Measure Up with a Homemade Thermometer
Are you eager to understand how everyday items work, or interested in making useful objects and instruments yourself? Have you ever imagined you could build your own thermometer? In this activity, you will make a liquid thermometer to track how temperatures vary with location, indoors or outdoors. What will turn out to be the hottest spot in your home? What about the coolest? Your very own homemade thermometer will be able to tell you!
- Clear plastic drinking straw, preferably a 0.2 inch diameter straw.
- Metric ruler
- Fine-tipped permanent marker
- Clean narrow-necked, small bottle with lid. Travel size bottles like these available from Amazon work well. Washed empty food coloring or vanilla extract bottles work well too.
- Rubbing alcohol
- A few drops or red, blue, or green food coloring. Liquid food coloring works best.
- Paper or cloth towels
- Modeling clay. Air-dry clay works well. Ultra-light modeling clay is easy to model and clean.
- Medicine dropper or syringe
- Small bowl
- Ice cubes
- Optional: Store-bought thermometer
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- Liquid-filled thermometers consist of a liquid-filled reservoir at the end of a narrow tube. The clear drinking straw will become the narrow tube of your thermometer. Use a permanent marker to mark the straw, from the bottom up, at half-centimeter intervals. These will serve as level marks on your thermometer. Alternating between two colors is not necessary, but can make it easier to read the thermometer.
- Modeling clay will seal the bottle's neck and hold the straw in place. Mold the clay until it feels soft and elastic, then form a ball and push it flat. This round, flat piece of clay should be bigger than the neck of your bottle. Use your straw to punch a hole in the middle of this round piece of clay, just big enough to allow to straw to go through. Remove any clay clogging the straw.
- Work in a well-ventilated room, and do not leave the bottle of rubbing alcohol uncovered.
- Add rubbing alcohol to the bottle, filling it about a third to halfway up. Reseal the bottle of rubbing alcohol immediately to minimize evaporation and exposure to fumes.
- Add a couple of drops of food coloring to the alcohol, close the bottle, and shake it so the liquids mix well. This will be the liquid-filled reservoir of your thermometer.Why do you think it is important to color the liquid?
- Open the bottle and fill a medicine dropper or syringe with the colored rubbing alcohol. Carefully set the medicine dropper or syringe aside, preferably on a paper towel or cloth in case any liquid leaks out. If the level of liquid in your bottle dropped below one quarter of the way up, add a little more rubbing alcohol to the bottle so the level is between one quarter and halfway up the bottle.
- Poke the straw through the hole in your modeling clay and place the clay on the bottle's neck so the straw hangs into the bottle. Adjust the straw so the end is immersed in the liquid, but does not touch the bottom of the bottle. The majority of the straw will be sticking out from the bottle.Why do you think the straw should not touch the bottom of your bottle?
- Use the clay to seal the bottle opening and hold the straw in place. Important: Make sure the clay forms an airtight seal around the straw and over the mouth of the bottle, but do not close off the straw's opening.Can you think of a reason why it is imperative that air cannot enter the bottle?
- Drip the content of your medicine dropper or syringe—drop by drop—into the straw. What happens?If liquid rises up the straw and stays there, why does it do that? If liquid drops down into the bottle, why does that happen?
- If your seal is airtight, no air can flow out of the bottle to make room for additional liquid, so the liquid drops down the straw and forms a column. If your seal is not airtight, liquid running down the straw flows into the reservoir, pushing air out of the bottle. If this happens, remove the clay and straw and go back to step 4, making sure the clay forms an airtight seal at the bottle's neck and around the straw when executing step 6.
- The fluid level in the column should reach about midway up the visible part of the straw. If needed, use the dropper or syringe to add more alcohol (uncolored is fine this time) to the straw.
- Observe the fluid level in the straw. This level indicates room temperature. Use the permanent marker to make a small symbol indicating this fluid level on your straw.How do you think the fluid level will change if the bottle is cooled or warmed?
- To test, add cold water to a bowl. Place ice cubes in the water so the water cools to 0 degrees Celsius (32 degrees Fahrenheit).What do you think will happen if you place your thermometer in this water bath?
