Standard Pressure

[wings folded]

This topic may well have been hammered to death already, but my pathetic mastery of search functions has ensured that I have never found the threads.

And I am not even sure to be in the right forum.

But.....

I think of atmospheric pressure as being one bar.

That is, I think, 1,000 millibars.

Why then is standard pressure such an ungainly number as 1013.25 and some more decimals?

1,000mB would be a lot easier.

(29.92 has a certain symmetrical elegance - a numerical palindrome even - is that that the reason?)

[enicalyth]

One bar is 100 000 Newtons per square metre which very well may be someone else's standard atmosphere. 1013.25mb is 29.921 inches of mercury at 0*C. Oops musn't say millibar the hectopascal police will have me.

Fahrenheit wanted the number of degrees between freezing and boiling to be 180 because that represented a phase change, a total about-face from one extreme to another. When it came to the actual physical numbers he wasn't bothered one whit. A pedant might have slid the scale so that 0 was freezing and 180 was boiling but he resisted the urge. wherever it landed up on the first tube of glass was good enough. Ice = 32? Boiling = 212? Fine, book it Danno.

Similarly someone might say why not tidy up the definition of "bar" to be exactly an ISA atmosphere. But Fahrenheit would say why should your atmosphere be any more important than anyone else's? Physics isn't tidy.

Personally I think the standard atmosphere should be 1 bar = pressure in the RSL, Alice Springs on Australia Day = absolute zero unless you are trying to get a drink.

[FCS Explorer]

perhaps 15°C and sea level were more important and millibar had already been invented!:8

[inbalance]

That´s a good question.

I believe that we have to blame the guys who defined the units for pressure.
I think they had more importent Pressures in mind then the airpressure.

But what about MSL. As the sealevel is rising we will have to ajust the standard airpressure in near future.

[tvrao]

The international standard atmosphere (ISA) as per ICAO standards defines standard atmospheric pressure as 1013.25 hectopascals and standard day temp 15degrees C.
Eventhough as per physics 1000 hectopascals is atmospheric presure which is 1bar.
ICAO standard atmosphere used for navigation and aircraft and engine performance computations and comparisons.

[BelArgUSA]

And if you want to be "metric", dont forget 760 mm/Hg for 1013.2 hPa...
Russia still uses that one for their QFE altimeter settings at domestic airfields.
xxx
:8
Happy contrails

[bookworm]

Why then is standard pressure such an ungainly number as 1013.25 and some more decimals?

The unit which the 1013.25 multiplies is already defined by other needs -- it's not arbitrary. 101325 pascals is 101325 kg / (m s^2) and the metre, kilogram and second are defined in ways that have nothing to do with atmospheric pressure. So the question is, in one sense, a bit like asking why the acceleration of freefall is 9.8 m/s^2. Wouldn't it be "neater" to make it exactly 10? Unfortunately, that would require the redefinition of either the metre or the second.

Of course that doesn't explain why 1013.25 hPa is the 'standard'. The standard atmosphere was originally defined as 760 mm of mercury (torr). OK, it has two significant digits, but it's a fairly round number. And making it 700 or 800 torr would make it quite inconsistent with the real-world average sea level pressure. And 760 torr was, to the nearest Pa, 101325 Pa. More recently, in an effort to put derived units on the basis of fundamental SI units, the standard atmosphere has been redefined as exactly 101325 Pa.

[wings folded]

Well ladies and gents, thank you for your replies, and for not totally scorning me for my ignorance.

On the topic of the Farenheit scale of temperature, I had heard the 180 theory before. I had also heard that the good scientist wanted 100 degrees as body temperature. Seems neat. Unfortunately he had a slight fever the day he signed off on his scale, and we are left with 98.4 as "normal" body temperature.

This may be an urban legend, but I like it anyway

[enicalyth]

I had quite forgotton the lure of body temperature being 100*F which I have heard too. But I like to think of fahrenheit saying "Whatever, have I got my bovvered face on?" At school great emphasis was laid on Avogadro's Hypothesis not his Theorem. The chemistry master would deduct marks. Similarly NTP meaning Normal Temperature and Pressure was predicated on
60*F and 30" of mercury whereas STP or Standard temperature and Pressure was 32*F and I suppose 30" of mercury and we had to do all that P1*V1 over T1 = P2*V2 over T2 stuff with Charles' Law, Boyle's Law and what's the difference between a Law and a Hypothesis Boy!! Wonderful stuff. The great man taught physics and chemistry with ease and arranged term trips to wonderful places such as sewage works, rum distilleries and so on. He had the knack of sensing when a class needed diversion and once arranged a sweepstake to see how far a cork could be shot. I suppose many a schoolmaster has done the same, it isn't a new trick by any means but when you are fourteen and discover old codgers can be irrepressibly juvenile it is a wonderful sign of hope. All my great schoolteachers gave been gathered in long since and my flying instructor and best man too yet it is comforting to know somehow that we still have time to wonder what temperature and pressure are all about and have a laugh over it. The old chap who taught us chemistry used to say that if he was alive then temperature and pressure were normal; when he died the school annual begged to record "...that Mr CDR reported T & P abnormal". What a way to go. But isn't it ironic that the same goes for flying? T & P abnormal. Sorry, wittering again and I don't mean the airbase of the A1.

