Air Velocity

Have you used a pitot tube to get the difference in water heights? This should give you and accurate reading if you can set it up.

Thats a good idea. I have a pitot tube available.

I just tried it. I used a pitot tube and got around 5.1" H2O. This seems reasonable since the air speed would convert to about 9000ft/min. I am still trying to understand why when I hooked up the vacuum directily to the manometer (by placing tape around the front opening and making a slit in the tape to insert the manometer tube) I get 18"H20.

I am curious to know if anyone has any idea what a "typical" air speed for a handheld vacuum (i.e dustbuster) would be?

Your input is helpful. Thank you.

Yes, the pitot tube is giving me a realistic value of about 5" H2O. Realistic because I have now measured the pressure of other similar hand held vacuums and they are giving me about the same number.

I believe my problem was I was measuring dynamic pressure (pitot tube) and static pressure (taping the end of the vacuum) and trying to compare the two values. This was noted in a previous response here on this question.

The pitot tube is the "real" air speed of the vacuum. I am still trying to wrap my head around the fact the supplier of this vacuum is so caught up on the air speed and not the suction power....Oh well.

The help is much appreciated.

Rick

You need to have a Pitot tube with 2 ports to be able to measure the dynamic pressure with a Pitot tube. If your Pitot tube has only one port in the center, it gives you the total pressure and you have to deduct the static pressure from the total pressure to find the dynamic pressure.

CG

I believe my problem was I was measuring dynamic pressure (pitot tube) and static pressure (taping the end of the vacuum) and trying to compare the two values. This was noted in a previous response here on this question.

Exactly. With the inlet taped over, there is, of course, no flow at all, thus no air velocity. One might think a vaccuum cleaner is "working harder" when is it blocked (as in lifting a bowling ball). The increase in the motor speed tells you otherwise.

Does the gauge you're using to get the 18" result impede the airflow?

No it does not. Actually, using a manometer to get the 18" water column produces no air flow at all, its all static pressure.

I only get 18" H2O with the manometer. The air flow gauge has lots of losses becasue the diameter of the gauge is larger than the opening of the nozzle end where I am taking the measurements.

The small hand held dust buster should draw about 50 " of water. Inches of water is how I was taught to test all vacuums.

You are mixing dynamic pressure and static pressure.

When you tape the opening and put the gage, you have no more flow this is the maximum static pressure that you can have.

The flow involve a dynamic pressure and I guess that what you measure with the air velocity gauge is closer to the reality. You have to understand that the air flow velocity will vary along the geometry of the opening so you will have to find and average speed. You can use a pitot tube. A good Pitot tube has 2 ports, 1 for total pressure and 1 for static pressure and the dynamic pressure is the difference between them. You have also to make the appropriate corrections to bring the results at the standardised conditions. The Pitot tube will give you a local speed and it will be very sensitive depending where it is placed in the opening. With the Pitot tube you will have to use a certain method to find and average air speed at the vacuum entry.

The Pitot tube has to be in the vacuum conduit so you have to drill some holes to have realistics measurements.

CG

The velocity of any fluid through a tube/pipe is a parabolic with zero velocity adjacent to interiors of tube/pipe and max. velocity at center. So by calculating the velocity by using the equation V = 1096.7 SQR (P/D), I think the result will mean the average velocity.

So if you have a such means to measure all velocities allover the parabolic, it will be more accurate.

I believe that if you measure the air Velocity using a Pitot tube or a handheld device ( I am thinking yacht anenometer here) you will see a marked diferrence depending on the degree of Vacuum being pulled. For example, close the end of the suction tube, the max vacuum that the mechanical device allows will be somewhere between 12-19" Hg, but the actual airflow will be ZERO, velocity will be ZERO, if you had a perfect pump and achieved absoloute Vacuum (I know, it wont happen) there will be no air to move... If you follow my thoughts,when pulling a lighter vacuum you are in effect operating the fan/impellor/pump in a reduced air atmosphere which will then make your readings from a pitot tube or measuring instruments innaccurate. Does that make sense? I work with large Roots Blowers in the Oil & Gas, air velocity and carrying capacity of the airstream is seen as critical, increasing the degree of Vacuum doesnt assist much, it is the mass and velocity of the air that does the job. --IMHO--

Mass flow meter (if available) and calculate for area of nozzle......