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What is an Alcohol Thermometer?

Mary McMahon
Updated May 21, 2024
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An alcohol thermometer is a thermometer which utilizes the expansion and contraction of alcohol in response to temperature changes to measure the temperature. A number of different alcohols can be used, depending on the environment where the thermometer is being utilized, with ethanol being among the most common. This type of thermometer is very popular because it is nontoxic, unlike a mercury-in-glass thermometer, and the contents will not pose a threat to human health or the environment if the thermometer is broken.

Since alcohols are clear, a dye is usually added to the alcohol used in the thermometer. Red is a common choice for dye, although other colors may be used as well, and the backing of the thermometer is usually colored to provide contrast so that the meniscus of the fluid will be clearly visible, allowing for accurate temperature readings. The thermometer may also be housed in a casing which protects it from impact and fluke temperature fluctuations, and to make it easier to handle.

Alcohol thermometers work by enclosing a narrow capillary attached to a bulb of reserve fluid. As the temperature heats up, the alcohol expands, rising up the capillary. As the temperature goes down, the fluid contracts, dropping down the capillary. Markers along the capillary indicate the temperature, with people reading the temperature by finding the marker which corresponds to the meniscus of the fluid inside the capillary. This can be tricky to do with a narrow capillary, as the thin thread of alcohol may appear almost invisible even with dye.

The earliest prototype of the alcohol thermometer appears to date to the 1600s. It is one among numerous devices which can be used to measure the temperature, and alcohol thermometers are utilized by a number of laboratories and scientific organizations for temperature readings. Different types of alcohol are used for different conditions; an ethanol alcohol thermometer, for example, cannot perform very well at temperatures too far over the boiling point of ethanol.

As with a mercury-in-glass thermometer, it is easy to skew the reading of an alcohol thermometer by heating or chilling the bulb which holds the reserve fluid. For this reason, it's important to avoid handling the bulb when working with this type of thermometer, and to avoid exposing the bulb to temperature extremes when trying to get an accurate reading. For example, an alcohol thermometer being used to measure indoor temperature should not be placed in a sunny spot in the house or in an area next to a stove or heater.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a All The Science researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By anon995137 — On Apr 06, 2016

If a newer thermometer contains a silvery liquid it is filled with galistan, a non-toxic alloy of tin, gallium and indium.

By Monika — On Sep 10, 2011

@ceilingcat - I vaguely remember using thermometers with some kind of red liquid in them when I was in middle school. And they were kind of difficult to read! I remember my teacher made a pretty big deal about taking the reading at the bottom part of the meniscus.

I must admit, I wasn't that diligent about doing that. I know incorrect temperature readings can affect an experiment, but I guess I didn't take it that seriously!

By ceilingcat — On Sep 10, 2011

I've never heard of this type of thermometer before. I remember when I was really little we had a mercury thermometer, then we switched right over to digital. I feel like maybe alcohol thermometers aren't that popular for household use.

I bet they aren't that expensive, so they're probably used in schools a lot. The only drawback to that is that it sounds like alcohol thermometers are kind of hard to read!

By dfoster85 — On Sep 09, 2011

@rugbygirl - Was it a mercury thermometer your mom broke? It's hard to believe something that dangerous was once standard in the house!

I sympathize with your desire to keep an accurate thermometer as long as it's not toxic. I'm not an expert, but mercury is silver in color and I don't think they dye it or anything for the thermometers. So if your thermometer has a silver column, it's probably mercury and you should dispose of it safely.

But if the column is red or some other color, then it's more likely an alcohol thermometer and safe to use - as long as you're patient enough to wait that long! Me, I go with the instant.

By rugbygirl — On Sep 08, 2011

How do you know what kind of glass thermometer you have? I have a glass thermometer that's several years old, but I don't remember how many. I'm not sure if it's got mercury in it or not.

I know I could just buy an instant thermometer, but I kind of like the old glass ones and I hate to throw anything away. I remember my mom once broke one when she was shaking it down - she accidentally banged it on the counter! - but I am much more careful.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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