What unit is used to measure pressure on the low side/suction side of an MGA?

The correct unit to measure pressure on the low side or suction side of a mechanical gas appliance (MGA) is mercury (Hg), which is commonly used in the context of vacuum measurements and low-pressure readings in HVAC applications. Measuring pressure in inches of mercury (inHg) is typical for low-pressure systems such as refrigerants in air conditioning systems or the suction side of refrigerators.

When dealing with HVAC systems, the suction side often operates at a slightly negative pressure relative to atmospheric pressure when measured in inches of mercury. The use of mercury as a reference is due to its density, which aligns well with the low pressures typically encountered in these systems, making it a practical choice for readings.

Other options, while legitimate units of pressure, serve differing contexts or applications. For example, PSIA measures absolute pressure, which includes atmospheric pressure in the reading. PASCAL is the SI unit for pressure, often used in higher-pressure contexts rather than the typical low-side measurements. PSIG measures gauge pressure, which compares the pressure to atmospheric pressure but is not typically the preferred unit for low-side readings where vacuum pressures are more common. Therefore, measuring in Hg is standard practice in the HVAC field for these specific applications.