CFM in isn't = cfm out?

[kenc184]

I picked up an anemometer for fun. I wanted to check it was functional, so using one of my Royal uprights - an 886 - I unplugged the bag and measured cfm out. First,let me say this was not done with the utmost precision, the vane type sensor was simply placed over the orifice and the orifice area used as the multiplier. 0.025 I think. No "cone" was used to adapt one diameter to the other, this is simply a "does this work" exercise. The measured cfm was insane - around 200cfm. So now, I'm thinking "this thing is garbage".
Royals have a short steel tube adapter for the hose, so I fitted this and measured input cfm, again no real effort was made for accuracy, sensor simply placed over the metal tube orifice - this time around 100cfm. Much more believable.
How can cfm in not equal cfm out?

 

[cheesewonton]

To calculate cfm, take the airspeed shown on your anemometer in feet per minute and multiply that by the area of the orifice the air is being drawn through or exhausted from in square feet. There are websites that will calculate surface area in square feet from the diameter of the orifice ( intake or exhaust ). Take that product and multiply it by 0.8. The reason for this is airspeed at the center of the opening is faster than airspeed at the sides of the opening due to the boundary layer of dead air that clings to the sides of a tube. Airspeed increases as you move towards the center of the airstream. The rough rule of thumb therefore is to take cfm you calculate using the highest airspeed value and reduce it by multiplying by 0.8.

See what you get using the correct values for the surface area of the suction inlet and exhaust. Then consider there will be some frictional losses along the way.

 

[kenc184]

Thanks, yes I followed the cfm debate - the blunderbusses as dawn would be more accurate between tourself and another gentleman. LOL.

MY point is WHY when I place the anemometer on the intake I get about half the cfm that I get when I put it over the fan case output (bag removed). Both inlet and output are within 1/8" of each other. I was not looking for a precise number just trying to figure out why cfm in was half of cfm out? Certainly cfm out feels significantly more fierce than the suction at the input but how can this be?

 

[cheesewonton]

Thanks, yes I followed the cfm debate - the blunderbusses as dawn would be more accurate between tourself and another gentleman. LOL.

MY point is WHY when I place the anemometer on the intake I get about half the cfm that I get when I put it over the fan case output (bag removed). Both inlet and output are within 1/8" of each other. I was not looking for a precise number just trying to figure out why cfm in was half of cfm out? Certainly cfm out feels significantly more fierce than the suction at the input but how can this be?

Are you measuring suction side cfm right at the opening in the fan case with the nozzle removed? And using the correct area of each orifice when making the calculation? If you are trying to measure airflow under the nozzle you will not get an accurate number.

 

[kenc184]

Both the outlet from the fan case, and the metal "stub"adapter for attaching a hose to a Royal inlet" have similar diameters. The Royal hose isn't attached like a Kirby. And again, I am not looking for a precise measurement at this point, just trying to understand WHY apparent input cfm is around 100 and the apparent output CFM is around 200. How can cfm in not be equal to cfm out. Certainly one can FEEL the difference, the input suction is stout, but the outlet is hurricane force! To reitierate: I am just trying to determine if this anemometer is behaving correctly or not, it's a $50 Amazon Chicom product so one never knows.
Once I develop some faith in it, I will start worrying about accuracy.

 

[cheesewonton]

Both the outlet from the fan case, and the metal "stub"adapter for attaching a hose to a Royal inlet" have similar diameters. The Royal hose isn't attached like a Kirby. And again, I am not looking for a precise measurement at this point, just trying to understand WHY apparent input cfm is around 100 and the apparent output CFM is around 200. How can cfm in not be equal to cfm out. Certainly one can FEEL the difference, the input suction is stout, but the outlet is hurricane force! To reitierate: I am just trying to determine if this anemometer is behaving correctly or not, it's a $50 Amazon Chicom product so one never knows.
Once I develop some faith in it, I will start worrying about accuracy.

There is a problem with how you are measuring airspeed and / or you are calculating the area of the inlet and exhaust openings incorrectly. May I see a photo of how you are measuring suction airspeed. I don't know Royals as well as I know Kirbys but you need to get the anemometer right in front of the fan on the suction side. If the nozzle is not removable like a Kirby, then you will have to remove the brush roll to gain clearance and hold the anemometer in front of the fan opening.

 

[kenc184]

Here you go:

 

[cheesewonton]

Here you go:

There is the problem. Try removing the brush roll, measure the diameter of the opening to the fan, calculate the surface area of that opening and then hold the anemometer right up to the opening in the fan case.

 

[kenc184]

That tube "stub" goes through to the fan case, it's sealed to the fancase with an o-ring.

 

[Hatsuwr]

Thanks, yes I followed the cfm debate - the blunderbusses as dawn would be more accurate between tourself and another gentleman. LOL.

MY point is WHY when I place the anemometer on the intake I get about half the cfm that I get when I put it over the fan case output (bag removed). Both inlet and output are within 1/8" of each other. I was not looking for a precise number just trying to figure out why cfm in was half of cfm out? Certainly cfm out feels significantly more fierce than the suction at the input but how can this be?

1/8" difference could explain a lot of what you are seeing, depending on which side is larger. I don't know how big those ports are, but a 1.125" diameter circle has over 26% more surface area than a 1" circle. Add in leaks and imprecise measurements and I don't think there's much mystery. If you want more accurate measurements, attach a short hose to each of those ports when measuring and seal it up with some tape, and also check for other air intake paths, like past the motor or around the interface between that intake port you showed and the roller brush area.

