[Catalist] Repairing and maintaining alcohol thermometers for student labs: A CONSTAWA Investigation sponsored by Modern Teaching Aids

[Leon Harris]

Greetings all,

Yesterdays CONSTAWA was, as usual, excellent and as usual provided many 
stimulating conversations.


During one of these Frank De Rooy, of Modern Teaching Aids fame was 
showing me some glass alcohol thermometers where the column of alcohol 
had broken and the upper portion of alcohol was close to the top.

Stuffed Thermometer

We normally regard these as hopelessly broken, but they are not.

This post aims to do two things:

1. show you how to prevent this type of damage

2. Show you how to fix it, as it is a common issue in lower school labs.


Part 1: how to prevent column breakage in the thermometer.

Always store your thermometers vertical position. Cut a 35cm length of 
90mm pvc pipe, cut out a piece of sponge to place in the bottom, and 
glue a 90mm end cap to it. Secure it so it can't fall over (eg by 
putting in a cardboard box packed with newspaper, or any other technique 
you fancy) and keep your thermometers upright in this. The sponge in the 
bottom is to protect the bulb end of the thermometer. When students 
return thermometers from labs, have them return them to vertical containers.


Part 2: how to fix it.

Fixing thermometers takes a little time and care. The principle you need 
to keep in mind is that you must always have a significant volume of air 
in the thermometer, or the bulb will shatter.

The first task is to break up the liquid at the top (ill call this the 
top line from here on) . To do this you will need to drop the 
thermometer onto a plastic surface, such as a lunchbox turned upside 
down, repeatedly. The goal is to change a top line that looks like this 
________ into one which looks like this - - --- -   - - . When you drop 
the thermometer onto this surface, small droplets of the top line are 
separate from the rest. Because the thermometer is vertical, these 
droplets move downwards under the influence of gravity, and become 
closer to the main mass of coloured liquid. You can see a person doing 
this step here: https://youtu.be/Be9BsV7KvfA

Once you have broken up the top line and have as much air between it as 
you can get, you can start to try to join the alcohol together.

Repeat this next step cautiously. You will need to do it several times 
if you are to avoid having your thermometer bub explode. To do this you 
need a low powered bunsen safety flame. You wave the bulb of the 
thermometer through the hot air above the safety flame (not the flame 
itself). You want to heat the thermometer as slowly as you can, and you 
MUST MAKE SURE THAT THERE IS ALWAYS AIR SPACE in the thermometer. 
Normally there is a little bulb in the top of the thermometer. As soon 
as you cannot see air space, the thermometer will shatter. As soon as 
you have heated the thermometer as much as you can, let it cool. If 
there are still droplets of alcohol that are separated, try tapping them 
a bit until the column breaks up, and then repeat the heating step.

I hope this helps with a common problem in our schools.

I thank Frank de Rooy and MTA for two samples of hopelessly damaged 
thermometers, one of which I destroyed to confirm my idea about 
incompressibility of fluids and broken thermometers.

I note in passing that such types of collaboration are one reason why 
face to face pd is essential, and will never be fully replaced by Zoom.


Have fun all,

Leon Harris


(Ps: apologies for the image, I shrunk it to 62Kb so as not to more 
space than a signatures logo)



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