[wings folded]

Ah, all this talk of Charles Law, Boyles Law makes me think of P1, V1, T1 and P2, V2 and T2 and all the rest.

Then there is Coles Law which has to do with shredded cabbage in a sort of mayonnaise

[mr. small fry]

What a great idea.

And why not make the answer to all ATP questions either 100, yes or no? That would simplify things too!

[barit1]

Why shouldn't your IAS be calibrated (e.g.) in megafurlongs per fortnight? :ok:

[bookworm]

Why shouldn't your IAS be calibrated (e.g.) in megafurlongs per fortnight?

AMC OPS.GEN.410(a)(4) Flight instruments and equipment - VFR flights
CALIBRATION OF THE INSTRUMENT INDICATING AIR SPEED - SAILPLANES, AEROPLANES AND
HELICOPTERS
The instrument indicating air speed should be calibrated in knots (kt). In the case of sailplanes with a maximum certificated take-off mass below 2 000 kg and aeroplanes other than complex motor-powered aeroplanes with a maximum certificated take-off mass below 2000 kg, calibration in kilometres (km) per hour is acceptable.

You have a few months to get your excellent suggestion incorporated into the AMC... ;)

[cortilla]

As to why a standard atmosphere is standard, here's a story i heard a little while ago, but i really can't vouch for the accuracy of the story, but i like it.

When a definition was required a few scientist types went to a beach in Brighton (sea level) and took measurements at lunch time.

Their results for that day? 15 degrees c 760mm/HG (equates to 1013.1 HP or 29.92 IN/HG) and they also measured the standard density, standard lapse rate etc etc. They liked the numbers and called that standard

Can't find any references to prove it though

[wings folded]

Like cortilla, I am not sure of my source, and cannot find it, but I am aware of a variant.

Same bunch of scientists, but in Paris, taking lunch (well you would, in Paris, wouldn't you?) in the restaurant part way up the Eiffel Tower.

They recorded the actuals and then demanded that the cheeseboard be brought back in, while one of them wrote it all down.

(OK, I made up the bit about the cheeseboard)

[john_tullamarine]

There are a few "standard" atmospheres in use. The usual ICAO standard atmosphere relates to a study done over US continental airspace a long time ago (can't put my hand on any documents to confirm just at the moment .. but) .. the mid 1940s rings a bell somewhere ?

Main things to keep in mind -

(a) doesn't really matter what your standard atmosphere is, so long as it is defined and we compare apples with apples when doing work relating to air activities

(b) probably helps if your standard atmosphere has a passing resemblance to the sort of atmosphere with which you are working .. hence a number of specialised standard atmospheres

[wings folded]

Well, you have all been very kind and helpful

If it is not too dull for you, can I extend my question?

When I inflate my tyres, the little sticker inside the door says 2.3bars or something like that.

(I am supposed to inflate to different pressures if I am on a motorway or not - puzzles me that one - "yes officer, I know I am parked on a motorway slip road, just adjusting my tyre pressures according to the manual, don't you know - soon be on my way")

Anyway, back to the issue.

If I have 2.3bar in my tyres do I have actually 3.3 bar, counteracted on the other side of the rubber by 1 bar atmospheric pressure?

The reason I ask is because I am puzzled, and I am puzzled because my lawnmower tyres (it is an American model) give the required pressure in psi.

When I convert to Bars, because my compressor only reads out Bars, I end up with less than unity. I have recalculated the equation many times; I still end up with say 0.7 Bar.

That sounds to me to be a partial vacuum unless it is really 0.7 plus 1 bar so 1.7 in absolute terms

Anybody still interested enough to help?

[Dont Hang Up]

Anybody still interested enough to help?


Don't see why not.

Your pump measures zero when it is disconnected. Just as it measures zero whan connected to a completely deflated tyre. And yet that deflated tyre must contain air at atmospheric (a puncture won't create a vacuum, it will just equalise internal and external pressure).

So the 0.7 bar you refer to is pressure above atmospheric.

[Flash2001]

If I may elaborate a little on DHU's answer. Often the letter "g" (gage) is appended to measurements above atmospheric whereas "a" is appended to absolute measurements. Tyre pressures are given as gage measurements.

After an excellent landing you can use the airplane again!

[wings folded]

DHU and Flash

Thanks, so I was sort of groping in the right area then.

DHU - your explanation was dead clear; you should be a teacher if you are not already one