One of my favorite things about lab work is the the opportunity to be creative in tiny ways. These "lab tricks" often go undocumented, though, so they are often lost to the ether when lab members move on to bigger and better things. The Yeung Lab's attempt to alleviate this problem will be occasional posts about challenges we've encountered and how we addressed them, which we hope will also help and amuse our fellow intrepid lab rats elsewhere. Today, I'll tell you how we slow down the leak rate on our vacuum sampling vessels so we can store them for up to six months—by filling up the valve stem with water.
It's not as simple as you think. Because of the tight tolerances in high-vacuum valves and the presence of waxy vacuum grease, water does not like to wet the part of the valve right next to the o-ring seal. Unfortunately, it is also the most important area to get wet: A bubble there is as bad as having the valve exposed to the atmosphere.
The challenge, then, is to get rid of all the air bubbles in the valve, and to do it efficiently. We had about 32 vessels to prepare for this round, and many more in the future, so it was an important problem to solve.
A little background
This idea of filling up the valve stem goes back at least as far as 2007, when Matthew Reuer (then a student of Michael Bender) published a paper studying biological productivity in the Southern Ocean. They used concentrations of dissolved oxygen in the surface ocean as a tracer of productivity; more oxygen means more photosynthesis, and a more productive ocean ecosystem. However, they needed to know if air was leaking into their vessels because they stored some of their samples for many months. So, they studied the leak rates through the valves they were using.
Reuer and his co-authors found, perhaps unsurprisingly, that water-filled valves were 3 - 4 times less leaky than their air-filled counterparts. It is, in essence, because the water displaces the air that would otherwise leak in. With water in the way, air has to diffuse through the water and through the o-ring to leak into the sample vessel. Plus, filling up the stem with water before the bottles gets in the field makes sampling a breeze.
I was originally taught to tap the neck of the bottle on the table until the bubbles came out. While it is a reasonable technique, its ineffectiveness became quite apparent as I was teaching Shuning and Boda how to do it. We were all having a lot of trouble tapping with enough force to dislodge the bubble without risking breaking the glass. So I found another solution.
How we do it now
We use a PVC endcap, which is usually only added when we're done to keep the water in the stem, to our advantage. After filling up the stem to the brim, we push the endcap on. The close fit between the endcap and the stem results in some resistance when pushing the cap on. This is a good thing.
That resistance indicates that you're making a weak seal with the valve stem, which can be used to pull the bubbles out.
We pull the cap off quickly. It makes a satisfying "pop" sound. That's the sound of the suction generated, which dislodges the bubble and may even degas the water a little (like when you open a bottle of soda). A bit of water often comes out of the stem but that's okay—it's just the cost of doing business, and a small price to pay for efficiency.
We found that the stem needs to be filled to the brim. The technique relies on the fact that air is compressible and water is not; if there is an air bubble at the tip of the stem, all you'll end up doing is compressing and decompressing that top bubble, and not any of the bubbles inside the stem.
Even with this technique, we found that we have to refill the stem and go about 5 or 6 cycles per bottle before all the bubbles are out. Yet, it is still much faster than the tapping method; I'd say maybe by 3 times or more.
Extra pro tip: Cutting foam
Something else we discovered this past month was how to cut thick packing foam: An electric knife. You know, the kind you might use to carve a turkey or slice some bread (ours came with a bread-slicing guide, even). It cost twenty bucks and probably saved us countless hours in labor.
Before we found this method, Ian was cutting foam by hand using his pocket knife. The electric knife is both safer and more fun to use.
There are other methods, like using a heated knife, but those only work with certain types of foam. The electric knife seems to be a generally useful tool that will no doubt receive a lot of love in the coming years.
Well, that's it for this installment of Lab Tricks. Hope you learned something!