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Determine the accuracy of various thermometers

Determine the accuracy of various thermometers

Introduction: (Initial Observation)

Thermometer has many industrial and scientific applications. Inaccuracy of a thermometer can fail a product or cause wrong results in a scientific research. At home thermometers are being used in refrigerators and ovens. The safety of our food depends on being kept in cold temperature and being cooked in a high temperature. To find out how cold or how hot these devices are, we rely on thermometers. Inaccurate results can put our safety in risk.

For our own safety it is important to test these thermometers at least once a year or any time it is has been dropped or subjected to abuse.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Information Gathering:

Find out about different thermometers and how they function. Read books, magazines or ask professionals who might know in order to find out what types of thermometers are more accurate and how the accuracy of a thermometer can be tested. Keep track of where you got your information from.

Following are samples of information that you may find.

One of the critical factors in controlling bacteria in food is controlling temperature. Pathogenic microorganisms grow very slowly at temperatures below 40 °F, multiply rapidly between 40 and 140 °F, and are destroyed at temperatures above 140 °F. For safety, foods must be held at proper cold temperatures in refrigerators or freezers and they must be cooked thoroughly. But how would a consumer know if the refrigerator was cold enough, or if the oven was heating at the proper temperature?

Appliance thermometers are specially designed to measure the temperature of the air in either the refrigerator/freezer or the oven. Some refrigerator thermometers have long metal probes and are similar in appearance to food thermometers. Other refrigerator thermometers, and most oven thermometers, are designed to hang from a wire rack or sit on a shelf.

Whether they measure the temperature in the oven or refrigerator/freezer, most appliance thermometers are either liquid-filled or bimetallic-coil thermometers:

  • Liquid-filled Thermometers, also called “spirit-filled” or “liquid in glass” thermometers, are the oldest types of thermometers used in home kitchens. As the temperature increases, the colored liquid (usually an alcohol solution) inside the thermometer expands and rises to indicate the temperature on a scale.
  • Bimetallic-coil Thermometers contain a coil made of two different metals with different rates of expansion that are bonded together. The bimetal element is coiled, fixed at one end, and attached to a pointer stem at the other end. As the temperature increases, the pointer will be rotated by the coiled bimetal element to indicate the temperature

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to find out the accuracy of thermometers. We need to come up with a test method and a way to describe inaccuracy.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

The independent variable (also known as manipulated variable) is the type or model of thermometers that we test.

The dependent variable (also known as responding variable) is the accuracy of each thermometer that we will calculate.

Controlled variables are room temperature and light.

Constants are the experiment method, instruments and procedures.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

I think the accuracy of thermometers depend on it’s usage and technology. For example thermometers used in laboratory and research are more accurate than thermometers used in ovens.

Another example of hypothesis:
I think the accuracy of thermometers depends on their range. Those who have a short range, for example 32°F to 100°F are more accurate than those in range of 0°F to 600°F.

Yet another hypothesis:
Cheep thermometers have a lower accuracy than expensive ones.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”


Testing Thermometers For Accuracy

In order to test a thermometer, we need a reliable reference. such reference can be a pre tested high accuracy thermometers or can be a known temperature reference such as freezing point or boiling point of water.

We will use freezing point and boiling point of water as our reference.

The thermometers that we will use to test are a kitchen thermometer and a room thermometer.

Boiling Water Test

The most common way to test a thermometer is to place it in boiling water. An accurate thermometer will read about 212*F in boiling water at sea level under normal atmospheric conditions.

To test your thermometer, bring a pot of water to a vigorous boil. Hold the thermometer stem or probe in the water, making sure not to touch the sides or bottom of the pot, and take your reading.

Remember that there are several factors that affect the boiling point of water:

  • As atmospheric pressure decreases, the boiling point decreases. Atmospheric pressure will vary depending on your altitude and local weather conditions.
  • Hard water boils at a temperature 1-2*F higher than soft water, due to dissolved mineral salts.
  • Using a tall, narrow pot will result in a boiling point about 1*F higher than a short, wide pot.

If you live at high altitude, you’ll need to take that into account when testing your thermometer. The table below lists the approximate boiling point for a number of different altitudes. As a general rule, the boiling point decreases by 1*F for about every 500 feet in altitude. Note that the actual boiling point may be higher or lower depending upon atmospheric pressure in your area on any given day.

