Question: What's the story with the Micron rating of a vacuum pump? I'm assuming that smaller is better, but what does it mean as far as practical application? I do my own AC on my car and have a 5CFM, 25 micron vacuum pump.
Vacuum pump manufacturers usually provide a factory measured rating for their pumps. Fewer microns are best. As for practical application, the best pumps can reach the deepest vacuum levels. A 5 CFM pump is a great choice for just about any mobile A/C job. The fact that it can reach 25 microns is even better!
Microns are not part of the International System of Units, but, that's O.K.. The concept of microns, as it relates to vacuum, is easy to understand. Atmospheric pressure measures around 759,968 microns. The deeper the vacuum, the closer we get to 0 microns. By that, we could look at the vacuum pump as a micron remover.
In case you're wondering, the micron gauge, also called a thermistor vacuum gauge, is used to measure vacuum in this precise manner. Unlike the compound gauge on your manifold set, a micron gauge can be used to determine how well your vacuum pump is operating. We can also use the micron gauge to monitor an a/c system for leaks. Using a standard gauge set, it would be very difficult to tell if 10,000 microns entered the system through a leak path. We would see that easily on the micron gauge.
Robinair has a very informative, well illustrated article about vacuum as it relates to the A/C system. Nice vacuum chart too!
Here's another must read... Geoge Goble covers vacuum pumps and microns at Yarchive