Evacuating / Pulling vacuum

[brewtoo]

When you pull down an AC system for recharge....

My vacuum pump pulls the pressure down to about -30 on the low-side gauge in just a few minutes.

But I've always heard you should let the pump run for quite some time.

If the pump has pulled the system to as low a vacuum as it can, what is the point of continuing to let it run? Or is there one?

[slk230red]

Originally Posted by brewtoo

When you pull down an AC system for recharge....

My vacuum pump pulls the pressure down to about -30 on the low-side gauge in just a few minutes.

But I've always heard you should let the pump run for quite some time.

If the pump has pulled the system to as low a vacuum as it can, what is the point of continuing to let it run? Or is there one?

I was always told it was to remove moisture.

[brewtoo]

I agree. But once the pressures equalize and the pump has lowered the pressure as far as it is able, can any more moisture be removed?

[slk230red]

I'm not sure...I have always let my pump run for 20 to 30 minutes and then charge the system. Never had a problem with this method and I always replace the drier/receiver when I open the system.

[just-n-time]

Originally Posted by slk230red

I'm not sure...I have always let my pump run for 20 to 30 minutes and then charge the system. Never had a problem with this method and I always replace the drier/receiver when I open the system.

What is it going to hurt? The vacuum will help pull all that moister ladened air from the lines and the dryer and the evaporator,my uncle was a votec teacher for a/c and he was always telling me to let it pump,the longer the better! most customers cant wait around but if you can let it rip.JNT

[Bruno_300TE]

Originally Posted by brewtoo

I agree. But once the pressures equalize and the pump has lowered the pressure as far as it is able, can any more moisture be removed?

The gauge commonly used for the low side are not accurate when the pressure almost reaches vacuum. There are special gauges for this purpose. They are quite pricey however. So in your case it could be that residual moisture is still boiling off without showing on your gauge. To make sure the system is ok I'd let the pump on for a longer time. After you have shut it off you can observe the gauge for some time (1 hour?) and see whether it says low (if yes -> add the refrigerant). If it moves a little bit and then stops, there is some residual moisture in the system (continue evacuation, maybe stronger pump is needed?). If it moves a lot, there is a leak (-> find and fix).
Bruno