As a primer to this post, I would not have done the evacuation quite this way, with multiple pulldown and isolations at 500 microns, the goal was to demonstrate the characteristics of a system with moisture and to prove that “hitting 500 microns” is not enough. Hitting 500 microns in no way shape or form guarantees that the system is clean and dry enough for the introduction of refrigerant. Adding a Shrader cap full of water to this dry recovery tank will demonstrate why. We are using the BluVac+ Professional vacuum gauge and a free app developed by MeasureQUICK that will demonstrate the strong visual indication of moisture in a system.
Above is an example of a clean dry recovery tank, at vacuum isolation, with the gauge located at the opposite end of the system from the pump. I am evacuating with a 1/2″ hose on the vapor side and measuring on the liquid side above a dip tube. This is how an evacuation should look on a system that is purged with nitrogen during installation, and the tubing kept dry during shipment and installation. It pulled down and held in about 6 minutes. At isolation there is no significant rise, the decay effectively flatlines at less than 0.0 microns/second. Waiting 10 minutes to see the final decay took longer than the evacuation itself.
Do demonstrate moisture contamination, with the tank under a vacuum, I introduced a Schrader cap full of moisture into the system, broke the vacuum to the atmosphere, and started again. With less than a thimble of water, you will notice that the evacuation time almost exactly doubled. It took 11 minutes and 46 seconds to hit the 500-micron mark, as removing water is work and takes time. After hitting 500 microns, at isolation, the tank pressure very quickly rises (decay). This shows hitting 500 did not do the job of dehydration. Even hitting 500 microns effectively at the far end of the system! The wet tank quickly decays over to 1000 microns before I restart the evacuation process. Likely it would have leveled out between 1500 and 2000 microns. The decay took 7.5 minutes. It should also be noted that the amount of moisture was so small that it did not stall the evacuation process. There is a small shift in the curve, and the time increased, but the vacuum pump and the rig were able to boil off any large amount of moisture quickly, and now we are seeing the effect of the residual moisture that bonded to the walls of the tank. Notice even after several minutes the decay rate is .3 microns/second. The tank is still outgassing significantly.
The evacuation was restarted, and even after second time hitting 500 microns, the tank still eventually decayed to over 1000 microns. The decay took 20 minutes, but regardless of the time, (and many technicians would not have waited this long) it is important to note that the tank is still very wet. Introducing refrigerant into this system over time would lead to a catastrophic failure. The decay time has increased significantly, but it is still going over 1000 microns, evidence that the tank it tight, but still very wet, and holding more moisture then we would want to depend on a filter dryer to remove, especially in a system containing POE oils.
Above, the third time we hit 500 microns, the tank is decaying to the 300-micron range, it is starting to dry out, but additional time on the pump in my mind is warranted. I have personally seen the effects of moisture in compressor autopsies. Getting the moisture level as low as possible will assure longevity of the system, minimizing acid and sludge production. Notice we have a stable decay, indicating the true level of vacuum in the tank.
Pulled it down one last time to 250 microns and now the tank is holding about 300 microns with a leak rate of 0.0 microns/second. The tank is dry and tight. While we could have let it run down lower and more quickly removed the moisture, it is still worth noting that the moisture removal process took over 2 hours on a tank sitting in a 78-degree room. Notice how quickly the tank decay stabilized to 0.0 microns once the moisture was removed.
Removing moisture takes time, and as demonstrated here, hitting 500 is no guarantee that the system is ready for refrigerant. I cannot stress enough, you have to measure vacuum with the system isolated and allow time for the decay to stabilize before you determine the final level of vacuum. The vacuum must be isolated with core tools from the pump and the hoses. Do not rely upon a vacuum pump blank off valve to hold vacuum. Don’t follow these procedures and you will deal with problems like those shown below.
Want to see the final report generated by the application? Final Report.
Next up, why you never should install your micron gauge at the vacuum pump!
That was really helpful thanks
I don’t miss troubleshooting, installing, sizing g units are determining airflow in systems. Retirement is wonderful.
However ever I did study and learn from the best in the ac business. Doolin tech. Dollin HVAC bible founder.
Have fun ladies and men. It’s a living
Thanks for the great article….This verifys , black and white print, my argument with certain people in the field. I’ve been performing AC service for years and have had great success ……..Fixing what others said couldn’t be and what others had “broken”.
Evacuation is as if not more important than refrigerant charge or properly setting airflow. The challenge is we cannot often see the damage that an improper evacuation has done until after the compressor fails. As efficiency losses are so gradual, they are often too overlooked.
What about the ‘triple’ vacuum? Pulling a vacuum then breaking with dry nitrogen at 100Kpa, then dumping after half an hour and vacuuming again? I’d like to see findings on this, have always been told that this is the only way to truly get rid of all the moisture.
We can demonstrate that. Give me a little time and I will do that for you.
Very good information cascade I put the system down to 125 to 150 microns let system stand for 30 min .This way you got a tight system that dry the refrigerant expensive nowadays.
All of this information is very good, but it is often the case, at least in my experience, that it is difficult to find a technician/ company that will go through all the necessary effort stated in this article. Most I have found will say this or that is good enough, or we have never had a failure doing it this way. Some will even get upset if you insist on going the extra mile because it is cutting into their time or resources, i.e. profit margin. Yet, in the end, if the system fails, they don’t mind charging you unnecessary labor charges because they can and claim that this is just the nature of the business. It’s all BS and the consumer all too often comes out on the losing end despite any apparent warranties.