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.