Hi

The three places of difference are: Range, scale and construction.

(NOTE [Maybe WARNING would be more appropriate]: I’m an American. All temperatures will be in Fahrenheit. I'm also pretty long-winded.)

Range: Lowest to highest temperature. Candy and frying thermometers, for instance, don't have to worry about temperatures below 100 degrees F. Refrigerator thermometers don't have to worry about temperatures ABOVE 100. Meat thermometers don’t have to worry about anything below 100 or over 250. A room thermometer that went from 160 to 250 degrees would be pretty useless. So would a general cooking thermometer that didn’t get down to poaching (100 or so) and up to frying (400 or so).

Scale: Candy thermometers tend to be HUGE, because sugar and water boils down pretty quickly, and a 2 degree change is very significant, so they want you to be able to SEE that 2 degree change. Frying thermometers are smaller, because 10 degrees isn’t that big a deal: 300 degrees, 325 degrees and 350 degrees are the kind of guidelines you find, and if you’re aiming at 300 degrees and the thermometer is so small that you can’t see the difference between 295 and 305 degrees, it’s not that big a deal. How fast (both temperature-wise and time-wise) will the target food change state? Knowing the temperature you want doesn’t help if you can’t SEE it.

Construction (1): Analog and digital thermometers are very different birds. Digital is cool, because you know the EXACT temperature, they don’t need calibration (very often), and they are as easy to read as the manufacturer wants them to be, BUT, it’s hard to gauge the RATE of change of the temperature of the material being measured. It’s all step, step, step, even if it’s stepstepstep or step ... step ... step ... .

Construction (2): Liquid-filled thermometers use tinted alcohol or mercury, both of which expand and contract under change of temperatures at a very regular, and therefore predictable, rate. You’ll find that most thermometers that go much over the boiling point of water will be mercury-filled. Alcohol boils at a lower temperature than water, and vapor pressure can mess up the reading.

Construction (3): Oven and ‘fridge thermometers and lots of spike-insertion/meat thermometers have a strip of metal, coiled up and one end attached to the body and the other attached to the pointer. I should actually say a strip of two different kinds of metals, which change length at different rates when the temperature changes, fused together. As a result, this fused strip deflects as one side expands and contracts more than the other as the temperature changes. The dial actually measures the change in the deflection of this bi-metal strip, which changes predictably as the temperature changes.

Construction (3’): Someone, at some point, noticed that some of these bi-metal thingies would, when heated, generate electricity! They ALSO noticed that they’d generate MORE electricity with more heat!! Hook one up to a milliampmeter, put the bi-metal thingy in freezing water around sea-level, mark that as 32, then put it into boiling water and mark that as 212. Mark the divisions, and continue the marks down to about –20 and up to about 500 (range remembered from a discussion with Pappa – he was probably correct, but my memory may be flawed), and you have a thermometer! Use a digital milliampmeter, and you have a digital thermometer!

Construction (4): The case or frame of the thermometer is important, too. Mamma’s old candy thermometer had, before I broke it in a gravity-related tragedy, a wooden frame. That let it sit on the bottom of the pan without measuring the temperature of the pan instead of the liquid IN the pan. Pretty handy, but kind of hard to clean (that "hard to clean" led to its demise). It also had candy-makers marks on the scale: Thread (230), soft ball (240), firm ball (245), hard ball (255), soft crack (280) and hard crack (300) (temperatures are approximate. Either that or wildly inaccurate) ... like that. Meat thermometers will often have "rare," "medium rare," "medium," "medium well" and "well done" marks on the scale. Few of them go all the way to "toast" as AB's seems to. ‘Fridge/oven thermometers frequently have metal cases so they can take more abuse (getting knocked to the ground while inserting/removing contents) and so they will test the temperature over a larger area, thereby avoiding spot-measurements. Ever been in an air-conditioned room with a hot-spot over my the far wall, or a nice, warm cozy bedroom with sub-freezing temperatures over by the window? ‘Fridge thermometers will often have blue on the scale below freezing, green in the safe zone, and red over about 45.

For candy making (up to about 300 degrees), look for mercury filled or dial (because of the heats involved) with those candy-makers marks, long and large (to facilitate safe use and handling as well as ease of reading) with a nice, big, easy to get a hold of, insulated handle at the top.

This is a personal opinion and is in NO way binding.

And please remember: Being spattered by sticky, gooey, thermally dense material heated to 300 degrees or so, thrown cielin-gwards by a long thermometer taking a triple gainer out of a candy pot has been know to cause candy-cooks to become downright adjectival! A candy thermometer almost ALWAYS come with a pan-clip (way I figure: No clip, NOT a candy thermometer). USE IT!!

Hope this helps, some. Hope you didn’t fall asleep, too…

Last Edited: 08/27/2010