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.

 

[cheesewonton]

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.

I am using a hot wire anemometer. It is just a simple narrow cylindrical probe with an slotted opening in the tip to expose the heated element to the airstream. I asked the manufacturer how to use it to measure vacuum airflow and their recommendation was to hold the probe right at the opening. No shroud or cone necessary since there is no rotating vane. There is a white dot on the tip to tell you which way to orient the probe ( dot facing into the moving air ) but I have tried it both ways and it doesn't seem to make a difference. You are just trying to get a raw airspeed measurement at the hose end. From there you multiply the airspeed by the area of the opening and then multiply that product by 0.8. Pretty straightforward.

 

[kenc184]

Ah right, yes, you DID answer this already, LOL.

 

[Vacuum Facts]

hold the probe right at the opening.

Make sure it's in the exact centre where the velocity is highest for your 0.8 correction factor to work. You'll get a reasonable approximation. 0.8 will work for vane anemometer as well. Accurate measurements will require one of the two methods above.

 

[cheesewonton]

Make sure it's in the exact centre where the velocity is highest for your 0.8 correction factor to work. You'll get a reasonable approximation. 0.8 will work for vane anemometer as well. Accurate measurements will require one of the two methods above.

Yep. Most of the vacuum hose openings are so small the only choice is to hold it centered.

 

[vap0rtranz]

Well then ... I'm late reading this debate. As @blackheart said, it's like watching smart kids arguing about different answers.

This thread is another reason why I hate theory. Practical applications matter most in life.

It doesn't matter what Baird meters, fan versus wire anemometers, etc. claim, or what vacuum manufacturers claim, or which equations claim to predict performance. What matters is what said vacuum actually does -- for us!

Real world use of vacuums are what affect my life. The vaccuum needs to pick up dust.

Cheesewonton claimed Kirbys can't pick up dust. OK, I've no brand allegiance nor do I care about the Cannister vs Upright War. But I'm going to need to see a YT test of how much dust is left behind by which Kirby and in which setup.

Bill has given the community evidence about 100s of vacuums so we can see how the machine cleans. 2 real world vacuum tests are great to see: sand for large particle, and flour or baking soda for fine particle. Compare how much of the particle was put into the carpet, and how much a machine removes.

And testing vacuuming methods matter too. Does 2 pass vacuuming work just as well as 3 or 4 passes? Those kinds of tests inform me as a user so I can make my own decision.

Is flour or baking soda the same as common household dander? No. Do I wish there were more dog hair tests? Yes. The tests aren't perfect. They're still synthetic. But these real world tests are MUCH more valuable to me than some damn CFM, water lift, or Airwatts debate.

One of my 1st computer science professors had an wise saying: "Without the users, we wouldn't have jobs." The idealized machine doesn't matter. A machine that works well for its human user matters more than the equations on the whiteboard.

I'm sure smart theorists will disagree with me in their pursuit of the perfect machine, but life is short and I need to vacuum my home.

 

[panasonicvac]

Well then ... I'm late reading this debate. As @blackheart said, it's like watching smart kids arguing about different answers.

This thread is another reason why I hate theory. Practical applications matter most in life.

It doesn't matter what Baird meters, fan versus wire anemometers, etc. claim, or what vacuum manufacturers claim, or which equations claim to predict performance. What matters is what said vacuum actually does -- for us!

Real world use of vacuums are what affect my life. The vaccuum needs to pick up dust.

Cheesewonton claimed Kirbys can't pick up dust. OK, I've no brand allegiance nor do I care about the Cannister vs Upright War. But I'm going to need to see a YT test of how much dust is left behind by which Kirby and in which setup.

Bill has given the community evidence about 100s of vacuums so we can see how the machine cleans. 2 real world vacuum tests are great to see: sand for large particle, and flour or baking soda for fine particle. Compare how much of the particle was put into the carpet, and how much a machine removes.

And testing vacuuming methods matter too. Does 2 pass vacuuming work just as well as 3 or 4 passes? Those kinds of tests inform me as a user so I can make my own decision.

Is flour or baking soda the same as common household dander? No. Do I wish there were more dog hair tests? Yes. The tests aren't perfect. They're still synthetic. But these real world tests are MUCH more valuable to me than some damn CFM, water lift, or Airwatts debate.

One of my 1st computer science professors had an wise saying: "Without the users, we wouldn't have jobs." The idealized machine doesn't matter. A machine that works well for its human user matters more than the equations on the whiteboard.

I'm sure smart theorists will disagree with me in their pursuit of the perfect machine, but life is short and I need to vacuum my home.

Jumping back to the thread even though I said I wasn't gonna view it anymore. But I completely agree here. The only best vacuum in your home is what works best for you. As for me, I already found one. However, no vacuum is perfect for everybody. Each has their pros and cons so everyone has different tastes. I was once convinced from my old local vacuum store that Kirby was the best in my buddy's opinion. Well, Kirbys does do a fine job with dusting from my experience but I don't find them to be the best on carpets. I will say both of the G4s' at my grandparents' house had trouble picking up heavier debris such as cat litter or rock salt because they lacked on suction, thought the Dirt Devil did a better job since it had more suction. There's a YouTube video of evidence by Performance Reviews when he did a comparison test between a Kirby and a Miele on hardwood floors which made total sense to me. Words are just words, I too would need evidence before coming to a conclusion. The reason why I said VW's tests are so misleading is because for example when he did a review video of a Sanitaire traditional commercial, it picked well over 100% of sand. I then knew something was off. Just like Consumer Reports, I also don't trust them either. They claim for example on canisters that Kenmores are the best, well where's the evidence in that? One vacuum store called GoVacuums on YouTube showed a complete opposite when they did a comparison test between a Kenmore and a Miele, that's what I call evidence.

 

[cheesewonton]

The reason I started testing vacuums is because I would read stuff on vacuum enthusiast websites that didn't match my own impressions of the very same vacuums, especially regarding the claims of the superiority of older vacuums compared to modern ones or the superiority of certain brands. I spent a career in an RDT&E environment where data has the last word on what works and what doesn't. No room for subjective impressions. In my previous life as a pilot I learned not to trust my senses all the time, they will play potentially deadly tricks on you, especially the inner ear but also your eyes. Read up on "autokinesis". And everyone has a blind spot dead ahead you work around without even knowing it. The flight physiologists demonstrated all the ways your senses can fool you, and kill you. Trust your gauges was our motto. The totality of this makes me immediately doubt subjective impressions of anything. Show me the data.

So I started to come up with the means to test vacuums to understand the truth. Along the way I learned some interesting, at least to me, things about the effects of larger diameter hoses and how the shapes and diameters of hose ends affect airflow. Some of the enthusiasts favorite hose handles do terrible things to airflow. Some canister swivel ends are pretty restrictive too. I also learned that some of the received wisdom of vacuum enthusiasts is not always born out by test data. Tristars and Patriots have lousy airflow and suction despite what so many enthusiasts and sales people claim. A lot of old vacuums, contrary to the enthusiasts, are not more powerful that modern vacuums from Miele or especially Kenmore. To me these things are interesting to know.