Thermometers and highschool science...

After some startlingly thin negatives, my suspicion grows that my thermometer is the culprit. I throw one into my sink full of water which water jackets the bottles, and another I use to check temps out of the faucet and in the tank. They've always agreed so I never looked at the one at the bottom of the sink anymore.

Now, they seem to disagree by close to two degrees. They're both mercury/glass. So I decided to try the freezing point calibration... for the first time ever. And here's where it gets fun... I had a blast setting up the experiment, with the ice/salt/water and I witnessed super-cooling for the first time. Bear in mind, I'm a pilot and we're taught that icing is bad, real bad. But what and how super-cooled water makes it so bad never enters the picture. Here, instead, I witnessed the water drop to -5.5 C and remain perfectly liquid. Then, very suddenly, the temperature shoots up to zero and, BAM, ice crystals everywhere.

That was so fascinating I had to do it three times!

But more to the point, at what point of the process am I supposed to be calibrating my thermometer on? Right after the heat discharge and the state of ice I presume? If that's the case, couldn't I just mix ice and water (no salt) and use that as my zero degrees?

If you are checking a thermometer you should not use an ice/salt bath since the final temperature can be a function of how much salt you use. Rather just use an ice/water bath. As long as there is unmelted ice the temperature of the bath is 32F. Make sure that you stir the bath when checking the temperature.

Actually it would be better if you could check the temperature at a temperature near where you usually develop film. But mercury thermometers are usually linear.

Thanks Gerald. I think I described the exercise poorly. The salt/ice mixture is used to cool a little test tube of clear water which is immersed in it. The thermometer is in the clear water and the freezing moment (when ice crystals form - which is incredibly sudden) it is - in my understanding, zero degrees.

The salt/ice/water mix was hovering around -10°C. It's quite a fun experiment, but as I had liquid water down to -6°C, I was unsure how the freezing point is defined. At least until I saw it happen a few times.

Other than against another thermometer, how would one test for accuracy around developing temps? Or at least get a second data point.

One method would be to choose a solid which melts around 20C. But practically the best choice would be comparison with another thermometer.

Absolute *best* calibration for a thermometer is the "triple point" of water. You can make a triple point cell by boiling water in a test tube, then sealing the tube while the water is still boiling, so the water vapor displaces all the air. Now, when you cool the cell, you can witness water continuing to boil as it cools, because the pressure will drop in lockstep with the temperature -- and at 0.1 C (IIRC) you'll get to see boiling water and ice at the same temperature. That's the triple point, and it's solely a function of the compound being examined (assuming it's pure, of course). There are several semi-convenient triple points that are used to calibrate master thermometers; that of water, of course, is the most convenient, but the triple point of carbon dioxide is around -78 C, IIRC, that of sulfur dioxide is somewhat higher, and that of anhydrous ammonia a good bit colder.

You can also use the melting points of a couple metals as calibration temperatures -- mercury at -39 C and gallium at 21 C are the most convenient (though gallium is a little hard to get and has a very strong tendency to supercool). Melting points are harder to use with materials that have low latent heats of fusion (compared to water), but the problem with water is that the melting point is so strongly dependent on pressure -- pure water will melt at -15 C under relatively modest pressure (which is why ice skates work).

For the experiment you set up, however, the pressure is well controlled (there's much less than 0.1 C variation in melting point over the normal range of atmospheric pressure in the regions where 95% of humanity lives). The problem is that the supercooling will only give a dependable 0 C if the water doesn't all freeze when the supercooled state collapses (i.e. the water suddenly freezes). What I'd suggest is to wait for the thermometer to read about -3 C, then stir or introduce a small crystal of ice from the freezer as a nucleation point; you want to get a freeze to slush, not to a solid block, and thus get water and ice at the same temperature -- which, at ordinary ambient pressures, has to be very-very close to 0 C.

Using ice cubes in water might not work, because the water can be a lot warmer than the cores of the cubes, and water is such a poor conductor that you can have 5 C variation over a few mm.

Donald,
Better late than never; thank you very much for that extremely informative explanation. I just reread it and it suddenly all makes sense. I like that triple point experiment - might have to try that one one day. (If only I had been listening in physics way back when... )
Grazie.

You can simulate triple point by just melting ice in water.
A slurry of crushed ice and water has a temperature between -0.3 and 0 C

I used that to calibrate thermocouples, and my 2nd point is boiling water.

Check your thermometer for air bubbles, if you have any, freeze it
and if these are still there try using a strong magnet to move the mercury down.

NikoSperi said:

I like that triple point experiment - might have to try that one one day. (If only I had been listening in physics way back when... )

Click to expand...

I first remember reading about triple points when I was about 6 or 7. Forty years later, I've never made a triple point cell -- doing it the way you would in physics/chemistry lab requires a glassblowing flame to draw a seal on a test tube while you keep the water inside barely boiling over a Bunsen burner or similar (tricky to do, because if you tip too far and the boiling water contacts the red-hot softened glass, you'll get a steam explosion driving very hot shattered glass everywhere -- not fun -- and of course you have to avoid building up too much steam pressure inside while the fresh seal is still soft, or you blow a big bubble in the glass). Given it's been around 28 years since I did any glass blowing, I probably won't try a triple point cell...