The other advantage is using it during dehydration of a system with a lot of moisture.
If you pull a vacuum to quickly and deep enough the moisture will freeze, leave your gas ballast open a pinch and you will pull a lot more moisture out without freezing it.
I had an issue with excessive moisture in a new install recently, and it was quite irritating. What point do you open or close the ballast? Does it add extra time? How effective is this method with relative humidity being >75%?
The purpose of a gas ballast in a vacuum pump is to prevent water vapors from condensing during the discharge stroke of the vacuum pump.
Water can only be removed from a system in vapor form. If the atmosphere that you are removing from the refrigeration system is moisture laden, as that moisture enters the pump it is in vapor form, it is in a state of equilibrium with the air in the system. This state of equilibrium is what is meant by the term ballast. (something that gives stability)
The ballast, when it is open introduces free air into the pump during the discharge stroke to keep this moisture in equilibrium. If the gas ballast is closed, the pressure created in the discharge stroke will condense the water vapor and drop the moisture out into the oil. Having the ballast open during the initial pull down of a wet system will help to prevent condensation within the pump. (keep it open until you are at 1500-1000 microns)
Moisture is what kills the vacuum pump oil. When oil is wet, the vapor pressure increases to a point where a deep vacuum can not be created. (wet oil is white oil) If the oil is wet, it is cheaper and faster to change the oil then to let the gas ballast work it out. That moisture will also damage your pump if left in so always change the oil if you are work on a wet system. The reason I recommend you always change the oil is it is hard to see how cloudy is is through a small unlit sight glass.
An open gas ballast prevents the pump from reaching its ultimate vacuum levels and should be closed after you reach 1500-1000 microns. The gas ballast used used only during the roughing period and only needed when there is moisture in the system.
One of the most important things you can do is to always nitrogen sweep or purge a system before performing an evacuation. This means push the nitrogen through the system, one side to the next WITHOUT significantly raising the system pressure. This will push out the moisture vapors with out dropping them out into the system in liquid form.
If you purge during assembly, and sweep the system with nitrogen prior to evacuation, you will likely not need to use the gas ballast at all. The gas ballast is only effective in removing small amounts of moisture, so a very wet system will require frequent oil changes if you want to make fast work of getting the job done.
Per the info at Ritchie Engineering website in link above, the gas ballast should remain open until 1,000 - 1,500 microns. See page 3, sections 4 & 5.
There is very little moisture left in the system once you reach 10,000-15,000 microns, having the ballast open will only slow the evacuation. You cannot get into a deep vacuum with it open at all. If there is little moisture, it is not even worth opening.
Some pumps also will not pull down with the ballast open. To see where your pump cuts off, connect the gauge to the pump and see how far it pulls down with the ballast open then see where it goes with the ballast closed. You might be surprised.
If there is moisture in the system you are often far ahead with a few oil changes then to let the gas ballast remove the moisture.