How to correct measured inlet pressure drop of 38 mm of h2o 36 deg centigrade to inlet pressure drop of ___?????__________ @20 deg Centigrade.

 

Have you looked at the Performance Correction Charts in the Operations and Service Manuals provided with the units?

I don't recall ever seeing a chart for inlet pressure drop at a particular ambient, but the Belfort Bunch might be supplying them with their F-class units (or BHEL, as the case may be). If you can't find the Charts in the Manuals, you can always ask GE or the turbine packager/supplier to see if they have one they can send you.

I do recall seeing separate Charts for inlet pressure drop, and ambient temperature correction--but not for the two combined. They're separate parameters.

And, the Charts are particular to specific machines (though they don't usually vary much for classes of machines, sometimes the compressor design can have a large impact on performance and on the corrections applied per the Chart(s)).

 

I know that industrial pressure transmitters provide a choice of head pressure units that can include water (density) at various temperatures, like mmH2O at 4 Deg C or at 0 Deg C.

I'll trust the on-line calculator at
http://antoine.frostburg.edu/chem/senese/javascript/water-density.html
for the values below (close enough)

Density of water:
At 20 Deg C 0.998207 g/cm3
At 38 Deg C 0.992968 g/cm3

So 38 deg C water is more dense by a factor of 1.00527 (0.998207/0.992968)

To correct 38 Deg H2O density to 20 Deg C H2O density, multiply by 0.99475 (0.992968/0.998207) or by the ratio of whatever source you use for the density of water at given temperatures.

However, if your measurement, 38mm H2O, is to two significant digits, can a correction factor that does not vary from unity until the 3rd digit to the right of the decimal point matter in the greater scheme of things?