - so I am assuming it is because the flow meter uses differential pressure.

It just seemed strange to me.

I would like to see the math behind it, but it might not be readily available.

 

[GHartmann]

Refer to the factors in the Tokyoeiso guidelines also.

The "rotameter" is really a "force balanced" meter and does involve some square root funtions.

Perhaps Perry or some other reference can guide you. McCabe and Smith has some formulas also, and show how they are derived.

Good luck.