fixing thermometer

[poco]

I just dropped my expensive color thermometer and now have separations in the "mercury" column. Any way of fixing this ...or did I just blow $70.00?

Thanks,


-Michael

[Ron Marshall]

Sometimes you can shake the mercury down, with repeated vigorous motion. If That doesn't work heat the thermometer in boiling water until the mercury merges. When it cools it will stay together.

[poco]

Thanks, Ron. The hot water did it. Whew!

[Donald Qualls]

Thanks, Ron. The hot water did it. Whew!

Do watch closely when doing that, though; I used to have a Delta floating thermometer (alcohol type) that accidentally got exposed to water about 20-30 degrees F hotter than its scale covered, and it popped the top off the capillary. Needless to say, I couldn't fix that one...

[poco]

Yeah, I know the dangers of overheating a thermometer. When I was a kid the rule was "no staying home from school without a high temperature." That's how I learned that thermometers and hot light bulbs don't mix ...was finding mercury pellets in my bed for a week.

[raucousimages]

Never use a light bulb on a thrememeter to stay home from school. Your mother will not believe you have a 125f fever! Just rub the bulb quickley on the sheets, you can raise it a couple degrees at a time. My kids realy hate it when I know the tricks.

[Donald Qualls]

My brother used to hold it in front of the heat register. Which did absolutely nothing, after we got the wood stove and the furnace wouldn't run at all overnight...

[Harold_4074]

A Kodak process control thermometer has a fairly limited range, and it may be possible to easily and safely fix a column separation by placing it in a deep freeze for a few hours. The temperature corresponding to the transition from bulb to stem is well above freezer temperature, so all of the mercury gets sucked down into the bulb, to re-emerge as a solid column on warming. Worked for me, anyway.

There's nothing quite like an etched-stem thermometer as a reliable basic reference, arguments for mercury-free electronic devices notwithstanding. Dial-type thermometers can suddenly develop errors of a few degrees, and I once bought a new one that was off by some eleven or twelve degrees. An electronic thermometer may or may not be better, but without a known-good reference, how would you know?

[Donald Qualls]

An electronic thermometer may or may not be better, but without a known-good reference, how would you know?

It's at least fairly simple to verify the electronic thermometer within a couple degrees F if you know you local elevation; all you need to do is boil some distilled water in your microwave (preferably in a clean glass graduate or beaker, to ensure a minimum of dissolved impurities that can change the boiling point, though Visions, Pyrex or Corningware cookware is a good substitute), then keep it barely boiling on a lab type hot plate or similar and measure the temperature of the water. Knowing the local elevation and, if you're *really* after precision, the barometer reading will let you calculate what the boiling temperature should be within a fraction of a degree F.

You can calibrate the other end of the scale with ice water -- again, if you start with distilled water for both the liquid and the ice and use a clean, non-metallic container, you can pretty confidently know the correct temperature to a fraction of a degree F.

No exotic equipment needed, and the tests can be repeated in half an hour, any time you feel like it.

OTOH, I'm not a big fan of equipment that needs a battery, if non-electronic stuff will do the same job. When I was in high school, we made simple air/gravity thermometers with capillary tubing fire polished shut on one end, tiny droplets of mercury drawn in by heating the tube and letting the cooling air draw in a short column, and the air trapped in the tube. Measuring the height of the air column at a known angle (where the weight of the mercury supplied a fixed gauge pressure) and correcting for barometer reading allowed simple calculation of ambient temperature using Boyle's law. Don't know where I'd find mercury to make one of those now, but the first place I'd look would be in the house thermostats, if I needed one...

[Harold_4074]

Donald--

While this is hardly the place to debate the niceties of experimental technique, I would like to point out that I once thought that ice and boiling points would be good, easily reproducible calibrations also. When my NBS-traceable platinum resistance thermometer disagreed, my colleagues at the time (at Hewlett-Packard Laboratories, back when the company was still very much an instrumentation outfit) gently pointed out that issues of superheating and stirring conspire to make fixed point measurements a lot harder than they look. Getting to within a degree or two is not so hard; getting down to fractional degrees (even if you have an instrument with that much resolution, rare for consumer electronics) is a lot tougher.

Certainly, the boiling/freezing points will tell you if your thermometer has gone seriously out of whack, which is all one needs for most photographic work. Thanks for pointing out the "kitchen physics" solution.

Regards,

Harold

[Donald Qualls]

Certainly, the boiling/freezing points will tell you if your thermometer has gone seriously out of whack, which is all one needs for most photographic work. Thanks for pointing out the "kitchen physics" solution.

Certainly, it's a different thing to be able to calculate the correct boiling and freezing points from local pressure, and being able to actually maintain conditions accurately enough to exhibit those, but as you say, if you actually do look up the accepted values in a table (there are engineering steam tables online, they're easy to find and not all that hard to use when all you want is the temperature that gives vapor pressure equal to ambient, or the saturation temperature of ice at ambient pressure), you shouldn't have much trouble verifying the thermometer within a degree or two, which is plenty good for photography.

If you want real accuracy and don't have a high precision barometer handy, there's always a triple point cell, which can pretty readily get you within 0.1 F, though few who don't have a calibrated thermometer to verify against are likely to have the glassworking equipment to make one of those, much less the technique to measure a temperature to that precision, with the implied accuracy.

[Harold_4074]

To give the dead horse just a few more whacks: another problem with using steam and ice to check a mechanical or electronic thermometer is that the two points are 180 Fahrenheit degrees apart. Depending on the shape of the error curve, this could easily place the maximum error in the vicinity of the range of photographically useful temperatures (about 68F--75F). In other words, it can be correct at both ends of the scale but if it isn't perfectly linear it can still be off everywhere in between.

One of the great virtues of the Kodak Process Thermometer is that the range is restricted to, and it is calibrated for, photographically useful temperatures. In fact, I understand that there were actually two versions, one for 68F monochrome processes and one for 75F color chemistry (I've never seen the color version, so maybe it differs in some other way).

I'm sure that some folks will argue this point, but I think a two-degree error it too much to accept, even for black-and-white. A few years ago my standardized Plus-X/HC110 development started giving blocked highlights. After agonizing for a while over agitation, water quality, light metering and such, it occurred to me to take the thermometer (a mechanical dial-type) and check it against a laboratory instrument. I had done this when I got it, so I know that it was originally OK. Sure enough, the readings (at 68--70 degrees) are now low by about 1-1/2 divisions, or three degrees. Allowing for this solved the problem, but if I hadn't tried it I wouldn't have believed that a "one division" error, not much more than the pointer width, would ever matter. The thermometer isn't adjustable, and I've since switched to a more forgiving film/developer combination, so I just live with it---but I still check it occasionally just to be sure that it hasn't suffered another concussion or something.