What's a micron gauge?

Above is a picture of a micron gauge. It's hooked up to the system through standard manifold gauges to measure the level of vacuum on the system. A vacuum has to be pulled anytime the copper lines that carry the refrigerant are opened up for repairs or installation. As you can see in the picture, the vacuum level on that system was 260 microns.

Opening the refrigerant lines allows moisture in. Even simple humidity is enough to contaminate the system and create acids. A vacuum of 500 microns or less must be pulled after sealing the refrigerant lines in order to fully evaporate and remove any moisture that may have entered the lines. 

The picture above is a close up of a typical gauge set used in our trade. The left dial measures low pressure and the right high pressure. Currently the left gauge's pointer is aimed at 0 PSI. When a vacuum is pulled, the pointer rotates counterclockwise towards the number 30. In order for that gauge to measure 260 microns, it would have to have nearly 3000 hash marks between the numbers 0 and 30! Obviously it's an impossible task for a standard gauge set. Hence the micron gauge is an important tool for the HVAC tradesman to own.

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