Wrote

The NASA equations align very well with ASHRAE formulas once a sign error was fixed.

What sign error ? in ASHREA. NASA & OACI agree closely.

I had a sign error and could not get ASHRAE & NASA to converge. Take alook at the sheet preciously attached, the function is there showing VERY small differences for Troposphere calculations. Use either or both references.

Wrote

I had a sign error and could not get ASHRAE

What is ASHREA doing in aviation ?
The Bible is OACI, and the extended NASA above 25 km,
... and more ISA from Ralph Charmichael
More bed time reading for the altitude correction.
From your document, would be wise to give the source as is
especially before freaking the units, gyzma #, color.
Modular native Smath is the key to any project.
Cheers ... Jean.

Inst_ISA [Landing altitude PDF].sm (309 КиБ) скачан 24 раз(а).
Inst_ISA [Landing altitude].sm (1 МиБ) скачан 24 раз(а).

* ASHRAE is NOT in aviation, it deals with air and compressed air. Air, at absolute (psia), is necessary for hydronic expansion tanks among other things.
* Never heard of OACI. You probably have aviation experience, not I.
* Most of us know you are not into units (gyzma). Many of us are and appreciate the fact that SMath, like MathCAD, assists in keeping things tidy internally. In this example, we see that NASA equation is in kPa, other references may be in Pa (like ASHRAE) or BAR. Unit's convert internally without a "lambda" gyzma correction somewhere after the fact that may be forgotten or missed.
* Heavy nighttime reading for later

BTW, the IP NASA equation solves for pressure in lbf/ft^2 (PSF) ... not the "more" traditional PSI. Plug in the function and convert the answer to PSI and WALLAH, more familiar units. Example: at sea level/zero elevation, 14.7 psi in lieu of 2116 psf.

Wrote

* Never heard of OACI.

Organisation de l'Aviation Civile Internationale.
For some years I worked next door to the Head Office [Montréal].
Pretty cold up there ! ... Jean

Wrote

Organisation de l'Aviation Civile Internationale.

So for the English-speaking world, the ICAO - International Civil Aviation Organisation.

I remember some 40-odd years ago arranging to meet someone at the WHO building in Geneva. He said all the buses from the part of town where I was went there. But all the buses I could see went to OMS. I felt very stupid when I finally realised that in Geneva the destination names would be in French - Organisation Mondial de Sante.

Wrote

This request comes up from time to time but as far as I can see it has never been ruled out as impossible or undesirable.

Could we please have a means to define "offset variables" like barg or psig (gauge pressure, i.e. the pressure differential between a measured process and the atmospheric pressure), and also to simplify using differential pressure (difference between two pressures). In the latter case, the pressure differences could be expressed as deltaUnit (ΔPa or Δbar or Δpsi, etc.), like SMath already does for Change temperatures. It would seem to be no more impossible to implement this, for pressures, than the already existing ability to use temperatures in degrees Celsius or degrees Farenheit, which are offsets from Kelvin and Rankine respectively.

Is there any reason why it cannot (or should not) be made available in SMath - apart from the developer's time to do it and his other priorities.

Gerard,
Back on your topic. Based on my 3 yr old question on this forum (Thank you Martin), I revamped this file with the NASA equations. Modify the equation to the SI equivalent (in the closed area) as appropriate.
I will reiterate that pressure is linear, so "delta" is not required, similar to length.
3*m-1*m is 2*m (m = meters) in SMath, not 2 Δm. You can always "define" Δ pressure units to your requirement
Pressure a2g_g2a.sm (35 КиБ) скачан 29 раз(а).

Wrote

I revamped this file with the NASA equations. Modify the equation to the SI equivalent (in the closed area) as appropriate.

Thank you, ElSid. As ever, this forum is full of very helpful people. Your worksheet got converting gauge pressures to absolute pressures and v/v for any defined altitude is very interesting, with the different methods of making the calculation.

The problem is that I expressed the question badly in the OP. What I am looking for is for a simple means to change the display units of pressure in SMath to gauge pressure, whether barg or psig. I do make all the calculations of steam properties in absolute units of pressure and temperature. But for presentation purposes it is useful to have a simple way to display certain pressure results as gauge pressures, even though they remain as absolute pressures in the calculation - the same as having a result in terms of temperature where one can define the result display unit as °C rather than K, for the benefit of less technical readers.

From Martin Kraska's response, he has shown there is a way to make gauge pressure units based on a fixed atmospheric pressure, i.e. at sea level, in the SMath configuration files. You and Jean Giraud have added to the formulations for determining atmospheric pressure for arbitrary altitudes; I like your a2g/g2a functions.

Wrote

... What I am looking for is for a simple means to change the display units of pressure in SMath to gauge pressure, whether barg or psig. ...

From Martin Kraska's response, he has shown there is a way to make gauge pressure units based on a fixed atmospheric pressure, i.e. at sea level, in the SMath configuration files. You and Jean Giraud have added to the formulations for determining atmospheric pressure for arbitrary altitudes; I like your a2g/g2a functions.

Not sure how you are working your sheets, but I see two clear options:

1. create your own absolute variable (' )barg or (' )Δbar
   
2. define the variable early in the calculation. EG for the purpose of this calculation, bar is used as absolute. At the end, you can just redefine the final solution per item 1 above
   
3. define bar:=barg or bar:=Δbarg

Just define psi:=psia somewhere or call it out. Personally, I would not mess with the SMath files. When you update, they may be overwritten. Psia cannot
The use of the (' ) allows us to define ANY unit we want. Personally, I use and define psi/bar as gauge at the top and then calculate to absolute (g2a function) since that changes with location. To make it clearer, I tend to define psi:=psig or bar:=barg and I'm off and running. In the units location, I can now use the units with the _g without issues.