Under Pressure?

Under Pressure to Know the Difference Between PSI and PSIA & PSIG?

Service information and data sheets refer to psi, psia or psig to denote pressure specifications. All of these refer to units of measure to indicate pressure on a surface. However, psi is not adequate for understanding the true value unless a suffix is added.

In a nutshell, here’s what they all mean:

PSI: Pounds per square inch. Psi starts at zero relative to a true vacuum. A gauge reading true psi will indicate 14.7 (approx.) at sea level.

PSIG: Pounds per square inch gauge. This term is correct to use when referring to a pressure gauge calibrated to read zero at sea level. Most pressure gauges start at zero and ignore atmospheric pressure. This type of gauge actually reads pressure above atmospheric pressure.

PSIA: Pounds per square inch absolute. Absolute pressure is atmospheric pressure (approximately 14.7 psi) plus gauge pressure.

For example: If a tire pressure gauge indicates 100, then:
PSIG = 100 lbs
PSIA = 114.7 lbs (PSIG plus 14.7 Lbs. atmospheric pressure)
PSI = 85.3 lbs (PSIGminus 14.7 Lbs atmospheric pressure) 

However, virtually all pressure specifications refer to differential of pressure to atmosphere, so gauges are set to zero, and indicate additional pressure above sea level atmospheric pressure of 14.7.  So, when you read a pressure gauge, technically you are actually reading PSIG rather than PSI.

About the Author

Curtis Bogert

Curtis (Curt) Bogert joined the Mitchell 1 Commercial Vehicle Group as an Associate Editor in 2013. His previous experience includes more than 35 five years in the commercial truck industry as a service technician, truck dealership service manager, factory service rep for a major commercial truck manufacturer and a commercial truck sales person. He has over 25 years as an ASE Certified Master Medium/Heavy Truck Technician, Master Truck Equipment Technician, Advanced Level Diesel Specialist and School Bus Technician. Curt also holds a certificate in diesel technology from San Diego City College.