Well I won’t argue (actually I would, but I’m not in a position to anymore) with the Air Force, but I will say they have been wrong more than once. But if everybody uses the same conversion then everyone’s measurements will agree, even if they are skewed slightly.
I really don’t think Dwyer knows that answer. And if they did at one time it was lost when the old fart finally retired.
You can download their current catalog
https://www.dwyer-inst.com/catalog/On page 524 there is a conversion table. (I am uploading that page) At the very bottom of the table they list the conversion from PSI to inches H2O as “PSI x 27.71 = in H2O”.
But if you look at the table, the ONLY point that applies to is 1.0 PSI.
Look down at the 10.0 PSI line on the chart and you can see that equals 276.8 in H2O. Even without pulling out my trusty old slide rule I can see that math is different.
I am uploading a pressure conversion chart from Sutron Corporation that throws even more confusion into the game.
They have reference tables for water at 4°C(39.2°F), 10°C(50°F), 60°F, 20°C(68°F) and 25°C(77°F).
Using Sutron’s numbers it looks like a Dwyer 1.0 PSI gage is referenced to 60°F and all the higher pressures are referenced to 4°C.
The conversion difference between 4°C and 68°F is only 0.18% so it really doesn’t make a significant difference for a 2% magnahelic.
So, my answer to your question is “I don’t know”, and I really doubt there is anyone left at Dwyer that knows either. Good luck with this one….
Well the Forum file size limitations won't allow me to attach the files but a good google search will find the Sutron conversion table for you.
Here is the link to their conversion chart:
https://www.sutron.com/wp-content/uploads/2013/12/PressureConversion.pdf