Pitot airspeed sensing

[Oznog]

I am now trying to figure out how to measure airspeed with pitot tubes, which measure a pressure difference between a closed tube facing towards the direction of travel and static air.

The pressure differences are quite small, under 1 psi. 30 mph equals 0.03 psi difference.

I need a differential (gauge) pressure sensor, and I'd like one with two hose barbs. Some (most) just have a vent in back, but I need to take it to a static source elsewhere so I need a second barb.

The problem is the sensors I've looked up have rather crummy offset to signal ratios. There's a common, economical 1.45 psi diff sensor which reads 12.5 mV diff output fullscale with a 5V supply, but there's an offset error of 0.5 mV, or 4%, in it. I believe this would be a problem since this correlates to around 60 mph.

If the offset is extremely consistent I'd have no trouble zeroing it, but I don't know how consistent that is. I'd hope to be within 2.5 mph here, preferrably less still. It's going to see lots of temperature and ambient pressure variations too. It just looks like I'm using far too little of the scale for this device and the offset is going to blow it completely.

Anybody got a better idea for sensing pressure? I'm still pretty set on sensing airspeed by pressure diff, but I need a more promising solution.

 

[John Sorensen]

Usually pressures in the range you are looking at (especially in an application like this) are expressed in Inches of Water (or Inches of Water Column, or "WC ). You may do well just to buy a transmitter intended for the purpose. Setra makes some inexpensive ones. For example, I have one here that is 0-0.5" WC (0-0.018psi) that has a 4-20 mA output.

BTW, "differential (gauge)," differential and gauge are technically different kinds of measurement. I suppose you could say that gauge is a special case of differential measurement. But you are definately looking at a differential measurement, not gauge, in a pitot tube application.

What are you doing, anyway? Sounds like fun...

j.

 

[Oznog]

It started out as an airspeed measurement for my ultralight. The FAA just created a new category of "Sport Pilot", which can go up to 138 mph level flight at sea level (diving and high altitude speeds may be substantially higher) and I may put it in one of those.

I found formulas and got 0.46 psi (12.7" H2O) for 160 mph. 1 psi is 234.5 mph. The scale is quite nonlinear. I will also be adjusting the calculation for air density. Thinner air lowers the pressure for a given speed.

I saw what you're saying- gauge is the type which ports in the back, differential has two barbs. Gauge would only work if I made the whole gauge airtight except for a port to static air, I can't do this.

The scale is quite nonlinear. 5 mph is only 0.00044 psi, or 0.01229" H2O. Some sensors have been so kind as to say what how much the offset can change with a shift in temp (even though they're listed as temp compensated devices, it is known that the compensation is imperfect).