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Ramius-II

Speed vs Pressure

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Hi All: :)

I've been searching for information on using a differential pressure sensor (Pitot vs Static pressure) to measure airspeed. I know the pressure is less than 1 psi, what I do not know is how much pressure represents what speed? The plan is to program the information into a PIC. Can anyone help or point me in the right direction?

Thanks, Ed

Ramius-ii-AT-earthlink.net

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http://www.grc.nasa.gov/WWW/K-12/airplane/pitot.html

I may be wrong, but by my calculations, at sea level and standard temp and pressure, that says the difference between pitot and static would be 0.00686 psi at 1mph and varies with the square of the velocity. Hope you have a very sensitive differential pressure sensor.

Jeff

Edited by jeffs555

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Wow! :)

Thanks Jeff! I have not done the math, and with 0-1 psi being represented by 0-5 volts and a 12 bit A-D I think it will work :rolleyes:

Thank you so much!

Best Ed B)

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Sound like an interesting project, let us know how you get on !

Terry

UK

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I had taken a more pragmatic method at calibration. On a calm moderate temperature day, I had a friend drive me at 30mph and 60mph on a long straight road. I simply observed the 10-bit voltages from the Pitot sensor. Then the data was scaled in software to match the car's speedometer.

Eventually everything was tried in a another vehicle and was confirmed. Seemed to work well, especially since the roads were nearly at sea level. This is also a good way to confirm/validate the strictly math methods.

I have no idea how accurate car speedo's are, but they seemed good enough for my needs.

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I had taken a more pragmatic method at calibration. On a calm moderate temperature day, I had a friend drive me at 30mph and 60mph on a long straight road. I simply observed the 10-bit voltages from the Pitot sensor. Then the data was scaled in software to match the car's speedometer.

Any more information on your speedo project? Particularly what pressure sensor(s) were tested, what accuracy were you able to get? I am also begining work on a similar project. Mostly still at the figure out what parts to use stage. Going to use a PIC with a 10bit ADC, and after that it is getting fuzzy. Looking at a motorola diff pressure sensor like MPXV4006.

David

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I used the Motorola MPX series. Through the use some instrumentation amps and some microcontroller controlled digital pots, the altimeter's resolution was about 1 ft. I do not recall the speedo's resolution. Precision was another thing -- it had a drift issue due to the digital pots and temperature.

This project was getting too costly and too time consuming for me. It is currently on the back shelf (has been for over two years). I do not know if I will get back on it since I have other things I would like to develop.

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Some notes for those trying airspeed measurements:

* Measure static outside the aircraft, preferrably on a pitot-static probe, mounted in clean air.

* Car measurements will be ok for spot checks, but very subject to turbulent air. In fact, one would need to mount the measurement probe high above or well in front of the car in order to clear the boundary layer. Like airspeed measurements, car testing will be subject to the wind direction and windspeed. Indicated airspeed is not the same as groundspeed.

* A good calibration method is to use a manometer.

* 1 psi is approximately 240mph

* Low speed measurements (<20mph) with a broad range sensor are VERY difficult.

Michael

Edited by Mluvara

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* 1 psi is approximately 240mph

Thats interesting !

Can you tell me it 0.5psi is 120mph or 60mph ?

Terry

UK

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Hmm... I just happen to have my physics book next to me.

I see:

v = sqrt((2*dp)/r)

where r = air volumic mass (for us) and dp = dynamic - static pressure difference.

v is the airspeed.

Excuse me if the terms are wrong ;)

Edit: Forgot to remind that the units are SI. So dp = Pa, r = kg/(m^3), v = m/s.

Now I have my tables I can say that r = 1.293.

Regards,

Kilrah

Edited by Kilrah

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Hi Guys:

I did a little more research and the pitot approach is the best for high speeds as there is a fair amount of pressure to work with. There is an alternative method which is via a "flow meter". Basically, it's a tube where the inside is shaped like a wing.

The advantage is by looking at the pressure comming into the tube there is a vacuum on the back side making the whole thing more sensitive.

The ratio of the inside of the tube (largest part) to the hole is 3:1. This comes from an air flow engineer at the Honeywell Corporation.

I have not tried this approach yet and plan to as a comparison to the pressure/static pressure method.

Best, Ed :ph34r:

Edited by Ramius-II

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Yes, it works by venturi effect and is seen on some old slow aircraft but I have never tried it at very small sizes.

I have made a very sensitive wind speed meter in the past using the hot wire method but it involves putting a very delicate wire in the air stream. It has been my concern that it may get damaged by a small fly and cause the plane to crash but I suppose that a fly could just as easily get stuck in the pitot tube ?

Terry

UK

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