Measuring CFM?

[Vacuum Facts]

Seems like lower velocity x larger area of vane = higher velocity x lower area of hose.
So is the vane anemometer somewhat accurate to first order? Or am I full of it?

Airflow in does equal airflow out (known as mass continuity). A reduction in diameter of the flow tube does increase air velocity (this is described by the Bernoulli effect). You’re possibly thinking that if the air velocity is reduced at the wider end because the reducer widens the hose to match the vane size, then you’re underestimating speed. That’s not actually the reason why readings are inaccurate. The vane anemometer expects laminar, uniform flow. If you don’t present that to the instrument, then it will give an incorrect reading. This error can occur if the anemometer doesn’t match the pipe diameter and therefore measures a different speed of the same airflow, or, even if it does match perfectly, the pipe won't generate flow that's laminar and uniform. This latter case is most common. I calculated in this video the amount by which it misreads and provided a detailed explanation of the science why. I also provided methods to correctly calculate airflow so each can compare their machines.

You can accurately time extraction of a known volume (need to use diverter valves to avoid motor startup time). Or, if you’re more scientifically inclined and have a ‘laboratory’ in your basement, you can measure pressure drop across a carefully machined orifice plate and use the Bernoulli equation (I can help with the design of this), which is also standard practice in industry and follows well known industry standards. You can even buy instruments to do the pressure measurements for you that spit out accurate airflow values if you have the correct experimental setup. If you’re more skilled, you can even build the pressure measurement instruments yourself using differential pressure transducers.

Ultimately, measuring airflow at an open hose when there is no resistive load is not representative or indicative of a machine's cleaning potential, and the science why is explained here. All the information needed to do what you want to achieve, and understand it all, is available to you now. If there are aspects to the science that are difficult to understand, I can talk it through further.

 

[cheesewonton]

I've been reading up on automotive flow benches shops use to test cylinder heads they are porting for improved airflow to see if there are any clues to be found in their airflow measuring techniques. Lots of variety in how flow benches are designed so no conclusions yet.

In the future however I will only measure airspeed directly, then multiply this by the area of the hose opening. I have a spreadsheet with the area for every hose inside diameter from 28 mm ( 1.1 inches ) up to 54 mm. Take the product of airspeed and area then multiply that by 0.8, the rule of thumb factor to compensate for the fact on such a small opening you are measuring airspeed in the center of the duct and not capturing the lower airspeeds towards the outer edges.

 

[kenc184]

Cheesewonton - Forgive me for asking what has probably already been answered. Are you using a closed sysem, such as the cone adapter the other fellow used, or do you just butt the hose up to the anamometer? I can see using the hose diameter in the latter case, but not in the former where the air velocities are different.