A Lab Gone Wrong Sometimes Teaches More

Sometimes adversity is the best teacher. Recently one of our first semester students learned more than he bargained for when he had multiple issues with his lab. The lab was to recover the refrigerant from a packaged unit, evacuate the system, and weigh the charge back in. The recovery process went well, but things got off track during the evacuation. The system would not pull down low enough for the vacuum gauge to register. After waiting a while, he decided he must have a leak, so he disassembled his evacuation setup and began to put nitrogen in the system. The leak showed right away – it was on the stem of the Schrader valve. It may have been created while he was connecting and disconnecting hoses. He brazed up the leak, reassembled his evacuation setup, and evacuated the system down to 250 microns. However, when he closed the valves on the core tools the vacuum started to rise. I told him a little rise is normal because the pressure back in the recesses of the system is a little higher than right at the point where you are pulling out the gas. However, it continued to rise past 1000 and kept going. At which point he decided he must have another leak. Once again, he disassembled his evacuation setup and charged the system with nitrogen. When he took off the vacuum gauge, he noticed that the rubber O-ring was missing. Most likely, that was his second leak. He left a note on the system stating the nitrogen pressure and ambient temperature so he can check it on Monday after a weekend of sitting. If the temperature changes, he will have an opportunity to use the gas laws to determine what the holding pressure should be. The lab was really intended for practice in refrigerant recovery, evacuation, and charging. He will get that, but he also receives the bonus of seeing first-hand the effect a leak can have on system evacuation. This also illustrates the value of leak testing and using vacuum gauges. Without the vacuum gauge he would have charged a leaky system and a good bit of the refrigerant would be gone in a few days. Vacuum gauges don’t cost time, they save it. Using a vacuum gauge is the only way of knowing when you have a vacuum and when you don’t. If you can’t pull a deep vacuum, you need to find the problem. The time you save by ignoring the problem will be far outweighed by the time it will cost chasing down the problem later.