Pulling a good vacuum before charging a new system with refrigerant or after a repair is essential for the proper operation of an air conditioning system. Proper operation, system longevity, and the time you spend at the jobsite all relate to dollars in your pocket. In addition to the vacuum pump,a micron gauge is an essential tool. Below is a description of how the gauge works and why it is used in the HVAC industry.

A typical micron gauge.

Preventing Problems

A micron gauge measures the amount of air or "noncondensables" and moisture in a system. The unit of measurement used is a "micron." The gauge is placed between the vacuum pump and the middle port of your manifold set. The lower the micron reading, the deeper the vacuum. The deeper the vacuum, the lower the amount of air and moisture in any given system. Why is this tool necessary? It can help you prevent problems like the following.

Potential Problem #1

If moisture is left in the system, some of it will turn to ice when refrigerant is introduced into the system. If enough moisture is left, larger ice particles will form and can block the cap tube or piston, causing a restriction in the metering device.

Potential Problem #2

If moisture and air are left in the system, they can form acids when combined with chlorine (which comes from CFC or HCFC refrigerants such as R-12 and R-22) and oils (from the compressor). Over time, these acids eat away at the compressor's motor windings and cause premature compressor failure.

Potential Problem #3

If moisture and air are left in the system, they can cause higher than normal head pressures. These pressures can mislead a technician and cause higher discharge valve temperatures, which again can cause premature compressor failure.

Potential Problem #4

If a micron gauge isnotused, a small leak will never be detected using conventional gauges. Conventional gauges use inches of mercury (Hg) to measure vacuum. Most gauges go down to 30 inches of Hg. If one inch of Hg (at 32 degrees F) is approximately equal to 25,400 microns of Hg (at 32 degrees F), you can see how much more accurate using the micron gauge tool can be. It is recommended pulling new systems down to 400 microns or even less if time allows. If a leak is left in the system, first you will experience indoor coil freeze-up, then no cooling and/or compressor damage.

Money In Your Pocket

Taking the time to use a micron gauge to check for these potential problems is not only a preventive measure, it can also benefit your reputation as a contractor. Most micron gauges can be purchased for $100-$400 depending on the model - a small investment up front that can save you a lot of money in the long run.

Two More Things

But you're not done yet. Two other practices are recommended in conjunction with using a micron gauge.

The first is purging the system one to three times with nitrogen before the pump-down. The nitrogen absorbs moisture and can be released into the atmosphere without any harm. This reduces the start-up time even more.

The second is changing the oil in your vacuum pump regularly. Vacuum pump manufacturers suggest changing it after every use. If the vacuum pump oil becomes cloudy or saturated with moisture, it will reduce pump efficiency and thus increase the time you are at the jobsite.

As you have read, pulling a good vacuum before charging is essential for many reasons. Yet these reasons boil down to money in your pocket. Those who use micron gauges and practice good pump-down techniques know this. Unfortunately, those who don't will eventually learn the hard way.

Justin F. Sales is a product support engineer/technical service with EMI International. For more information, visit www.ecrinternational.com.

Publication date: 12/29/2003