The vacuum gauge/indicator is used to measure lower-than atmospheric pressures (vacuums). An accurate vacuum gauge is needed to measure the 500-micron deep vacuum levels that must be achieved to properly evacuate and dehydrate air conditioning and heat pump systems.
Even though the compound gauge on a gauge manifold set is capable of measuring a vacuum, it should not be used for this purpose because the scale calibration is not accurate enough to read the specific vacuum levels needed in the evacuation/dehydration process. U-tube manometers can be used, but they are more suitable for use in the laboratory than for field service.
When servicing air conditioning and heat pump systems in the field, most technicians use electronic vacuum gauge/indicators. Two common types of electronic vacuum gauge/indicators are shown in Figures 1-18 and 1-19. One displays the measured vacuum on a dial indicator.
The other uses a display consisting of several light emitting diode (LED) indicators. The electronic vacuum gauge/indicator measures absolute pressure in microns. As the vacuum levels go deeper, the needle on the dial moves, indicating lower and lower micron readings. For the LED gauge, the LED indicators, each representing a specific vacuum level in microns, turn off sequentially from the highest micron level to the lowest as the system vacuum pressure goes deeper. For both types of indicators, the lower the measured vacuum reading in microns, the deeper the vacuum being measured.