- Place your thermometer reservoir in the ice cold water and let the thermometer adjust to its new situation.What happens? Is it similar or different from what you expected? Could this measurement help you add a scale to your thermometer?
- Mark the level of the liquid in the narrow tube indicating 0 degrees Celsius/32 degrees Fahrenheit.
- To test what happens if the reservoir is warmed up, hold your hands around the bottle and observe the results. Be patient, as it might take time for the fluid in the reservoir to adjust to its new situation.Does the fluid level in the straw change? Does it change a lot or just a little? Why do you think this happens?
- Put some medium-hot tap water in a small bowl and place your thermometer reservoir in the water. Give the thermometer time to work. Be sure to pay close attention and take the thermometer out of the water bath if the fluid rises to a level close to the top of your straw.How does the fluid level in the straw change? Did it change more than when you heated the bottle with your hands? Why? What do you think could cause the fluid to flow over the edge of the straw? What could you change in your thermometer to prevent this from happening?
- Take your thermometer on a tour of your home or yard. Remember to give it time to respond to each new environment, keeping in mind there might not be noticeable differences if the temperature is relatively similar in different places.Are areas in direct sunlight warmer than nearby places in the shade? Is one spot the warmest? What about the coolest?
- To dispose of the rubbing alcohol in the thermometer, pour the alcohol down the sink with plenty of running water. Flush well so no alcohol lingers in the pipe.
Did you see the liquid rise as you heated the reservoir and fall as you cooled it?
As the temperature of the fluid (liquid and air) in the reservoir increases, it expands, and the only place it can go is up the narrow tube. If the temperature of the fluid decreases, it contracts, thereby allowing more fluid to collect in the reservoir. The level in the narrow tube then falls to a lower level. Once a thermometer is built, each level in the narrow tube corresponds to exactly one volume of fluid, and thus, to exactly one temperature.
Liquid-filled thermometers have been used for centuries to measure temperature. They consist of a liquid-filled reservoir at the end of a narrow tube. If the temperature of the liquid in the reservoir increases, it expands and rises into the narrow tube. When the liquid cools, it contracts, allowing the liquid in the tube to fall to a lower level. Therefore, high levels of liquid in the tube indicate a higher temperature and low levels indicate a lower temperature. When calibrating the thermometer, the specific temperatures corresponding to the levels of the liquid in the narrow tube are marked on the outside of the tube. Now you understand how a liquid thermometer works, but why does it work?
Scientists refer to the tendency of matter to change volume as a result of a temperature change as "thermal expansion and contraction." Gases (e.g. air) expand and contract drastically. Liquids expand and contract, too, but their volume change is more moderate. Even solids change volume when they are heated or chilled. For example, bridges are a little bit longer on a hot day. The change in volume of solids is too subtle, though, to work in an everyday thermometer.
Various liquids can fill the reservoir of a thermometer. How much the liquid expands for each gained degree in temperature as well as its freezing and boiling point are all factors to consider when choosing a fluid for a liquid thermometer. Whether and to what degree air is trapped in the reservoir is another factor to consider. The size of the reservoir and the relative proportion of air inside the reservoir largely determines how fast the thermometer adjusts to a change in temperature.
A narrower tube results in a more accurate thermometer because the same expansion or contraction of fluid will cause a rise or fall over a longer distance. As a drawback, the narrow straw might decrease the maximum temperature that the thermometer can reach and can make it harder to drop the liquid in the tube.
Ask an Expert
For Further Exploration
- Make a scale on your thermometer (also called "calibrating" the thermometer) with the help of a store-bought thermometer. Could you use the fact that in this temperature range, alcohol and air expand by equal amounts for each gained degree in temperature to make calibration easier?
- You created this thermometer with a particular straw width. Try to build an identical thermometer, now with a narrower or wider straw. How do you expect the measurements to be different? Will this new thermometer be more or less accurate compared with your first thermometer?
- You used rubbing alcohol for this thermometer. Try to make an identical thermometer, now using water as a fluid. Unlike water, which freezes at 0 degrees C and boils at 100 degrees C, rubbing alcohol freezes at -89 degrees C and boils at 82.5 degrees C. Which fluid would you choose to make a thermometer to measure the temperature in your freezer? What about in the kitchen to cook something on the stovetop?