Venturi calibration

I have an original air speed indicator from a vintage Klemm aircraft which I need to check the calibration off. This ASI is unusual in that instead of being operated by the more common Pitot/Static probe it uses a Venturi tube that generates a vacuum. I've measured the relationship of the indicated airspeed to that of the vacuum pressure for the ASI and this appears to be largely linear. So this makes me believe that the vacuum pressure generated by a Venturi is largely proportional to air velocity. However I am finding it difficult to extract this information from the complex equations available which also usually deal with incompressible fluids rather than air. Another factor I'm struggling with is that, unlike a pitot/static probe whose performance is insensitive to port dimensions, a Venturi performance may depend on the max/min diameters. Venturi for aircraft are usually used to provide a vacuum for air powered gyro instruments so their vacuum pressure/airspeed relationship is not usually provided as a number but just that they are big enough to power one or two instrument.

If anyone has information about this or can point me in the right direction that would be greatly appreciated.

Clive

At lowish speeds you can ignore density changes. Just use bernoulli. The throat velocity is inlet velocity*inlet area /throat area. Pressure should be proportional to velocity squared

Clive

I don't know how large the venturi is but is there any merit in attaching it to a vehicle, out of the vehicle's local airstream, and measuring the vacuum produced at a number of fixed speeds. A simple manometer could be used to measure the vacuum at the different speeds and then using the same manometer the reading of vacuum measured at each speed could be reproduced and applied to the indicator unit. The actual value of the vacuum won't necessarily be known as this method will only produce relative displacement levels for each speed point, unless of course you choose to calibrate the manometer.

Just a thought

Martin

PS

Assistant will be required to monitor the manometer.

Edited By Martin W on 31/01/2021 18:22:04

Beg, buy, borrow or steal a look at BS1042.

It'll tell you what you need to know.

Many thanks Duncan for confirming that the relationship between inlet velocity and throat depression is linear and I'll try to use the Bernoulli equations to work out the Venturi constant from its dimensions.

I have tried a calibration using my car but the results were inconclusive due to turbulence. I might try using a pitot/static probe and ASI to gather with the Venturi and its associated ASI to compare one against the other. However I'm not sure my journey would be classed as essential.

I think I might have to steal a copy of BS 1042 as at £190 it's a bit steep for a one off calibration.

Many thanks again Clive

I didn't say that the relationship between inlet velocity and throat depression is linear, the relation twixt pressure and velocity squared should be.

unless you have a very fast car or a very slow aeroplane using car to calibrate plane sounds a bit iffy. Wait till the plane is flying and calibrate against gps on your phone?

Hi Duncan

I'm sorry I misinterpreted your reply. However I don't understand why, in the case of a Venturi , the pressure is the square of the velocity when the sensing port in the throat is at right angles to the flow so shouldn't be sensing any velocity component. My reasoning is that lift generated by an aircraft wings upper surface, which is effectively half a Venturi, is proportional to airspeed. I don't remember, a least in my aeroplane, ever experiencing a square law increase in lift with increase in airspeed although when fully loaded I wish I had.

I'm not trying to calibrate the ASI using a car but just to confirm the Venturi law and constant as I don't have a wind tunnel to hand. For checking the calibration of ASI's I use a published equation relating pitot pressure and airspeed together with a precision capacitance manometer and a certified ASI as a double check.

Clive

Have a look at bernoulli

The air in the throat is going faster than at the inlet, and you can re-arrange Bernoulli to get

DeltaP = (V2^2 - V1^2)*density/2

remember pressure acts equally in all directions

see also venturi