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.

* 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.

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.

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.