Millibars to PSI
 

Hi

I'm trying to figure out atmospheric pressure in terms of PSI. Is there a table one can refer to or simply convert millbars to PSI somehow?

On another note, does anyone know the PSI at the pitot for differnet airspeeds?

VT

Standard atmosphere is 14.7 psi.

As for different PSI at the pitot head for different airspeeds? It's not as simple as that as it depends on altitude.

Remember that pitot pressure is TOTAL pressure, which is the sum of ram air pressure plus static pressure. Therefore total pressure will vary as both airspeed or altitude varies.

There are tables in a/c Maintenance manuals which provide various values for P and S to give corresponding altitudes and/or airspeeds.

Thanks mono, I am trying to figure the required range a digital pressure sensor must be to be used on an aircraft and how sensitive it must be.

VT

I think 2.8 PSI = 1 bar. I'll have to look it up to be certain though.

From my little black book:-
1 Atmosphere=14.7 p.s.i. =1013 mb (or Hectapascals) = 760 mm of mercury = 33.95 feet of water head =10.35 metres of water head.

regards

If 1 atmosphere at sea level = 14.7 psi

and Standard atmosphere at sea level = 1013.25mb

then 1 psi = 1.01325/14.7 bar
and 1 psi = 0.0689285 bar

I think that's right, but do we care?:confused: :confused: :confused:

From handy book of conversion tables (there are also a few pretty good conversion tools on the net):

1 mbar = 0.0145038 lbf/sq in

As for a pressure sensor to measure at the pitot tube, are you trying to get airspeed?

On the helo I'm testing on at the moment we have a differential pressure sensor for airspeed recording (connections to both pitot and static) and an absolute sensor for static (altitude) recording.

We used Setra transducers, but the ones we have are only good for up to about 200 kts (differential transducer) and 10,000 ft (absolute transducer).

Our required pressure range (for airspeed) was calculated using:

q = 0.5 * atmospheric density * (velocity squared)

(Difficult to write equations in here isn't it?)

q = Dynamic pressure (lb/ (ft squared))
v = Velocity (ft/sec) (ie the max velocity measurements are to be taken at)
and atmospheric density is in 'slugs/ft cubed'

If you want it in PSI mulitply answer (which is lb/ft sqd) by 0.00694444 to get lb/in sqd.

For atmospheric density we used sea level value (0.002378 slugs/ft cubed). Though I imagine this would still be the worst case (highest atmospheric pressure) for the total pressure at the pitot as well, since you start of with the highest atmospheric pressure, and then add the effects of airspeeds onto that.

As I said this was done for a helo, so I can't say it will any good for faster aircraft, as I'm not sure how compressability would affect the calc (we ignore it in helos, it makes things much easier, and its negligable anyway at our sort of speeds). You don't say what speed range you are looking to measure over, but I hope the above helps.

It might be worth asking on the Flight Testing Forum, I'm sure some the guys there have also had to work this sort of thing out for instrumentation purposes.

Edited to add:
Try this website for converting stuff to other stuff-
http://www.calculator.org/properties.html

Last time I looked a pound was exactly 444822.16152605 dynes (i.e. 453.59237 times 980.665). And a square inch is exactly 6.4516 square centimeters, so one psi is (the former divided by the latter) about 68947. 57293 16836 13367 dynes/sq cm. And a millibar is exactly 1000 dynes per sq cm, isn't it?

NOTE

1 bar =one atmosphere=14.7psi

and we do care if pressure vessels are involved, from aircraft fusalages on up.

Anyway they have changed to kPa where 100kPa=1bar in europe just to make sure you refer to the books before each calculation.