 

[kenc184]

1/8" difference could explain a lot of what you are seeing, depending on which side is larger. I don't know how big those ports are, but a 1.125" diameter circle has over 26% more surface area than a 1" circle. Add in leaks and imprecise measurements and I don't think there's much mystery. If you want more accurate measurements, attach a short hose to each of those ports when measuring and seal it up with some tape, and also check for other air intake paths, like past the motor or around the interface between that intake port you showed and the roller brush area.

From memory one port is 2-1/4 and the other 2-3/8 so a very marginal difference, not one to explain away a 2:1 ratio of cfm.

 

[cheesewonton]

Let me ask a question. Are you reading CFM right off the anemometer, or are you reading airspeed in feet per minute and calculating airflow based on the areas of each opening? Those fan type anemometers base their "airflow" number on the surface area of that fan, which is much larger than the surface area of the two orifices, suction and exhaust, you are measuring. If you are not calculating the airflow based on airspeed times surface area times 0.8 you are getting incorrect airflow readings.

 

[kenc184]

I am inputting an approximate number of .025 for area into the anamometer which dispalys cfm directly. . Frankly, though it is not relevant in the presence of a 100% difference between input cfm and output cfm. The cross sectional areas are approximately equal and slightly smaller than the vane diameter. I could use 10 square feet and it wouldn't change the fact that there is a 2:1 difference in displayed CFM.
And, as I hope I am implying, the 0.8 factor and other corrections to give an accurate number are also not relevant. I don't care if it reads 1cfm or 1000cfm, I did expect to get two close readings, I don't consider 100% close, and it certainly isn't because 2-1/4" isnt equal to 2-3/8" diameter. 0.0276 vs 0.0308 sq ft won't give a 100% error. I thnk that would be about 10%

I must be expalining myself really badly. The prurpose was to get an idea of the accuracy of this anemometer., not a precise measure of vacuum cfm. Since input cfm should equal output cfm then I expected to read approximately the same number both times. I am not interested in 1% accuracy. I just want someone to come up with a theory for why input does not equal output. I can multiply both numbers by 0.8 and they will still be 100% different.

 

[cheesewonton]

I am inputting an approximate number of .025 for area into the anamometer which dispalys cfm directly. . Frankly, though it is not relevant in the presence of a 100% difference between input cfm and output cfm. The cross sectional areas are approximately equal and slightly smaller than the vane diameter. I could use 10 square feet and it wouldn't change the fact that there is a 2:1 difference in displayed CFM.
And, as I hope I am implying, the 0.8 factor and other corrections to give an accurate number are also not relevant. I don't care if it reads 1cfm or 1000cfm, I did expect to get two close readings, I don't consider 100% close, and it certainly isn't because 2-1/4" isnt equal to 2-3/8" diameter. 0.0276 vs 0.0308 sq ft won't give a 100% error. I thnk that would be about 10%

I must be expalining myself really badly. The prurpose was to get an idea of the accuracy of this anemometer., not a precise measure of vacuum cfm. Since input cfm should equal output cfm then I expected to read approximately the same number both times. I am not interested in 1% accuracy. I just want someone to come up with a theory for why input does not equal output. I can multiply both numbers by 0.8 and they will still be 100% different.

You have to use the airspeed measured by the anemometer in feet per minute and calculate the airflow using the measured inside diameter of each orifice to calculate the surface area in square feet. Do not use the airflow calculated by the anemometer.

 

[kenc184]

That's exactly what it is doing.It is only measuring air speed, what else could it do? . I input the area and it does a simple bit of arithmetic.

But again, why the emphasis on accuracy? It is unimportant at this stage. A ratio of 2:1 in cfm is not explained by small errors in area.

And to remind you, the outlet air FEELS like twice the velocity of the inlet air, so I don't think the machine is necessarily lying.

 

[Hatsuwr]

You're still talking about 10%+ difference in cross section just from your orifice estimates. Add in the leaks mentioned earlier and imprecise measuring and it really isn't surprising.

It seems like at this point, plenty of valid potential answers have been presented, and to figure out exactly which ones are the cause and to what degree, you will have to start addressing them and remeasuring.

 

[kenc184]

I couldn't disagree more. You're talking about minutiae. Absolutely no valid answers have been presented.
It's impossible to confuse 10% with 100%.
And what leaks? Each orifice is smaller than the vane, so 100% of the air passes through the anemometer.

 

[Hatsuwr]

Well, prove us wrong then. Eliminate those variables and add some consistency to your measurements and let's see the effects.

Obviously the amount of air going in is equal to the amount of air going out, so the only answer to the question of "why am I measuring a difference" is that your measurements (or calculations) are wrong.

• You may have changed the state of the vacuum between the two measurements.
• There are very likely leaks on the suction side that you are not accounting for.
• We already know your calculations don't take into account the different orifice sizes.
  • Related, you are obstructing the cross sections to a varying degree with the body of the gauge.
• You aren't considering differences in the turbulence of the airflow and how that affects your gauge.
  • Also related, you aren't considering the differing flow paths of the air and how that affects your gauge.
• You have a cheap gauge and, as you said, you haven't made an effort at precision.

Or maybe it's just the vacuum fairy, idk.