Calculating CFM if inches of H20 known

[Mike Rappold]

For my flow bench, I have a hand held digital manometer that can be used. I have also built some homemade water manometers.

The question I have is that the hand held digital manometer reads in inches H20. If I know the test pressure and differential pressure, both in inches of H20, what is the formula to calculate cfm from this?

Thanks in advance.

Mike

[slracer]

Mike, Go to the "Formulas" section of the forum. When you open the section, you will find only 1 or 2 entries. At the bottom of the list, there is a dark blue band with some white boxes. In the last box, pull down the menu "the beginning", then click "go". You will find what you want there (check the thread fromTony), then check out the rest. Almost any one that you want is there (or in the spreadsheets, etc section).

Good luck, Doug

[bruce]

I edited that section so it now shows "from the beginning".

I'd do this to all the sections but it would be Information Overload.

[86rocco1]

Basically, you need 4 pieces of information, the pressure differential across the orifice, the area of the orifice opening, the discharge coefficient of the orifice and the air density basically you plug those values int this formula

Where Cf is the discharge coefficient
Ao is the orifice area
ΔP is the pressure differential
ρ is air density.

This is actually an overly simplified formula, just to make it easy to read and understand. You'll probably need add additional coefficients depending on which units you're measuring in. As you look at that formula, you're probably wondering where you get the Cf value from, that's where one of Bruce's calibration plates come into the picture, you flow the plate then you adjust your Cf in the formula until it gives you numbers that match Bruce's calibration. For a detailed formula, look at the explanation in my spreadsheet found in .

[brain]

I'm new to this forum and I've been browsing it for the past couple of weeks and I'm still confused how some of this stuff works. I've been trying to understand how to generate flow curves for all the different orifices sizes in the flowbench.

My flow bench uses two Ametek 115334 motors two 24" Dwyer Manometer and a Dwyer 246 6" Incline Manomoter. It looks similar to a SF-600, but doesn't have near the flow (I need more motors - maybe that's part of my problem). I have various orfice sizes on the internal plate and I have a Superflow calibrated oriface plate from a SF-110, it has 1.875" and .312" holes in the plate and is calibrated to 153 CFM at 10". I've included a link to pictures of the bench.

I've been trying to understand how all the formulas work so I can generate my own flowrate look-up charts based on the different orifice sizes. I've downloaded most of the spreadsheets and have spent a few hours running over calculations trying to understand them - but I'm still lost. Can anyone help?

Brian

[bruce]

Checkout this thread for a percent scale to replace the inch scale you have now on your 246

[brain]

I already downloaded the spreadsheet that allows me to convert inches to percent. Are these the correct numbers for the Dwyer 246 (6") incline manometer?

Tube I.D. 0.125
Reservoir I.D. 0.75
Scale Length 20.0
Scale Vetical Height 6.0
Fluid density 0.826
Scale Angle 17.45760312

I'm a little confused by the numbers, I thought 20" (100%) would correlate to 6" WC, not 5.4". It looks like the tube ID has a direct effect on this? Am I using the correct ID?

% of flow Ds (in) Dp (WC in)
1.0% 0.0020 0.0005
2.0% 0.0080 0.0022
3.0% 0.0180 0.0049
.
.
.
99.0% 19.6020 5.3071
99.5% 19.8005 5.3609
100.0% 20.0000 5.4149


If I'm flowing 10" on my 24" manometer and the incline manometer reads 50% (assuming the above info is correct) is the flow difference 5" across my orifice plate?

Brian

[86rocco1]

[brain]

Thanks for the feedback, 1.875" makes more sense to me too. I'll see if I can't find any other threads on this to confirm this number.

I see the error in my previous post - 50% (1.5") is the difference across the orifice, so the backside of the orifice would be 11.5" and the front side would be 10". I do understand the squared relationship, I've already contacted gofaster@heads1st.com to get the % scale that is adjusted for the squared relationship rather than use the spreadsheet in combination with the 6" scale that came on the incline manometer.

I read one of your posts from May of last year which included a spreadsheet with the following simplifed equation:

flow = c* orifice area*(pressure difference^0.5) and c=.62

I assume this equation will be a close approximation, but to have a calibrated airflow I would need the calibrated reference plate to calculate the variation in my bench and incorporate that factor into the math?

[thomasvaught-1]

The dimension should be 19.85" for the scale vs 20" also. Try that and see what you get.


Tom Vaught

[brain]

I've attached the Dwyer datasheet, it says the 246 has a 20" scale. I'll measure it tomorrow.

[brain]

Sorry Tom, but I measure 20" on the Dwyer 246. Pictures are attached.

[brain]

Now for the other end of the tape measure, sorry the other picture didn't show up when I attached it.

[brain]

My flow bench is up and running, I may have some very minor leaks and probably will need to add another motor or two.

I've been trying to correlate the formulas on the flowbench_util.xls spreadsheet with some data I took off my flow bench. I tried to check this with my SF-110 reference plate to see how close it flowed to 154.8cfm @ 10".

Using the 2.043 orifice plate on my flow bench and a 10" WC on the 48" verticle manometer I measure about 2.1" pressure difference across the orifice plate which correlates to about 32-33% flow based on the spreadsheet (Dwyer 246).

2.043 = 300cfm @ 28" or 189cfm @ 10" (calculated from sqrt(10/28)x300). Do I take 190cfm x .33 for my flow rate? 62cfm seems too low. The formula says 82cfm. Both of these are nowhere near the SF-110 154.8cfm reference plate is calibrate to?

I assume some of this error is due to leaks, and I'm not sure where the best place is to locate the incline manometer tubes on either side of the orifice plate to measure the pressure difference. Can anyone shed some light on where I might be able correct such a large error. Am I applying the formulas and math correctly?

[bruce]

I'll assume the 2.043 plate is the one inside your flowbench? Assuming this a 2.043" plate should flow max of 138.6 @ 6" Dp (this is the rise of your inclined manometer).

Your plate flowed on the SF110 is rated for 154.8 @10" so in theory you should have moved all the fluid up and out of your inclined when you tested at 10 Sp.

In order for the SF plate to flow 100% or 6" of your scale you need an internal plate that is 2.159" in dia. 154.8 @6"

A word of advice, when dealing with calibration issues work with the actual numbers you have and not numbers calculated from some other numbers. Also deal with the leaks, find them and seal them and then work on the calibration otherwise you will be "chasing your tail". Saves a lot of headaches . . .



Edited By bruce on 1234184621