Saturday, March 29, 2014

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.

3 comments:

  1. Carter

    So true! In addition, the soundness of the evacuation equipment and the monitor needs to be verified before it's use. On many occasions, we find that the micron gauge connections are poor. Pulling air in to a dry system creates problems that may not have existed before the service event started, so make sure pumps are blanked off and checked before each and every use. The 5 minutes this takes shows from the start that the tools we need to rely on are accurate and we go on from there.

    Good timing and example of what we do to learn!

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  2. I had a student once who it seemed like every time he went to do a lab something else went wrong that he also had to fix. In those cases I tell the student 'look at all this extra education you're getting and we're not even going to charge you more.'

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  3. That's right, with mistakes and failures we also learn. In case of AC, some trouble will not show up unless problem shows.

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