Vienna Standard Mean Ocean Water

In order to do this, they basically are taking something that thermodynamically is an invariant point where water vapor, ice, and liquid water coexist. You can basically take these little glass tubes and you can put some dry ice down this little opening, and you will get to the point where all three of them will coexist. The masses of the three phases may change over time. You can put this in a little incubator-type thing and it will hold the temperature for several weeks. You put your little thermometer that you want to calibrate down inside this thing, and you will know — it turns out the triple point of water is at about a tenth of a degree centigrade. But they don't just use any water. What do you think they use? It's not just deionized — it's the Vienna Standard Mean Ocean Water sample.