Flow Bench Instrumentation 1

[matthewx]

Our company is working with a Spartan budget and I have been asked to develop INEXPENSIVE flow measurement for air flow into our product. We have air actuated tools that we want to check the air flow consumption of in order to provide a guide to our customers of the minimum air requirements to operate our equipment. This is necessary because our tool is often used in remote areas with limited air options so we want to give our customer guidelines on what they need to provide for full tool capability and how much to der-rate the tool's capability for lower than optimal air supply.

The least expensive route appears to be to measure the Differential pressure over a 15 foot stretch of straight pipe while the tool is in operation. Using the formula on this site, it appears that around 55 cfm and 90 psi (Our expected compressor pressure for our flow bench) we would have around 20 inches of pressure drop across this 15 foot section of straight pipe. Lower flows will obviously result in a lower differential pressure reading and of course higher ones would show a higher pressure drop as well. The idea is to just use a long section of straight pipe instead of using an orifice plate.

My question is whether this method will provide a reliable flow reading. If so, we will get a Differential Pressure transmitter with display readout and use the formula to determine our flow based on the Differential pressure readout. We don't require super accuracy but we do hope to have reasonable accuracy (Say within 5%). If not, is there a better means that isn't cost prohibitive ? Thanks for the help. Our pipe size is 3/4".

 

[VEBill]

Many automobiles are equipped with mass airflow measurement systems that are inexpensive, accurate and (sometimes) reliable.

 

[VEBill]

I missed the 3/4" on the first reading. Auto MAF sensors would not be suitable for such small pipes.

 

[DRWeig]

Sounds reasonable to me.

Your accuracy will be limited to either: 1) the accuracy of the assumptions you will make in characterizing your length of pipe, or 2) the accuracy with which you calibrate your transmitter (flow computer). Or both.

Your method will be very repeatable. The challenge is getting an accurate flow at a single point so you can calibrate it. If you know true flow at one measured pressure drop, you should be good to go as long as your density (temperature, moisture content) doesn't change much. If you're concerned about true accuracy, see if you can calibrate it closely somehow. Rent or borrow an accurate flow meter? Blow air into a calibrated balloon or container?

That's my take.

Good on ya,

Goober Dave

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[danw2]

The automotive MAF's are thermal dispersion meters. Vendors like Sierra, Kurz, Brooks, EFM, and a dozen others offer thermal dispersion flow meters that size to the pipe size.

 

[MiketheEngineer]

I suppose you could run a compressor up to say 100 psi. Now you know the pressure and the total volume of air.

Turn off the compressor. Run the the tool for say 15-30 seconds. Check the new tank pressure - do the math!!

Not all that accurate - but very CHEAP and might be good enough??

 

[matthewx]

Thanks for the input. It looks like we will probably use a DP sensor for this project as that looks like the least expensive option. However, there also appears to be an inexpensive turbine flow meter and we will look at that option too because it will be a bit more User-Friendly.

 

[roydm]

Why not use a vortex shedding flowmeter, they work well on air. It will give you air velocity which you can convert to volume or mass.
Roy