Posted Thu, Jul 18th, 2013
By Jason S. Herrington
LC/MS/MS Performance Tips

The oil in a rotary vane rough pump acts as a sink for compounds in the vapor stream you are vacuuming. This ties in nicely with my most recent blog in which we learned that the color of your rough pump oil is not always a reliable indicator of the oil's quality (i.e., measured by pumping capacity and/or the ability to achieve ultimate vacuum). Well of course this makes sense right!?!?

If the oil acts as a sink for compounds in your vapor stream and said vapor stream contains components with a propensity to discolor the oil, you may wind up with "dirty" looking oil in a relatively short period of time. However, the oil quality (again, as measured by vacuum performance) may be just fine. The problem with this scenario is that one may be duped into changing their oil prematurely. Not the end of the world.

However, the converse may happen in which the vapor stream is free of components that discolor the oil. In this scenario, one may use their oil to the point of degradation (and beyond) and pump performance suffers. This scenario seems to be more common than the aforementioned. This scenario also carries more significant implications, as discussed in my previous blog.

Now... time to talk about BALLAST. Old and new rough pumps come equipped with a ballast cap/valve (see picture below). When you open this valve the oil is allowed to "breathe" (or at least this is what I call it).

Rough Pump

The ballast is particularly important when dealing with the two following scenarios:

Your vapor stream carries solvent vapors that condense in the rough pump oil and ruin ultimate vacuum via loss of lubrication by dilution.Your vapor stream carries water vapor, which has the same implication as the aforementioned scenario.

In both of the above mentioned cases, periodically crack open your ballast (as per the owner's manual recommended procedures), which will allow your rough pump's oil to breathe and the solvent and/or water vapor will be "baked" out of the oil. How often you do this and for how long will be dependent upon your application; and just like changing your oil, I recommend you get on a schedule with this. One tell-tale sign your rough pump is filling with solvent and/or water is that you will see the oil level increase over time (this is not supposed to happen). In one case I saw a rough pump fill up with so much water it had backed up into the inlet (see picture below). Another sign is a loss in vacuum performance. There is also the scenario in which you may see bubbles in your oil... again, crack open the ballast. It is important to note that this is an excellent maintenance step; however, it does not excuse you from regularly performing oil changes on your rough pump.

Rough Pump

In the end, do not be afraid to familiarize yourself with your rough pump and your particular application. This may require a little trial-and-error. And do not hesitate to open up the owner's manual. Lastly, the term "rough" pump does not refer to how the pump should be treated.