Would You Like Ice in Your Drink?

Q: “Would you like ice in your drink?
A: “Yes, but make it ice Ih, please.”

Many of us may recall our high school or college physics class, and learning about water and its physical characteristics. We learned from our instructors that it has three phases: liquid, ice, and vapor; and that it has a triple point: a single specific pressure and temperature at which all three phases of water can coexist, any of which can abruptly change from one state to the other.

They lied.

As vacuum engineers, many of our applications deal with water in one of its forms. It may be drying, distilling, or packaging, to name a few. Thus understanding its characteristics is important.

Water is a very complex compound, and we are still discovering new things about it. For example, we now have identified at least 14 phases of water, not just the three we were taught about in school. Other than the liquid and vapor phases, all other phases are of different forms of ice. These phases depend on pressure and temperature. Each phase of ice is called by its number (e.g., ice VI is ice six, ice XI is ice eleven). The ice you may be enjoying in your iced-tea glass is called ice Ih, as in “one h” (the “h” stands for hexagonal—the shape of the crystals; it’s why snow crystals have six sides). Crystals in some forms of ice are cubes (but that’s not why we have ice cubes), such as ice Ic; some are trigonal, some are tetragonal.

We were taught also that ice floats in water, with nine-tenths below the surface; that’s because it has a density of 0.92, which is lower than liquid water’s 1.0. But some ices would sink straight to the bottom, because their density is far greater than liquid water—up to 2.51. Higher density ice is created by higher pressures—the kind of pressures that might be found at the bottom of 800+ miles of ice on other planets.

Rather than the single triple point, we now know that there are at least 12 triple points for ice and water. Most of these triple points are for the various phases of ice itself. Alas, all these other triple points and phases of water will remain a mystery for most of us in our day-to-day engineering; we’ll still be dealing with the triple point and three states of water that we were taught about in school.

But, the next time you are asked if you want ice in your drink, you can now say that you want ice Ih.

Then, sit back and chill.

A dirty vacuum guy

This is a beginning. Of what, I don't know yet. I intend to write about the world of industrial vacuum. From time to time, as subjects or applications arise I will post some notes. I don't know how I'm going to do it without giving away too much detail (industrial spies, you know), which could affect my work, but I'll try. It will be a learning process, for sure.

I have spent 40 years in the field, working for a number of companies that produce a wide variety of types of vacuum pumps. The one thing that they all had in common was that the pumps they produced were used in the rough vacuum field. This covered vacuum levels from atmospheric to 29.9 in.HgV (0.5 torr). We would occasionally combine them with roots-type blowers to pump down to about 20 microns.

In my experience, folks who work in the high vacuum field tend to look at us with scorn, since we are in the "dirty" vacuum range. But, I would contend that they would have difficulty with many applications in our range. Some things get more complicated in our world; take pump-down.

Pump-down equations use average volumetric flow (acfm) to determine time to evacuate a closed chamber. this is OK when a pump that can pull down to a deep vacuum (low absolute pressure) is used, since the acfm curve is flat, thus the average acfm is, well, average. What I mean is that it stays constant, whereas a rough vacuum pump may have a curve that is, well, a curve. Being a curve, the average is a variable. In order to calculate the average it is necessary to perform the calculations in steps, then total all the averages. The points between the two pressures must become narrower as you approach the pump's ultimate pressure, since pump-down is logarithmic. It should be noted that the higher the pump's ultimate absolute pressure, the more pronounced the curve.

For the high-vacuum guys, the equation is simple. They perform it one time. For us dirty guys, we must perform it many times. A spreadsheet makes this process quite easy, however. It is also possible to create a list of correction factors, based on the pump's ultimate pressure. This can get complicated, however, since there are many possible ultimate pressures.

Anyway, that's the first post. I welcome your comments--if you don't mind getting "dirty".