Venturi Vacuum

[Speedy]

I have an application where I want to confirm air-flow in an extraction duct. At present we use a diaphragm type vacuum switch which measures the vacuum from a smaller L-shaped pipe fixed in the middle of the air flow, i.e the venturi principle.

The vacuum switch is unreliable. I have measured the vacuum at 55mm water. This with a simple water tube. This equates to 5.5 milibar (if my calculations are correct).
The range on the vac switch is 5~50 milibar.

My question is how can I increase the vacuum to bring it closer to the switch mid-range?

Would a funnel type opening on the end of the venturi pipe help?

Regards,
Speedy

 

[tlee123]

Look at:

http://www.efunda.com/formulae/fluids/venturi_flowmeter.cfm

If you reduce the diameter at the throat, you'll decrease the pressure more (better vacuum).

 

[Compositepro]

Speedy, what you describe is a Pitot tube, not a venturi. Are you sensing differential pressure between the tip of the Pitot tube and the wall of the duct. If not, static pressure changes in the duct will affect your reading.

 

[MintJulep]

More correctly, it sounds like a total pressure probe.

If you simply need to know flow or no flow, a sail switch might be a better choice.

 

[Speedy]

Thanks for your replies.

To clarify - it is in fact a pitot tube, with no static pressure measurement. It's function is to clarify FLOW / No FLOW.

I like the idea of a sail switch, but I have to find one with ATEX approval.

Any suggestions,
Speedy