Venturi or pitot for measuring air flow?

[bbbrad]

Hi, Im working on a project that requires a need for me to measure airflow into an engine. Im after a mechanical means to do this rather than an automotive electronic style air flow meter. As the title states, what would give me the most acurate, or strongest signals out of a venturi or pitot tube setup? Thanks in advance.
Brad

 

[Engdoitbetter]

Hi bbbrad,

Pitot tube isn't usually employed for various reasons (low turndown ratio, fouling etc.) but the main one is that measuring local velocity in order to calculate global flow rate isn't practical as you should know the velocity profile in the pipe. The usual solution is the annubar, which measures the average pressure in pipe section.

Anyway, I would suggest you to employ an ISO 5167 Venturi tube for the following reasons:

- design is standardised, but not below 2" diameter (ID 50 mm) pipe. In this case you have to perform an individual calibration of the flow element;
- lowest permanent pressure drop among standard diff. pressure flow elements;
- upstream & downstream straight run requirement is the lowest among standard diff. pressure flow elements;
- within the limits of ISO 5167, accuracy is easily calculated and predictable, due to the large amount of data available.

Anyway, keep in mind that a Venturi Tube is usually long compared to pipe diameter, due to the little angles of convergent and divergent cones.

Hope it helps.

Stefano

 

[dgallup]

You don't necessarily want the "strongest signal" out of the device because that implies significant pressure drop which is to be avoided on an engine inlet. When I did it 30 years ago we always used a venturi connected to an inclined water manometer. By slanting the manometer at about 10 degrees from horizontal it could be read to much higher precision. I supposed today you would use a sensitive differential pressure gage.

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[bbbrad]

Hi again thanks for the replies. I thought the venturi would be a good bet. Im a little concerned about the restriction it may cause. For exampe, having an engine feeding from a 2in pipe, and then putting a 2in venturi in front that has a smaller inside bore will obviously restrict it. So does that mean I should put an even larger venturi in its place with a 2in inside bore to stop the restriction? So then i will have in order....say a 3in start, tapering down to 2in, flaring back to 3in, than back down to 2in for the engines intake.!!!
Is the section where the venturi flares back out important in its operation? Could I just do the following... 3in start, tapering down to 2in(where my mesuring takes place, then straight into my engine at the 2in size? Rather than going back out to the 3in again???
I hope that make sense.
Brad

 

[Engdoitbetter]

I'm not sure if I completely understand your point. As far as I know, Venturi outlet and inlet diameter are usually the same, but in one on my references it is written that it's possible to eliminate the divergent cone (this will increase the pressure drop); furthermore, in order to ensure correct measurement, a certain length of straight pipe is required downstream and upstream of the device. Finally, Venturi throat diameter must be calculated starting from the required differential pressure between upstream and throat taps.

Hope it helps.

Regards,

Stefano