Compressed Air Flow measuring

High School Science Fair time, cool! Two ways to do this. One get an altinator from a junkyard. Have a friend run the car at different RPM's You hook up electric multimeter and get reading at X RPMs you have Y output. After a few minutes you have a RPM to output graph. Now go to your lawn place and you'll find those weatherveins with a windmills. Attach said to altinator windmill infront of pipe cut at 30 degrees so cup of windmill can pass infront of pipe as close as possible. If cup is at least 1/2 of diameter pipe reading should be accurate.

The second is to go to Us.gov. Type in weights and measures as search and send an e-mail. The stuff you're looking for was done during the Civil War for determining steam pipes for trains and boats. Boiler pipe charts still used today. CFM to pressure for pipe sizes charts should be found there.

1. The air flow rate in any pipe is largely a factor of pressure, allowable pressure drop and length of the pipe.
2. Take the pipe you wish to gauge for pressure drop and flow and connect the specific length of that pipe to a large reservoir, say 250 gallon (33.42 ft³)
3. Put a full flow ball valve on both inlet and outlet. The outlet valve must be as large as the biggest pipe you will test. Bush down for smaller pipes. Also connect an accurate air pressure gauge to the reservoir, at least 4" dia with a full scale of 200 psig. Gauges are most accurate in mid range.
4. Connect your test pipe to the outlet ball valve with one end open to atmosphere and be sure the outlet valve is closed.
5. Open the inlet ball valve and fill the reservoir to 100 psig then close the inlet valve.
6. Be ready to time the flow with a stop watch.
7. Open the outlet valve and start the timer simultaneously.
8. Watch the gauge until it drops to 90 psig.
9. Observe the elapsed time and close the outlet valve to save the rest of the air
10. Repeat steps 5 to 9 three or more times then average the decay time.
11. The flow to atmosphere was 22.72 Std. Cubic feet. (Be quick on large pipe)
12. Divide the flow, 22.7 by the time in seconds x 60 sec/min. for approx SCFM to atmosphere which is sonic, chocked or maximum flow for that pipe with 100% pressure drop.
13. Divide the flow from step 12 by 53.13, Q scfm/53.13 = Cv The Cv for that pipe size at the length tested.
14. This link will take you to my white paper in the education section of the International Fluid Power Society Web page. This particular WP "Approximate Cv" will give you formulas to calculate the flow for your Cv numbers for a pressure drop of either 2, 5 or 10 PSI. http://www.ifps.org/Education/WhitePapers/ApproximateCv.htm

to tom krehers reply. im an engineering student and need to calculate the flow rate of compressed air in much the same way as you have suggested. just wondering if you could discribe the procedure for calculating more clearly as i am struggleing to find where the 22.72 value in part 11 comes from and also the 53.13 that you use in 13 to get the qscfm

i would be very grateful for a reply.

thanks mike.

Is this for an industrial application, like for your job? Something you'll do lot's of? How accurate? I may be missing something here and don't know if my comments are of interest.

I mention this because modern hot wire anemometer, insertion type flow meters are what we use. The electronics are PC configurable for different conditions (pipe size, media, etc.) and require only a 1/2" NPT tap in the line for insertion. If you get one with a long stem, it can be portable and used on anything from 1/2" pipe or tube to however long the stem is.

The other alternative , if you are just testing different setups and/or very small tubing, is to buy one that comes with a length of pipe or tube with the sensors in the tube. You break into your line, install the meter run, read the flow, remove the meter run.

The street price for these is anywhere from $1500 to $3,000, depending on requirements and options. The nice thing is they typically have a turn down ratio of 100:1 or so with around 1/4% accuracy. The other good thing is that they automatically compensate for gas pressure and temperature ( within broad limits) due to their design.

The negative is they need to be factory or otherwise re-calibrated once a year to maintain accuracy certification. A simple orifice rig, a known flow rate fixture, will get around this if your only concern is base accuracy for your own use.

Just Google "thermal mass flow meters". Check Ebay as they are fairly common there, cheap.

Insertion probes: the flexible choice
For various pipe sizes, insertion type thermal flow sensors are one of the most convenient solutions. To insert these devices, you can make a hot tap connection or, if you can depressurize, you can (let someone) weld a connection directly on the pipe.

Installtion procedure
The insertion probe is inserted through a ball valve and can be retracted after use. As long as the line pressure is below 10 bar / 145 psi the force on a 0,5 inch (12,7 mm) probe is below 10 Kg / 22 pounds so you should be able to hold this. But always use an additional safety device, because insertion sensors can become dangerous projectiles!

Configuration
An insertion probe is a spot velocity measurement device (unless you are using a multi point probe). The probe measures the velocity and one point in the tube. After programming in the tube diameter, the probe can display the mass flow rate.

Installation effects
Please note that installation effects and flow profile effects will influence the accuracy of this mass flow indication. This applies for every spot measuring device: Thermal mass flow, ultrasonic single path, vortex, delta p and so on. So nothing new here. Make sure you have basic knowledge about fluid flow through pipes (check efunda.com or engineeringtoolbox.com).

Data readout
Most thermal mass flow probes offer a local display, 4..20 mA, pulse and digital (RS232, RS485) outputs. Our product offers all three.

Limitations and other technologies
Always check flow, pressure and temperature ranges to make sure you choose the right model.

For high temperature applications, a vortex flow meter can be a better choice, although there are some specialized companies that offer thermal mass flow for high temperature as well.

Orifice based differential pressure measurement is an often used to test compressors, But it should not be used in permanent compressed air monitoring installations, as pressure loss is a waste of energy.

Turbine meters can be a safe choice for fiscal metering, but take care with humidity, large pressure and flow pulsations as this may shorten the accuracy and the lifetime.