I doubt that many of you live at elevations above 7,500 feet, but I’ve included those numbers anyway for your interest.

Altitude Boiling Point (F/C)
Sea Level 212/100
2,000 ft. 208/198
5,000 ft. 203/95
7,500 ft. 198/92
10,000 ft. 194/90
15,000 ft. 185/85
30,000 ft. 158/70
  • To determine your barometric pressure, visit The Weather Channel.
    • Enter your city name or ZIP code on the home page to look up your current weather conditions.
    • Note your barometric pressure.
    • You can also get your current pressure from The Weather Channel on television.
  • To determine your elevation in the United States, try using the USGS Geographic Names Information System.
    • Enter your city name in the “Feature Name” field.
    • Select your state from the “State or Territory” list.
    • Select “populated place” from the “Feature Type” list.
    • Click “Send Query”.
    • If your city is found, click on it to view detailed information.
    • Note your elevation.

Ice Bath Test

If your thermometer measures temperatures of 32*F or lower, you can test it using an ice bath. Fill a Styrofoam cup with crushed ice, then add cold water. Insert the thermometer stem or probe into the ice bath and stir continuously. An accurate thermometer will read 32*F.

One advantage of using this method is that you don’t have to take atmospheric pressure into account. An accurate thermometer will read 32*F in an ice bath at any altitude or atmospheric pressure.

Materials and Equipment:

(Adult help and supervision is required.)

The final list of material depends on the material and equipment that are available to you and changes that you may make to the experiment procedures.

Following is a sample list of material:

  1. Glass alcohol thermometer (Part#GAT20110 MiniScience.com)
  2. Large dial thermometer (Part#TDIAL220FL MiniScience.com)
  3. Wide range food thermometer (from kitchen ware department of big stores)
  4. Ice
  5. Cups
  6. Aluminum pot for boiling water

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.

Repeat the ice bath and boiling water experiment a few times (for example 3 times) with each thermometer and record the results in a table like this.

Ice Bath test 1 Ice Bath test 2 Ice Bath test 3 Boiling test 1 Boiling test 2 Boiling test 3
Thermometer 1  35°F  36°F  36°F  202°F  204°F  203°F
Thermometer 2 
Thermometer 3

Then make another table showing how much off is each number from what it should be.

For example boiling point should be 212°F, if it is 202°F then it is +10°F off. in other words we can add 10 to take in account it’s inaccuracy. When the numbers are negative it means that we should deduct that number to get to an accurate number.


Ice Bath test 1 Ice Bath test 2 Ice Bath test 3 Boiling test 1 Boiling test 2 Boiling test 3 Maximum inaccuracy*
Thermometer 1  -3  -4  -4  +10  +8  +9  +14
Thermometer 2
Thermometer 3

* The maximum inaccuracy is the difference between the smallest inaccuracy and the largest inaccuracy. For thermometer one these two numbers are -4 and +10.

Make a graph:

Make a bar graph to show your experiment results. Use one vertical bar for each thermometer that you are testing. The height of each bar shows the maximum inaccuracy range for that specific thermometer. Write that number above each bar.


Take the boiling test results of each thermometer and subtract it from 212 to find out how much off it is:

Thermometer 1 is off by: 212-203 = +9 in Boiling test

repeat the same for all thermometers.

Take the Ice Bath test results of each thermometer and subtract it from 32 to find out how much off it is:
Thermometer 1 is off by: 32-35 = -3 in Ice Bath test

repeat the same for all thermometers.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Medical and laboratory grade thermometers have a higher accuracy than industrial thermometers. Even high-quality, industrial-grade thermometers are only accurate to +/-1% of their scale. This means that at a “normal” 212*F boiling point, these thermometers may read up to 2*F above or below the actual temperature and still be within manufacturing specifications.

If an industrial thermometer like those used in ovens and refrigerators is off by a few degrees, make sure you take those few degrees into account when reading your thermometer.

If your thermometer is off by more than a few degrees and it has an adjustment nut on the back of the dial, you can try to recalibrate it. Use a wrench to turn the nut, adjusting the dial pointer. Repeat the process of testing and recalibrating until you get an accurate result.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.