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Once the refrigeration machine has been leak pressure tested, repaired, and dehydrated, now comes the moment of truth…The Standing Vacuum Test!
With all the other checks and tests combined, there is no other method that positively, beyond a show of doubt, proves a system is dry and tight. (If the test is properly performed).
You perform a standing vacuum test in lieu of the 24-hour standing vacuum test or the 24-hour temperature compensated standing pressure test because simple monitoring of the system the first couple of hours will tell you what you need to know once you know what to look for.
The standing vacuum test, is a test, it doesn’t benefit the mechanical procedure, nor dry the machine, it’s value is that of time and expense.
You perform the test using a vacuum pump that can achieve anything below 500 microns, the bigger the pump, the faster it will pull down, assuming you properly dehydrated the system prior to this step. (See Rapid Dehydration.)
Once the system has reached a vacuum of 1000 microns, or less, isolate the system with a digital vacuum gauge attached to it1. Begin plotting your readings over time on the chart below every 10 minutes.
The chart is a plot of the vapor pressure of water (light blue) and ice (dark blue) evaporating in a vacuum over time. The first point you will make will be at the “Minutes 0” (far left side) and the appropriate pressure (somewhere below a 1000 microns).
As the minutes’ progress, continue to plot the readings from the digital vacuum tester and a “trend” will emerge, a trend is just a line displaying the direction the test is going, a flat horizontal line for 90 minutes would be outstanding results, but what you will likely see looks like the chart below:
At “0 Minutes”, the system was isolated at a pressure of 500 microns. Once data was plotted, up to 45 minutes, the line trended up a little, then got steady at 800 microns and continued there for the duration of the test. This would be an example of a good test. The early trend up was the system reaching equilibrium from the deep vacuum and some extremely small, but acceptable, amount of moisture, once at 800 microns, the vacuum holds. This is a good vacuum and pretty dry system. The vacuum level should be below 1000 micron for most systems, some systems (capillary tubes) should be taken to 500 microns2.
Below are some other ways the test could go.
If your trend starts upward at a fairly steep incline, like the yellow line, the system still has a leak. The steeper the upward trend, the bigger the leak.
If your trend slowly progresses up and then follows the pitch of the blue lines, like the red line above, the system is wet. When the vacuum is first shut off, and the system isolated, there is a brief period of stabilization when the atmosphere in the system reaches equilibrium (balances out). After that, the trend follows the vapor pressure (blue line) until the final level of moisture is reached, once there, the trend will flatten out at the final pressure and remain there, the value of the flat part is the final vacuum level and moisture level. A system in this condition needs further dehydrate. A system that has standing water will follow the blue line.
So, a short system test can alleviate the 24-hour standing vacuum test that some recommend and can prevent putting a system into service that still has problems.
1. Use only a digital vacuum gauge that can be attached and left on the system. You do not want to disturb the vacuum in any way during the test. Do not use a wet bulb device or other fluid filled thermometers.
2. Always use the evacuation and dehydration levels recommended by the manufacturer.
I worked for over thirty years in the HVACR industry. I have designed, installed, serviced, and trouble shot units of various types throughout the years. The posts here are information based on that experience, I hope you find them useful. If you have a different experience, please comment.