Should A/C vacuum pump run continously?

[mthomas58]

Just got everything connected and I'm pulling a vacuum unsing the Robinair 15434. Turned pump on about 5 minutes ago and it is running continously. Is this normal? PSI reads neg 30. I'll go shut of the maniold gauges and the pump now to see if it holds a vacuum or I have a leak.

Thanks!

[tsuter]

Yes - normal.

No - do not close the manifold gauges to check for vacuum leaks. Leave them open.

Close the Iso-Valve on the 15434 only. That will isolate the pump from the system. Then turn the pump off.

Watch the vacuum gauge reading over nite for movement indicating a leak.

Before charging, pull vacuum overnite. 5 minutes or 30 minutes won't do the trick except for a basic leak test.

[mthomas58]

OK thanks. I've had the pump shut off for 30 minutes now and its holding steady at -30 PSI. I've got a nitrogen bottle that I'm going to pressure test at 150 -175 PSI for leaks. If no leaks, I'll pull anther vacuum for 3 hours or so then charge and I should be OK.

[mthomas58]

Holding steady on Nitrogen pressure test at 155 PSI - NO LEAKS!!!!!

[billybek]

Leak checking with lots of pressure can be a bit deceiving.
Most of the leaks on these systems can be pretty small. You may not notice a pressure drop on the gauges for a quite some period of time.
As mentioned before, a vacuum test is a great way of determining if the system is tight. Pull your best vacuum and isolate your vac pump. Look for a loss of vacuum overnight. On small leaks it will be more evident with a vacuum than with pressure.
If you find that you can't hold a vacuum, then nitrogen and trace gas in the system and a good electronic leak detector is your friend...
Don't forget to consider lubricant (I know how that sounds) when adding refrigerant to the system. You will lose oil when you lose gas... (Oil is a great indicator of a refrigerant leak)

[Tigerrat]

When I ran this drill, I was surprised at the static pressure variation with temperature. So if you do leave it pressurized over nite you will have to check it at the same temp the next day as your earlier reading. Ken

[firemed51]

Originally Posted by Tigerrat

When I ran this drill, I was surprised at the static pressure variation with temperature. So if you do leave it pressurized over nite you will have to check it at the same temp the next day as your earlier reading. Ken

The great thing about nitrogen is temperature actually doesn't matter - that is why it is used in racing tires. 150 lbs of pressure is sufficient as you are mainly testing the front seal and o-rings. As we know, pulling a vacuum might actually seal a o-ring or front seal where pushing might move it and show the leak too.


Leak testing is done both in a vacuum and pressure. The best available tool for checking under vacuum is a micron gauge - it splits the last 12 inches of mercury into 300,000 microns - I normally pull a car down to 250 microns. Yacht refrigeration manufacturers only warranty units that have been pulled to 250 microns x 4 hrs and I use a dial recorder and add the wheel to the receipts.

http://www.achrnews.com/Articles/How_To/b1a8ec2f93f5a010VgnVCM100000f932a8c0____


Anyway - pull the vacuum overnight - the freon permiates into the oil - along with moisture - and pulling the freon out of the oil is only done by holding a heavy vacuum over time. if your gauges don't move for 30 minutes, after closing the manifold, you only know you don't have a bad leak. There is no such thing as a good leak....
The micron gauge is the only effective way to find out how much of a leak you really have. The gauge can be a bit deceiving as your hoses permiate and there condition can actually be read, even when the job is properly done.


With the funds you have invested in the nitrogen, vacuum pump, manifold set, and your education - you'll find the micron gauge to be a great tool.

http://www.acsource.com/index.asp?PageAction=VIEWPROD&ProdID=561

There are cheaper ones out there - this is just like the model I've been using for 10+ years.

Sea Ya,
michael

[billybek]

+1 on the vac gauge.. It is the best way after a good pull on the vacuum pump to determine if you have any leaks. Also a great way to test your vacuum pumps performance.
One thing to do is change your pump oil regularly after use (clean oil for times between use/ no contaminants left in pump). Change it hot!!
Also change your oil before you do your final pull down on the system. The pump will perform better.
Back in the day, we would do a triple evac. Breaking vacuum with refrigerant to atmospheric and pulling a vacuum again. Can't do that now though....

[firemed51]

Originally Posted by billybek

+1 on the vac gauge.. It is the best way after a good pull on the vacuum pump to determine if you have any leaks. Also a great way to test your vacuum pumps performance.
One thing to do is change your pump oil regularly after use (clean oil for times between use/ no contaminants left in pump). Change it hot!!
Also change your oil before you do your final pull down on the system. The pump will perform better.
Back in the day, we would do a triple evac. Breaking vacuum with refrigerant to atmospheric and pulling a vacuum again. Can't do that now though....

Yep - some days big brother gets in the way of progress........

[bazar01]

Nice to have all the tools you got. You are still missing an important tool. Electronic leak detector. You charge a little refrigerant and pressurize with nitrogen and use the electronic leak detector. I don't trust the vacuum test.