Fundamentals of Refrigeration: Absolute Pressure and Gauge Pressure

While the pressure that the air exerts over the surface of the earth is approximately 14.7 pounds per square inch, most pressure gauges will read ‘0’ psi when open to the atmosphere. This is because pressure gauges measure a “relative pressure” compared to the surrounding atmospheric pressure.

When a gauge is put on a tire and reads 35 psi, that means that the pressure inside the tire is 35 psi higher than the atmospheric pressure. So if the atmospheric pressure is 14.7 psi, then the total pressure, or “absolute pressure”, is the sum of the gauge pressure and atmospheric pressure, which is 49.7 psi in this example.

It is important in refrigeration applications to differentiate between the pressure measured by a gauge, which we call “gauge pressure”, or psig, and the “absolute pressure”, or psia.

Converting from gauge pressure to absolute pressure simply requires adding the atmospheric pressure to the gauge pressure value. For most applications, 14.7 psi is an acceptable approximation for atmospheric pressure.

Example: A refrigeration system has a suction pressure of 20 psig. What is the absolute pressure?

This can be calculated by adding 14.7 psi to the 20 psi gauge pressure. The absolute pressure is 34.7 psia. 

Example: The absolute pressure inside a condenser is 150 psia. What pressure would be displayed on a pressure gauge?

This time, the absolute value is known, so to calculate the gauge pressure, 14.7 psi must be subtracted to obtain the answer of 135.3 psig

Here are some important takeaways regarding gauge and absolute pressure:

• Absolute pressure measures from zero pressure and uses the units “psia”.
• Absolute pressure cannot be a negative number since it is measured from zero.
• Gauge pressure measures from atmospheric pressure and uses the units “psig”.  
• The term “vacuum” is used anytime the gauge pressure is less than atmospheric pressure. When this is the case, the gauge pressure will be a negative value in terms of psig. 
• As a common convention in refrigeration applications, “psig” is used for positive gauge pressures, and “inches Hg” is used for negative gauge pressures.
• “Hg” is the symbol for mercury on the periodic table. The unit “inches Hg” represents the column of liquid mercury measured in a manometer and can be easily converted to psi. One inch of mercury is equivalent to 0.491 psi.