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Re: [Phys-l] turn up the heat



The Jewett reference, as presented to the list, is incorrect. It does not matter whether a thermometer is mercury, alcohol, or some other liquid (like mineral oil, kerosene, pentane). In all certified thermometers an immersion depth is specified, and this depth is marked right on the thermometer.

The two most common immersion methods are (1) total immersion and (2) an immersion of 76 mm (about 3 inches). Other depths are available, but much less common. In any event, the non-total-immersion thermometers should have a ring engraved around the stem to indicate the correct immersion depth

The most common glass thermometer used in chemistry labs has a range from -10 to 110 Celsius and is 76 mm immersion.

In analytical chemistry one of the lessons for using glass thermometers is how to make "stem corrections." This correction is necessary if you want to achieve the stated accuracy of the thermometer, but for some reason you have to use the thermometer with a different immersion depth than specified by the manufacturer. Generally this happens if you have a total immersion thermometer and you have to use it with partial immersion. If you Google [thermometer + "stem correction"] you will get about 700 hits explaining this.

I am curious to know if this is one of those things chemists learn, but physicists do not. I went through both programs in college and learned about stem corrections in freshman chemistry. For many years I was department chair, and now I am "Director of Laboratories" for a science department that includes, biology, chemistry, and physics. This means I have been purchasing all the thermometers for the department for the last 20 years or so. I almost always buy 76-mm immersion, but I have to ask the requesting professor what the intended use is. I've sometimes had to buy immersion depths longer or shorter than 76 mm. You can go to a vendor such as Fisher Scientific ( www.fishersci.com ). If you go there they request a login, but you do not need to, just search on thermometer and you'll find about 70 mercury, 50 non-mercury, and about 70 digital.

Michael D. Edmiston, Ph.D.
Professor of Physics and Chemistry
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu