MoTeC Global Forum

What is the best way to convert pitot tube inputs to speed units? Are their any preconfigured functions/configs/tables etc.?

thanks for the tips,

Eric Schieb
Electron Speed

Pitot tube inputs? Ive used AV input to read a differential pressure sensor......

Made a pitot tube, with a "micro switch" 5inch H20 Differential pressure sensor and a medical needle.
You have to work out the diffrential pressure for a given speed and put that in as a calibration in your AV input.

Im not an aerodynamicist, but thats the way i did it....


Martin

Normally you would get a differential pressure reading from a transducer attached to the pitot tube, which would be wired into an AV input as Martin says. This then needs to be converted into an Air Speed value, but this is not necessarily a straightforward calculation as Air Speed is dependent on Air Density. The formula for calculating Air Speed is:

Air Speed = sqrt((2*Pitot Pressure)/Air Density)

This would normally be done in a maths function in i2 Pro, taking into account that Air Density depends on ambient air temperature and atmospheric pressure. The density is thus calculated from those two values, which need to be recorded for each session, and could be saved using the setup sheets functionality in i2.

You can also calculate an approximate Air Speed value in the ADL2 by using a 2D table to equate pitot pressure to a calculated Air Speed worked out in MS Excel. However you will need to apply a compensation factor to take into account different air temperature and pressure values for each run.

As you can see, a pitot tube is not generally a "fit and forget" sensor, it needs attention to consistently get accurate data from the sensor

Hope this helps....

Spreadsheet was made by a friend, Sashen.

All,

Thanks for the push on the differential sensor. I originally assumed two ports on the Motec Aero 4. However, since we are only using a Pitot tube, why not connect the static ports to the reference ports and use only one channel as a differential sensor. This will cut down on my error sources. Thanks.

I did a bit of studying since my first post.

Chris,

Your equation is the same one I am using. Since this installation needed to be a "fit and forget" kind of thing, I took the formula and tried it at different elevations and temperatures. If I am doing the math right, a 1000 foot elevation change is about a 3% error in speed (i.e. 3 mph at 100 mph). Since the customer both supplied the hardware and also professed their lack of desire to mess with it all the time, I am hoping this is good enough for him.

Martin,

I spot checked your spreadsheet with my/Chris's equation and they seem to match. I will look into how you got from the pretty looking equation in your sheet to the numbers. I like your equation better.

Eric Schieb
Electron Speed