A/C : Pulling a Vacuum
Thread Starter
Drifting
Joined: Feb 2005
Posts: 1,583
Likes: 5
From: Richardson Tx.
A/C : Pulling a Vacuum
Ok, I admit this is not for the Vette but for my '91 Riviera. Its about 110 degrees here so forgive me 
I'm told that the amount of vacuum that should be pulled on the gauge is about 30". After about an hour on the vacuum pump this is all that I can get. What could be the cause and solution to get this to pull a deeper vac?
I'm told that the amount of vacuum that should be pulled on the gauge is about 30". After about an hour on the vacuum pump this is all that I can get. What could be the cause and solution to get this to pull a deeper vac?
Drifting
Joined: Oct 2005
Posts: 1,257
Likes: 4
From: North Carolina
I got 30+ on the machine and 15 on a this simple pump we used first. The pump was on the car for 4 hours.
If it holds vacuum, their is no air in the system and no leaks,... your good to go!
Dan
If it holds vacuum, their is no air in the system and no leaks,... your good to go!
Dan
Melting Slicks
Joined: Dec 2000
Posts: 3,407
Likes: 2
From: Carmichael CA
That looks fine...it's not the amount of vacuum in the system, but the amount of time that the vacuum is present. I believe the minimum vacuum is 16inHG but would have to be maintained for 4 hours. At 30inHG you should be able to get by with about 30 minutes like everyone else said. The pressure reading is basically a count of the number of times a molecule hits the plunger. There will still be some molecules bouncing around in other areas of the system and you have to wait until the pressure balances out.
It's also important to remove the vacuum pump and close off the system and wait a couple of hours to make sure the vacuum will stay put...ideally there should be zero leakage in those two hours before you fill it with refrigerant.
It's also important to remove the vacuum pump and close off the system and wait a couple of hours to make sure the vacuum will stay put...ideally there should be zero leakage in those two hours before you fill it with refrigerant.
Last edited by Rockn-Roll; Jul 20, 2006 at 02:02 PM.
Racer
Joined: Dec 2005
Posts: 325
Likes: 0
From: Partlow VA
29.9 “Hgv is the maximum any pump can do. Most gauges will show 30”. Pull a 30 minute vacuum, close off the system and note the gauge reading, wait 30 minutes and see if the gauge has dropped. If the reading is the same as when you closed it off. Then start the vacuum process again for at least another 30 minutes, and then charge the system. If you lost vacuum start checking for leaks in the system and or your gauges. 
Racer
Joined: Jul 2006
Posts: 337
Likes: 10
Just a note on evacuating a system. Pulling a vacuum not only removes air, it removes moisture. Deep vacuum is required to make moisture droplets "boil off" and the resulting vapor is removed through the pump. Removing water is critical, as any left in the system will convert to acid and attack parts of the system. Moisture left in the system can also freeze at the metering device and stop or limit refrigerant flow.
The only way to ensure complete water removal is to use a vacuum gauge. Short of this, leave the vacuum pump on the system for 2 to 3 hours or more to ensure all moisture boil off. Change your vacuum pump oil after every use to ensure a long life and maximum vacuum levels.
Also, a rise a vacuum pressure after pulling a vacuum can not only indicate a leak, but can also indicate moisture is still in the system.
The only way to ensure complete water removal is to use a vacuum gauge. Short of this, leave the vacuum pump on the system for 2 to 3 hours or more to ensure all moisture boil off. Change your vacuum pump oil after every use to ensure a long life and maximum vacuum levels.
Also, a rise a vacuum pressure after pulling a vacuum can not only indicate a leak, but can also indicate moisture is still in the system.
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Race Director
Joined: Mar 2006
Posts: 14,112
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From: Florida
Originally Posted by sjmaster
Just a note on evacuating a system. Pulling a vacuum not only removes air, it removes moisture. Deep vacuum is required to make moisture droplets "boil off" and the resulting vapor is removed through the pump. Removing water is critical, as any left in the system will convert to acid and attack parts of the system. Moisture left in the system can also freeze at the metering device and stop or limit refrigerant flow.
The only way to ensure complete water removal is to use a vacuum gauge. Short of this, leave the vacuum pump on the system for 2 to 3 hours or more to ensure all moisture boil off. Change your vacuum pump oil after every use to ensure a long life and maximum vacuum levels.
Also, a rise a vacuum pressure after pulling a vacuum can not only indicate a leak, but can also indicate moisture is still in the system.
The only way to ensure complete water removal is to use a vacuum gauge. Short of this, leave the vacuum pump on the system for 2 to 3 hours or more to ensure all moisture boil off. Change your vacuum pump oil after every use to ensure a long life and maximum vacuum levels.
Also, a rise a vacuum pressure after pulling a vacuum can not only indicate a leak, but can also indicate moisture is still in the system.
I let it run overnite on my own stuff, just for the fun of it.
You should really use a Thermistor Vacuum Gauge. It reads in microns. Water boils and is evacuated at 1500 microns and you should try and get to 700 microns.
But, forgetting all the mumbo jumbo, just leave the pump on a long time and you should be fine.
Racer
Joined: Dec 2005
Posts: 325
Likes: 0
From: Partlow VA
Originally Posted by sjmaster
Just a note on evacuating a system. Pulling a vacuum not only removes air, it removes moisture. Deep vacuum is required to make moisture droplets "boil off" and the resulting vapor is removed through the pump. Removing water is critical, as any left in the system will convert to acid and attack parts of the system. Moisture left in the system can also freeze at the metering device and stop or limit refrigerant flow.
The only way to ensure complete water removal is to use a vacuum gauge. Short of this, leave the vacuum pump on the system for 2 to 3 hours or more to ensure all moisture boil off. Change your vacuum pump oil after every use to ensure a long life and maximum vacuum levels.
Also, a rise a vacuum pressure after pulling a vacuum can not only indicate a leak, but can also indicate moisture is still in the system.
The only way to ensure complete water removal is to use a vacuum gauge. Short of this, leave the vacuum pump on the system for 2 to 3 hours or more to ensure all moisture boil off. Change your vacuum pump oil after every use to ensure a long life and maximum vacuum levels.
Also, a rise a vacuum pressure after pulling a vacuum can not only indicate a leak, but can also indicate moisture is still in the system.
You are correct. Well written
Pro
Joined: Sep 2002
Posts: 624
Likes: 1
From: Melbourne, Fla. 6 months- New Middletown, Ohio 6 months
UV dye
Be sure to charge with freon that contains a UV dye. The vacuum procedures discussed above only tell you if you have a leak under vacuum. It does not tell you if you have a leak under pressure.
Years ago I pumped down a system and it held for a couple hours with no problem since the vacuum pulled the o-ring connections tightly together. As soon as I started to let freon into the system I had the leak. One fitting was actually only finger tight. Talking to a friend, not paying attention to what I was doing.
The dye will show any pressure leaks. You need a UV light to see the residue from any leaks but the nice thing is that the dye will still work years down the road if a leak should occur. You can buy a pencil type UV light fairly cheaply at most auto parts stores.
Years ago I pumped down a system and it held for a couple hours with no problem since the vacuum pulled the o-ring connections tightly together. As soon as I started to let freon into the system I had the leak. One fitting was actually only finger tight. Talking to a friend, not paying attention to what I was doing.
The dye will show any pressure leaks. You need a UV light to see the residue from any leaks but the nice thing is that the dye will still work years down the road if a leak should occur. You can buy a pencil type UV light fairly cheaply at most auto parts stores.
