Standard pressure and temperature

[Robert Myers]

OK now.  If you really want to get picky let's inject the concept of 
significant figures.  How closely can you read the gauge?  How far apart 
are the numbers on the gauge.  0.1 bar increments?  Under ordinary driving 
conditions you will be lucky to be able to determine whether the gauge 
reads 1.70 bar or 1.75 bar.  That last figure (the 0 or the 5) is just a 
guess.  Your best estimate of the manifold pressure obtained from this 
gauge might be "slightly over 1.7 bar".  That is only two (or perhaps if 
you might feel justified to fudge just a bit, perhaps 2.5) significant 
figures.  Reporting your pressure reading as 25.046 psia (= 1.75 * 14.312) 
says that you have made a measurement which has a much higher precision 
than your actual measurement technique supports.  25.046 has 5 significant 
figures.  Your measurement supports only two significant figures or the 
manifold pressure should be reported as 25 psia.  If you want to fudge you 
might report 25.0 psia but a real stickler would not accept that report 
because it has too many (three) significant figures.  Then, of course, you 
might prefer to report the manifold pressure as psig rather than psia so 
you would subtract 14.312 from 25.046 obtaining 10.734 from your little 
handy-dandy pocket calculator.  How much boost can you legitimately say you 
have?  10 psig or 11 psig is about all you can say.

Fine, you have a nice precise conversion factor and you can (and perhaps 
should) use the precise value for your intermediate calculations but when 
the result is reported you are entitled to use only two significant 
figures.  That being the case, who gives a rat's posterior about the 
1.01325 bar per atmosphere vs. 1.0 bar per atmosphere?  The measurement 
techniques are incapable of detecting the difference.

At 07:09 PM 3/21/01 -0500, Lawrence C Leung wrote:

>Picky, Picky, Picky =-)  !
>
>LL - NY
>
>On Wed, 21 Mar 2001 00:59:45 -0500 Burl Vibert <blur at sympatico.ca>
>writes:
> >
> >
> >Lawrence C Leung wrote:
> >>
> >> The gauge is absolute, so the standard it compares things to is a
> >vacuum.
> >> Thus, no pressure at all (relative to a vacuum) is 0.0 bar.
> >Atmospheric
> >> pressure at sea level is 1.0 bar, which is the pressure of the
> >> surrounding air, caused by the weight of the atmosphere above sea
> >level.
> >> 2.0 bar means that the manifold is at 14.7 lbs/in/in (1.0 bar)
> >ABOVE
> >> normal atmospheric pressure. It's actually a fairly sizable amount
> >of
> >> pressure.
> >
> >
> >Just to be picky,  it's 1.01325 Bar at standard  pressure (14.696
> >psi, sea level).   2 Bar absolute is actually 14.312 PSIG at sea
> >level.
> >
> >
> >Burl Vibert
> >1987 5kcstq
> >Kingston, Ontario

Bob
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