How Accurate Is Using a DIY Pitot Tube to Measure Vacuum Pump Airflow?

[yessiko]

Dear all,

I have a simple experiment to make a cheap pitot tube using 3" PVC pipe to measure vacuum pump air flow. But in the middle of my experiment I have difficulty to calculate air flow in the pipe, because I try to use bernouli equation for standard pitot tube and the result still not same with pump factory refference.

May some body help me?

Rgrds,

Yessiko

 

[edgepflow]

Is your pitot tube reading high? There may be an entrance loss on your pitot tube that causes the deviatiation from ideal Bernouli flow.

 

[FredGarvin]

Make sure your pitot tube is well aligned with the inlet air flow. Also, chamfer the inlet with a 30° chamfer. This will help with the probe being less sensitive to yaw errors.

 

[yessiko]

This is my actual experiment device;

https://www.physicsforums.com/attachment.php?attachmentid=20265&stc=1&d=1251352815

But I still confused because my calculation and actual condition is not sinkron (at least will approach to actual value). I don't know how much factors working in there. May somebody can help me to make an applicable formula to find velocity value?

 

[Cyrus]

Interesting setup. It will be impossible to help you without seeing your actual calculation. I'm guessing you have a unit wrong somewhere.

 

[FredGarvin]

The pictures are too small to tell anything.

 

[dopeyranger]

Hello
I am undergoing a similar experiment at university and I am making an L shaped simple pitot tube (not pitot static tube). Are there any limits on the dimensions? Such as the ratio of the length of the base to the diameter of the tube? I have done research but cannot seem to find anywhere that says anything about that for a simple pitot tube. For a pitot static tube the ratio of 'the distance from the static holes to the 90 degree bend' to the 'diameter of the tube' is best being around 12. But for a simple pitot tube, are there any such limitations?

L shaped simple pitot tube:

|
|
|
|
|
|
'--------
\______/
^
Base

 

[FredGarvin]

It depends on the flow velocities but a good rule of thumb is to make the base length at least 4 times the tube OD.

 

[dr dodge]

because atmospheric pressure is the pump, not the vacumn, I would think you should need to correct for air density variations real time. A set of "runs" will be relatively meaningless without environmental conditions being recorded

dr