Using Pressure Gauge for Flow Measurement
 

I am looking into building a test rig for WB reviews and I was wondering of the accuracy I could get with a pressure gauge that would measure from 0 to 7 psi.. This is about the range that we see in WC loops. 10PSI is for md30 and above.

So my question is will the psi rating work correctly with the corelation to the pq curve of the pump so that I can get flow ratings. Or is there another factor that I am not accounting for with this idea.

Any input would be great.

Depends.

For a start it would have to be a differential pressure gauge, ie you are measuring inlet to outlet.
Secondly how are you planning to adjust the flow? If it is with a valve this might work but if you adjust the pump speed the curve is different. I don't think it will be very accurate but is maybe worth trying.

A better way would be to measure pressure drop across a known restriction. This is how many flowmeters work. The problem is that the restriction itself might be undesirable. If it is low the pressure drop will be very small and you will need a much more sensitive pressure gauge, e.g. 1psi FS. If it was high enough to measure accurately with a 7psi FS pressure gauge the restriction would be massive.

I am thinking a lot about this myself. Been considering doing just this actually but I don't like the restriction problem.

well they have water pressure gauges for this sort of thing.. it would be right on the outlet.. i would change flow by pinching down on the pump inlet a few inches back... between the res and the pump. if it cavitates i would squeeze it before the res and that wouldnt be a problem.

well mesuring presssure drop across a known restriction is... well its alot more complicated than my method and would not be cost effective from what it seems. correct me if im wrong but i think im right on this.

but anyway.. the measurment point would be right at the pump outlet as i said as this would be where the measurment would be accurate... anywhere else and it would incur the presure drop from the tubing lenght, blocks and whatnot and screw up the calculations.

thanks for the input incorherent.

but stil the question remains will it work?

It'll work, but it relies on the curve being correct, which is iffy (i.e. if your voltages are off, the actual curve won't match, inducing some kind of error).

I also prefer a differential gauge; it's just simpler, and reduces the error.

Make sure that whatever you end up using has a decent resolution; if the graduations (analog) or resolution (digital) are +/- 1 psi, it'll be pretty useless.

I use convert-me.com to convert psi's into something I can relate to.

A flowmeter is really simpler. I picked up one on eBay dirt cheap. Otherwise the best I've seen yet is the Swissflow unit.

http://wbta.us/forums/index.php?showtopic=57

ben can you help me look on ebay to see any models in particular that would soo what i need. Not to sure on test equipment as I am new to the whole test right thing.

btw that is quite an amazing setup you got there.

What's your budget?

lol... well 0 really.. but this will be a beg borrow and stealing thing. Im a college student who doesnt work, so money is in short supply.

over 50 dollars for the flow meter would not really be acceptable i think. anything below is fine. I would love to get a flow meter like the one big ben has, but that thing looks like it costs more than my tuition.

He he, surprisingly, I got it for $31.00, plus shipping.

As I pointed out somewhere else, the trick was to search eBay for the expression "flow meter" and to exclude the word "flowmeter" (no space); it pulls up items that are seen by fewer people, because the search is more complex.

The explosion-proof housing isn't necessary, but I'm not complaining, for that price.
(Claimed accuracy of 1%).

It took me a few months of searching through eBay, but patience prevailed.

wow... ben. that thing looks like it costs about 1000 dollars... u got the meter and the big round and the reader for it for 31... i might almost give my soul for that kind of deal.

I was ready to bid higher, it just never came up. :D

Either way, use my search tips on eBay, and you'll be up and running shortly.

Good Hunting!

well i tried ur tip.. nothing that was reasonable for the flow rates we need. but im on my way to having a temp sensor.. have the bread bored just about done.. just awaiting the maxim chip.

Actually, measuring the dP between inlet and outlet is rather mandatory here, this is what the P-Q curve are based on. Also, in a closed loop, the inlet pressure wont be zero (wrt to atm), unless you pump directly from a (not too tall) res.

Good point, I keep forgetting about that one, even though I've reminded so many... :rolleyes:

Keep at it; you'll eventually find something, like I did.

ok then... well ill keep trying... for now my reviews will have to suffer the mercy of lack of testing gear.. it really sucks as my reviews will make it seem like these blocks do better than they usually will as most ppl wont have an iwaki. but ohwell.

Well a nice chap over at XS is sending me two flow meters for free. So no more need for searching. They are nice little analog units with a range from .5 to 5gpm. They seeem to have an average of .5gpm flow loss.

What accuracy rate to these flow meters have?

not sure.. i asked for some numbers.. we will see.

You mean rotameters?

http://blue-white.com/

accuract yis +-3% full range if anyone cares

...but graduations are +/- 0.2 gpm.

im working with what i got here ben... lol.. cut me some slack.. when i can afford some better test gear I will get better test gear.

I just got a Dwyer VFC-141 off e-bay. Mine is 2% through the scale. It also have conections at the bottom and top instead of the side which takes 2 restrictive 90's out of the loop. And it is 1" connections so I am not expecting a killer flow loss. I plan on using my Hydor500GPH pump (same as the one pH uses) so power shouldn't be to much a problem anyway. That thing has some serious pressure.

http://www.dwyer-inst.com/htdocs/FLO...-VFCIISpec.CFM


Anyway as with you i am going to work with what I got untill I can get around to ordering the parts to my "good" test bench. :D Looks like finding a distributor for the SwissFlow SF800 meter might be a problem...

hmm.. even better accuracy.

Well the one im getting has 3/8npt has either end. So i am getting 3/8npt fittings with 5/8 barbs. With those fittings it should reduce the head loss by alot.

As Bill pointed out, remember that these require a straight run before and after. Typically 20d (d=diameter) at the inlet, and 10 d at the outlet, check the manufacturer for the setup requirements.