ATPL exam question that's bugging me!
 

Hiya,

I've just taken the August exams and one question is still on my mind. Here it is:

In what units are pressure measured?

1. PSI

2. N/dm squared

3. something else


Now, strictly speaking the SI unit of pressure is the Pascal (N/m squared), but here there appear to be two possible answers. PSI is the Imperial unit, and still used widely. But the "N / decimetre squared" is also correct, but just not the correct SI format!

Any suggestions ????

cc

Not an expert, but would have picked a. Would be mighty p*ssed off if it was wrong.

same for me, A.

everything is in PSI (pounds per square inch) in aviation ;) I don't think that they really care about the International Standards (scientific standard).

Was option 2
n/m squared
or as you have typed
N/dm squared ?

I would have gone erm... P=F/A = N/m2 which I would have gone for but if it wasnt there and it was "N/dm squared " as you typed then def PSI.

CHeers

Flystudent

You've hit the nail right on the head there. PSI was the only possible answer- the exam option WAS N/dm.

Ciao!
CC

Chose PSI myself,strange Q's though.

"N / dm squared" was definately there. A "dm" is standard abbreviation for a decimetre - ie 0.1m.

The units of pressure are of force per area. And N/dm squared is just that, a pressure. But like you guys I opted for PSI! Thought it was the MOST correct answer. But i wouldn't be surprised if i got it wrong in their eyes. As Europe (hence JAR) is metricified (is that a word?!!! )

Cheers.

Considering the moments in a 737-400 for the M&B exam are in kg inches (makes me cringe as a science graduate) it is very unfair of them to expect you to discard N/dm^2 as an option, at least it is genuinely a measure of pressure and consistently metric! PSI must be what they are looking for, but it does make you wonder about the intelligence of those writing the questions.

PSI for aviation units,
or second right choice N/cm square.so it's PSI!

In reply to Send Clowns:

I don't wonder about the intelligence of those writing the questions. I have already formed quite a firm opinion about it.

posted 15th August 2003 19:51
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In reply to Send Clowns:

I don't wonder about the intelligence of those writing the questions. I have already formed quite a firm opinion about it.




They do not write the questions, they take it from a databank.Some questions are 35 years old.

Awww come on boys! whats pressure measured in??


I can not believe that anyone would not know the answer to this, I'm sorry but you deserve my complete and utter derision.

ps its psi

No it s not PSI,a pressure is given in pascal, the standard unit for pressure is the Pascal, which is a Newton per square meter


as I have said before this question date of 1947!!not surprising why they are all bolt with grey hair at the CAA!;)

And do not be confused with PSI(pound per square inch) which is not ISO...

The SI unit of pressure is the pascal, symbol Pa, the special name given to a pressure of one newton per square metre (N/m2). The relationships between the pascal and some other pressure units are shown in the table but note that not all are, or can be, expressed exactly. Note also that the term standard atmosphere is not a pressure unit(5).

Unit Symbol No. of Pascals
bar bar 1 x 105 Pa (exactly)
millibar(1) mbar 100 Pa (exactly)
hectopascal(1) hPa 100 Pa (exactly)

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conventional millimetre of mercury(2,3) mmHg 133.322... Pa
conventional inch of mercury(2,3) inHg 3 386.39... Pa
torr(4) torr 101325/760 Pa (exactly)
pound-force per square inch lbf/in2 6 894.76... Pa

RE Units of pressure.

I used to work for a major motor manufacturer and I often couldn't see the logic in having so many ways of discussing such a simple value as pressure - "force per unit area".

Without trying to split hairs pounds (avoirdupois) are units of weight (mass) and not force although pounds are often reffered as a force. As such psi is often used - and we're all familiar with tyre pressure etc which are in psi.

Ideally you would like to answer this question with "force" (i.e newtons per unit area)....as newtons/decimetre squared most acurately fit the definition of pressure.

Even though psi is not an si unit as such it still remains a convenient way of measuring pressure - the likes of Boeing and Airbus talk in terms of psi when discussing pressurisation of various components. If I am not mistaken B737 refer to max diff cabin pressure as "8.65 psi" and normal hydraulic system pressure as being "3000psi", for example, so psi is alive and well in avaition.

Here's a quick pressure conversion tool

Edited to include another conversion tool