What is the correct method to convert from PSIA to PSIG?

To convert from PSIA (pounds per square inch absolute) to PSIG (pounds per square inch gauge), it's essential to understand the difference between these two measurement types. PSIA measures pressure relative to a perfect vacuum, while PSIG measures pressure relative to the atmospheric pressure.

Atmospheric pressure at sea level is approximately 14.7 PSI. Therefore, to find the gauge pressure (PSIG), you need to remove the impact of atmospheric pressure from the absolute pressure reading (PSIA). This is accomplished by subtracting the standard atmospheric pressure of 14.7 PSI from the PSIA value.

By applying this subtraction, you effectively convert the absolute measurement to a gauge measurement, which reflects the pressure above atmospheric levels. This is why the correct method is to subtract 14.7 from the PSIA reading to arrive at the PSIG value.