Airflow tests flawed? Lack of calibration!

[blackheart]

I purchased a new anemometer and found the vane defective so i ordered another one which worked.
Just to ensure it worked i put the vane to the air conditioner then i put my old one to it and found about a 100 ft/min in difference which would be 2.61 CFM in difference.
I thought It'd be a good idea to perform a trial on an actual vacuum to see what difference is made in measurement. So i threw my Eureka on the box (measured previous at 141.27) I did not change it's bag since i'm not concerned with achieving it's highest possible number I simply wanted to see how different the measurements would be between the anemometers.

In an attempt to eliminate variables I used the same vane on all 3 anemometers. I also re-did the test reconfiguring the panels. I did use the bags to attempt to seal the rounded corners in the 1st test they're just not in the picture.

Here's what i got, again the bag isn't new.
Anemometer 1: 4823 ft/min 125.88CFM
Anemometer 2: 4961 ft/min 129.47CFM
Anemometer 3: 5020 ft.min 131.02CFM

These findings annoy me. I get that the anemometer is cheap they've cost me under $20 but with a nearly 4% change between the meters I kind of feel as though my data is invalid. It's hard to say which of these meters is closest to being correct

 

[huskyvacs]

You're doing scientific research, but I don't think you're going to get accurate answers with consumer grade tools, especially Chinese knockoffs. That range of variance seems normal with tools like that, as they are all roughly calibrated for accuracy on the assembly line, but it's not down to a specific degree. Did you see if there's any way you can calibrate them by hand with a known object? They probably have some sort of potentiometer on the circuit board I bet.

Even with my house power voltage, the power company allowed +/- 5% in the 120v supplied voltage. Right now my house is at 122v.

I'd try sealing all your joints on your air box with foil HVAC tape just to make sure you don't have any air leakage. Maybe a foam gasket around the top to get a better seal when the vacuum is on?

 

[vaclab]

In order to fully check consistency you must

Use all three vanes on one anemometer and then repeat the same tests with the other two anemomters. That would be 9 sets of "mix and match".

And while you're testing for max peak airflow, you must test at least three times over a period of several minutes. Why? Because the vacuum will change its characteristics as things heat up and cool down.

Oh, and you must verify your line voltage isn't changing between testing sessions. Even a 1 volt difference can skew results a bit.

Just an FYI. I usually take at least 30 minutes before I generate even one solid CFM reading.

Good luck!

Bill

 

[broomvac]

I hate to break it to you: Air is a compressible fluid. This is a critical piece to your puzzle. What you are looking at throughout your tests is only half of the picture: volumetric flow.

For example, 100 CFM of air @ 101,325 Pa is not the same as 100 CFM of air at half that pressure, such as what might be experienced at the head of a vacuum cleaner. Mass flow rate of air will not be the same between these two cases, and therefore the work performed by the vacuum cleaner per unit time is not the same.

Take a couple of fluid mechanics courses and you will discover that what ultimately matters is the ability for a fluid stream to do work in a given amount of time. That is all a vacuum cleaner is trying to do, right? A vacuum cleaner with a higher air wattage--a watt is a unit of work per unit time (J/s), also known as power--has the ability to do more work. Fact. This is why it has become the standard measure of vacuum cleaner performance. Just the same way that a car with more power can do more work per unit time. Or a heater with a higher power rating can heat a room more quickly.

I am sorry to say that chasing CFM numbers alone cannot fully paint the picture you are looking for.

 

[vaclab]

Broomvac, what a strange comment

His conundrum is inconsistency of measuring devices, not whether or not air is a compressible fluid. Which it is NOT at normal atmospheric pressure and typical vacuum air speeds.

But don't believe me, how about Kim Aaron, PhD in fluid dynamics from Caltech.

"For low subsonic flow (less than about Mach 0.3), we usually treat air as being incompressible."

Can you imagine if a vacuum moved air at a such a speed that it turned into a liquid? That would be problematic.

Bill