Being able to make a thermometer is a handy thing for a Scout to know. You can make a reasonably accurate room thermometer out of some rough and ready materials. In fact, if you’re feeling creative, you can make some downright crazy thermometers. So what if they don’t hold up to scientific scrutiny. What you lose in accuracy you gain in uniqueness. Here’s a simple one to get you started.
A very early history
You may be surprised to know that early thermometers did not have any scales for reading temperatures. Officially they were not really thermometers at all because they didn’t have the metering element. Instead they were known as thermoscopes.
When markings were added it was the human body temperature and the boiling and freezing points of water that made it first. However, it was not uncommon to see arbitrary values like hot, cold, warm etc.
Numerical scales did become more commonplace but every thermometer manufacturer used their own. If the thermometer was for personal use, say for observing changes in local temperature, it didn’t matter much, but the lack of standardisation was a big headache for scientists that wanted to make comparisons.
Water – Not an ideal thermometer liquid for multiple reasons, but its free, safe and mostly works. We’re not trying to make a scientific instrument after all.
Jar or bottle – I’ve used a 400ml glass jar. But any container that can be made watertight is suitable – bottles, food jars, even soft drink cans are all viable.
Drinking straws – Ideally transparent but coloured will suffice. Mine were 6mm in diameter and 20cm long. Alternatively clear narrow tubing will work and save you some build time.
Food colouring – any colour you like.
Blu-tack – Used as a sealant. Plasticine or epoxy resin are alternatives.
Permanent marker – Fine tipped black.
Adhesive – Waterproof, with a setting time of a few minutes.
Vegetable oil (optional) – Any kind will do.
NOTE: Before commencing this activity fill the jar with water, add a few drops of food colouring, then place in the fridge for a several hours or overnight. Chances are it will end up very close to the desired 5oC minimum temperature. This means the lowest height of the thermometer will be automatically set and calibration will be partly complete.
The thermometer neck
The thermometer neck is the tube in which liquid rises and lowers in relation to the surrounding temperature.
The neck is made by joining several plastic straws together. The number required will vary but 3 or 4 is a good ballpark. Bendy sections can make the neck unstable so you might like to cut those off.
Join the straws by flaring (widening) the end of one straw so it is possible to slide it over another. Do this to a depth of about 1cm. I found gently rotating the head of a Philips screwdriver into the straw worked quite well, as did rotating and wiggling the end of a teaspoon.
It is extremely important to make the joint watertight. This can be achieved by putting a little adhesive over the unflared end and joining the straws by sliding and rotating them together. Avoid fast drying superglue because it will set almost on contact and certainly before the required depth has been reached (yep, I tried it). However, superglue can be used to create a bead that overlaps a dry joint and make it watertight.
Shortcuts that failed:
1) Cutting into the end of a straw so it will crush fit into another.
2) Sealing the joint with sticky tape.
3) Combining points 1 & 2. (I persevered with this for way too long!)
Dismantling and rebuilding a leaking thermometer takes way more time than gluing it properly in the first instance.
Test the seals by placing a finger over one end of your super-straw and blow through the other. Listen for hissing or feel over the joints for signs of air escaping. Dabbing a bit of spit over the joint will create bubbles when the straw is under pressure.
The thermometer bulb
The thermometer bulb contains a supply of liquid that takes on the temperature of the surrounding air. As the temperature increases the liquid expands and is pushed up the neck of the thermometer.
Bear in mind the larger the bulb the more liquid will expand up the neck. So you may end up with a tall thermometer or will need wider straws to accommodate it.
On a similar note, as a large bulb holds more liquid it will take longer for it react to changes in temperature. Don’t expect a response anything like a small alcohol thermometer.
Puncture or drill a hole into the lid of the jar. A Philips screwdriver makes a perfect size hole. Pass the straw a few centimetres through the hole and then pack around the straw on both sides with blu-tack to make it water tight. On the top of the lid make the packing fairly high (at least 2-3cm) to prevent straw movement from breaking the seal.
I used fast setting epoxy resin, which is ideal. It will maintain a seal even if the straw gets wiggled about a lot, which tends to happen despite taking care.
Add a few drops of food colouring to the bulb and fill it with water until it is overflowing. Carefully screw on the lid. Hopefully the bulb contains no air and is completely watertight.
