Well I"ll be

Techman

darned.Just as I have always suspected,the micron reading at the vac pump is not the same as the micron reading at the evap coil.We hooked up 2 of the same  micron gauges, 1 at the vac pump & 1 at the TXV external equializer connection.Line size is 3/8 & 3/4" ,30' long.After 15-20min of  startup of vac pump,there was 750 microns at the pump and 6500 @ the evap coil.After another 10-15 min the readings were 175-200 pump & 850-900 @ the evap.We broke the vac w/ nitroge ,reversed the micron gauges and restarted the vac.4-5 min we had 125 @ the pump & 350 @ the evap coil.Conclusion, well I'll be darned!

JStar

...

Are you reading it with the pump still running? I always isolate and shut down the pump when I'm testing with a micron gauge, or else you'll get the exact (false) results that you've experienced.

Techman

evac

Morning to u JStar, that part of the evacuation was with the vacpump still running.As you stated a normal shut down and wait and hold period followed to check the microns.Being that this test was on a totally new system,everything was clean and dry and nitrogen purged during brazing the 4 joints,I'm very courious to see the differance between the 2 micron readings on a known,or suspected,contaminated system,on a system that does not have a schrader acess at the evap coil. We will of course put an acess fitting in.Being the so many systems donot have a schrader at the evap coil,that means that all of the micron tests are done at tha vac pump or vac hoses.I've seen more than a few mechanics get 500 or 350 microns at the pump and off they go, charging the system.I've done that myself ,in the past,that is

JStar

I think...

...the evaporator reading is the true reading, and the condenser gauge is reading the power of the pump's vacuum ability. If you put a valved hose on the pump side of the micron gauge, you should see the same reading at both locations.