![]() In the micron vacuum scale, we start at 760,000 microns at sea-level atmospheric pressure and work down towards a perfect vacuum of 0 microns or 0″ Hg. One inch of mercury (Hg) is equal to 25,400 microns (of mercury). It goes down to -29.92 because it is PSIG, not PSIA. So, when the suction/compound/blue gauge goes into a vacuum, it reads in the “negative” Hg scale. When we read pressure as a tech with a gauge, we read it in PSIG, which means it is already set to zero at 14.7 PSIA and 29.92″ Hg. Therefore, a perfect vacuum can be thought of as 0 Hg, although a “perfect” vacuum can never be achieved. The force of the atmosphere around us is equal to 29.92 inches of mercury (Hg) or 14.7 PSIA. (We will use “Hg where applicable for the remainder of this article.)ġ” Hg is equal to. We often represent the unit “inches of mercury” with the abbreviation inHg or “Hg. “Inches of mercury” is a more rough measure of pressure, usually vacuum or even barometric pressure or altitude. In fact, a micron is one millionth of a meter of mercury displacement. Larger units of measure are better suited for higher pressure and greater differentials smaller units of measure are better suited for lower pressures or smaller, more critical differentials.Ī micron of mercury (or micron) is a very small/fine unit of measure related to the displacement of a mercury column by atmospheric pressure, thus the distance part. The trick is knowing which is best suited for which and the size of the scale. REALLY, it is a measurement of distance.įirst, any scale CAN be used to measure vacuum (negative pressure) and positive pressure. Open the tank, the pressure difference between the evacuated tank and pressurized system should be enough to recover the entire charge in most cases.Īt this point, you will need to install permanent access valves to the system for future reference, testing, recovery and evacuation.To answer the question in the title, we use it as a measurement of pressure. When the tank has achieved a deep vacuum, attach your charging hose to the system via the temporary access valve (pliers) and attach the other side of your hose to the tank. Run your pump and pull your tank into a deep vacuum, you can use a micron gauge positioned on the tank to ensure a deep vacuum has been achieved. Set up a vacuum rated hose to your empty tank and attach the other end to a vacuum pump (using a manifold gauge is not necessary). A simple evacuated recovery tank should be able to remove the charge. Here's a tip, because these types of systems contain only a few ounces of refrigerant, hauling around a recovery machine is not always necessary. Once you have determined the issue, you will need to recover the refrigerant to install permanent access valves. You can use one for the high side and one for the low side if you desire. Simply set the pinch depth and grip the pliers onto the system piping. It's a set of pliers, the tip of the pliers have a piercing needle and rubber seat, on the other side of the needle and seat is a 1/4" service valve. Yellow Jacket makes an incredible tool that will gain you temporary access to the system. If you come across an appliance that is not cooling effectively and you suspect a leak, a restriction or a refrigerant related issue, you will need to add access fittings. This makes it difficult to troubleshoot if there is ever an issue with the system, but it also helps prevent leaks as there are less potential leak points. When working on small, self contained reach in coolers and freezers, you’ll notice many of them are not equipped with service access valves for testing, evacuation and recovery. ![]() Gary McCreadie is an HVAC tech, the creator of and the HVAC Know It All Podcast
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