Measuring actual air CFM?

[BioHazard]

I've come to learn that almost all (below industrial grade) compressor MFGs lie about their actual CFM output, and almost all tool MGFs lie about their tool air consumption.

Is there some way a person can measure actual CFM input/output at a given pressure? Kind of like an amp meter for air - a CFM-o-meter. Is there something I can buy like that, that doesn't cost a million bucks?

 

[geko29]

It's got to be possible--the gas company meters your usage by volume (which they convert into therms). All that would be needed would be to add a time dimension to the volume recording, to get to the flow rate.

I can't tell you where to find it, but something to do this should exist.

 

[djjsr]

Google 'air flow meter'

 

[BioHazard]

Google 'air flow meter'

I tried, all I got was HVAC and engine air intake stuff.

 

[z28snksknr]

To really evaluate the flow of the compressor at 90 psig, you would have to install a valve that you throttle back the flow out of the compressor (no regulator) to the point that the pressure remains constant with the compressor on and air flowing out. That way, your flow rate of air out of the comressor tank is the same as the air flow rate from the compressor. Then, you can measure the amount of time it takes to fill a certain volume. Using a light weight trash bag would be a good way. If you calc the volume of the filled trashbag and the amount of time it took to fill it, you will have CFM at atmospheric conditions. Capture the temperature and barometric pressure at that time.

Then, all you would need to do is use the following formula:

P1*V1 = P2*V2

Where:
P1 = barometric pressure converted to psia (not psig)
V1 = trash bag volumetric fill rate (cubic feet per minute)
T1 = Ambient temp

P2 = 104.7 psia (90 psig + 14.7psi (atmospheric pressure))
V2 = what you are solving for - the volumetric flow rate @ 90 psig in cubic feet per min

So it simplifies to:

P1*V1 / P2 = V2

There are alot of other factors involved here- compressor heat input resulting in air temp being different than ambient temp (difficult to measure adequately) innacuracies with your throttling valve setting, etc., but this should give you a good idea of the flowrate of air that you will actually be using for your tools.

I can give a much more detailed experiment and calculation, but the end result (answer) may only vary by 10% or so IMO.

Does that make any sense?

 

[sberry]

Most of the output specs on comps are fairly accurate, the way they do horsepower is a different matter. But, about 4 cfm per real hp for 2 stage units, roughly maybe an amp and a half per cfm. As for tools, if they are listed as avg cfm about 3 times that for actual air consumption. This is just generalization but really close enough for most cases.

 

[larry_g]

Search out 'Dwyer'. They are in the flowmeter business, as well as others.

lg
no neat sig line

 

[W-Cummins]

The easy way to do some thing like this would be to time to fill the compressor from tank at atmospheric pressure to say 100psi. You know the tank volume and you could calculate the CFM from 0-100 psi. Better yet you could use this to compare 2 compressors. I also use this to tell how the machine is running, when you get a new compressor time it from start to shut off psi and record this value to use to check if your machine is getting "slow"

William......

 

[Mstrfxit12]

This might help you.
http://www.hedland.com/index.cfm?fa=prd.detail&sec=384

 

[gearbuilder]

I bought this flow meter from ebay. It will measure up to 48 scfm. I rebuilt an old Gardner Denver compressor but it still didn't pump like I thought it should so I bought this to check it but haven't got around to it yet.

I haven't figured out how to install it between the pump and the tank to measure the output of the pump with a tank pressure of say 90# or 150#.
It is made of plastic and may not like the temp. of the air as it leaves the pump.

Jamie



View media item 3842

 

[njfl]

I bought this flow meter from ebay. It will measure up to 48 scfm. I rebuilt an old Gardner Denver compressor but it still didn't pump like I thought it should so I bought this to check it but haven't got around to it yet.

I haven't figured out how to install it between the pump and the tank to measure the output of the pump with a tank pressure of say 90# or 150#.
It is made of plastic and may not like the temp. of the air as it leaves the pump.

Jamie



View media item 3842

Just a thought, consider getting two of these and putting T's on the top and bottom. Then the airflow through them would be in parallel. I think that if you get 48SCFM with one, then you can get a max of 96 (48x2) with two in parallel.

 

[bdog]

This may not be 100% correct but you can get a close estimate with a stopwatch and a calculator.

Look at your pressure gauge on your tank and write down what pressure it kicks on at and kicks off at. Subtract this to find the PSI change.

Now time how long in seconds it takes your compressor to go from on to off.

Look and see the capacity of your tank in gallons.

Now for the math.

1) Convert gallons to Cubic feet. Simply divide the number of gallons by 7.48.

2) Divide psi difference number by 14.7 (atmospheric pressure).

3) Multiply 1 & 2.

4) Convert the pump up time in seconds to minutes. Take your answer from 3, divide it by the number of seconds then multiply by 60. This is the approximate CFM of your compressor

 

[gearbuilder]

Just a thought, consider getting two of these and putting T's on the top and bottom. Then the airflow through them would be in parallel. I think that if you get 48SCFM with one, then you can get a max of 96 (48x2) with two in parallel.

The flow capacity of the meter is adequate at 48 scfm. The output of this 5-horsepower compressor should probably be around 17 scfm at 150 psi. My problem is that the compressor has rigid tubing with cooling fins between the pump and the tank to cool the air. I haven’t come up with a way to splice in there without damaging the tubing.

Jamie

 

[JerseyJim]

My company actually runs tests that determine air leakage of various things using a UL certified air leakage chamber. The method used would be suitable for this purpose. A pressure gauge (water column) and a flow meter are used. The idea is to determine the maximum flow while maintaining a constant pressure. We don't typically run it at the pressure the compressor is rated for. But the method would be the same. You would select a pressure and then charge the chamber. With air being fed through the flow meter, a discharge valve on the chamber is then opened and the flow through this valve is allowed to increase until the pressure cannot be maintained and begins to drop. The amount of air required to make up the lost air while maintaining pressure is measured and recorded as the leakage in our case... But would represent the compressed air system's maximum capacity at any given pressure.