Question Regarding Measurement of Vacuum Pump Air Flow Using Anemometer

Hello everyone, I am new to Instructables and am loving it so far.
I was reading through some literature on a piece of equipment we have at work and it requires 18"Hg @ 25CFM. We have flow gauges at work, so it is just a matter of using one to determine the flow but it got me thinking...
What is the most effective method of measuring airflow of a vacuum pump without a flow gauge. All I have is an anemometer, a basic understanding of physics and time. So I came up with this method and am looking for some folks to poke holes in it:
I placed the anemometer at the end of a 36" piece of PVC with a 1.6" I.D. sealing the anemometer to the PVC to ensure no air gets pulled around the fan of the anemometer. The other end of the pipe is reduced to a 1/4" line (.16" I.D.) which is connected to the vacuum pump.
Turning on the vacuum pump yields a steady 3.5MPH (307.98 ft/m)on the anemometer.
Doing the math, I come up with 4.3CFM (307.98ft/m*.013962634ft^2). 
Unfortunately, I cannot bring a gauge home from work to compare and I cannot bring my set up, as glorious as it may be. So this is more a thinking exercise.
So what sort of accuracy would you guys think this 'rig' has? Can you think of some method for me to verify it's accuracy (or, more likely, lack thereof)?

NachoMahma7 years ago
lemonie7 years ago

What is the device for which you need to measure the flow, and what type of pump do you have?
Remember that if air is going into a pump it will also be going out, so you should be able to measure flow on the exhaust.