In Power Engineering and Thermodynamics it is confusing to determine why we use certain units. With temperature we see Celsius, Kelvin and Fahrenheit, with pressure we see kilo pascals, mega pascals, pounds per square inch and some may be gauge and some may include atmospheric pressure to be termed absolute.
There is a methodology to this all and this video lesson is an example of how to work with ideal gases, and standard conditions.
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Mike Grossman, P. Eng.