Vacuum Facts
Active member
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.