Your thermometer is now assembled all that remains is to set up a temperature scale.
The temperature scale
To ensure the thermometer can measure the required range of temperatures we need to determine it’s low and high points. All that then remains is to create a scale in between those points.
Set low-point (min temperature)
If the water in the bulb has been in the fridge for several hours it’s likely to be close to 5oC. This is the lowest temperature that can be measured with water as a thermometer liquid. So whatever the level in the neck it certainly won’t go any lower.
If the water is extremely low in the neck you may like to add a few drops down the straw to raise it a couple of centimetres. Do this very carefully – it’s very easy to add too much.
Now, with a permanent marker draw a line at the water level in the straw. This is our minimum or 5oC line.
Set high-point (max temperature)
Now technically it’s possible to forgo this step, but I know you’re keen to do the job properly. After all, who want’s a thermometer that that’s sized for an impossible 500oC?
As our thermometer is designed for air temperature there’s no benefit in it measuring beyond 50oC. With the aid of a reference thermometer we can determine when that has been reached. However, if you do not have a second thermometer your only option is to raise the maximum temperature to a fixed value. i.e. boiling point, which we know to be 100oC or in practical terms 95oC.
Place your thermometer into a saucepan and fill it with water so the bulb is almost submerged. Slowly raise the temperature to 50oC and hold it there for 30 minutes or so. It will take time for the bulb temperature to equal the pan temperature, but a good indicator is for the water to remain stationary in the neck for a several minutes.
Now, with a permanent marker draw a line at the water level in the straw. This is the maximum or 50oC line.
It is now safe to trim the the neck of the thermometer to just above the maximum temperature line.
Creating a scale
You can mark temperature intervals directly onto the neck of the thermometer with a permanent marker. Although it will be much better if you use a piece of card as you can add more detail. Attach it behind the straw with some adhesive or fasten it to the lid.
Your thermometer should have a range of 45oC. So what’s the process for marking the temperatures between 5oC and 50oC? I would love to say simply divide up the distance between the low and high points into 45 equal segments and consider them as 1oC increments – but water makes things a little complicated.
The problem with water
Unlike mercury and alcohol, two commonly used thermometer liquids, water does not expand in a linear fashion. It’s maximum density (i.e lowest level in neck) is at about 5oC. Below 4oC, as it starts turning to ice, it actually starts expanding and goes higher up the neck.
The image below shows how water expanded up a straw at 5oC intervals. If it was linear each 5oC interval would have been equally spaced.
One way to set a scale is to calibrate the thermometer at fixed intervals as shown in the above image. It’s a bit tedious, but can be easily done whilst establishing the high-point. It’s a long way from perfect, but does get you in the ballpark with the Celsius scale.
Invent your own scale
Who needs Celsius anyway? If you’re not interested in comparing temperatures with anyone else just invent your own scale – just like the early thermometer manufacturers. I created degrees Jeremy (oJ). I did this by setting 1oJ to be the low point then added a mark every 1cm up the neck. Each 1cm is equal to 1oJ. Easily good enough for comparing temperatures from one day to the next
The next scale uses the letters of the alphabet. Well why not? I made it from a kitchen roll tube cut in half, so it also has a nice curved shape to wrap around the thermometer neck.
Even simpler forgo a numeric scale all together. Just describe the temperature – cold, cool, warm, hot etc. Record any highs and lows on there too. As long as it makes sense to you – that’s all that matters. It’s your thermometer after all.
Over time the water in the thermometer will slowly evaporate through the straw and it will become inaccurate. You can prevent this by placing a drop of vegetable oil down the straw. It will float on top of the water and stop it from evaporating.
Finally, it’s worth mentioning that the thermometer readings may vary from day to day due to something called atmospheric pressure. It’s possible to make another device called a barometer that measures this. The way to combat atmospheric pressure is to remove the air from the neck and seal the top of it. Not really possible with a plastic straw. If you suck the air out of a straw you’ll notice the thin walls collapse. It’s the atmospheric pressure making that happen.
How did you get on?
Remember, your thermometer does not have to be vertical; you could make it go sideways or in loops. If you use a very big container for a bulb you could make a thermometer as high as your ceiling.
Let me know how you got on. Or send me a photo.