Measuring temperature with calibrated instruments
Compare temperature-sensitive properties, calibrate a scale with fixed points, and choose between a liquid-in-glass thermometer and a thermocouple.
An original 0652 lesson for the temperature-measurement subsection. Written apparatus descriptions do not replace supervised practical work.
Before you begin
- Read a scale with labelled units
- Describe expansion using particles
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is calibration?
What you will learn
- Explain how a changing physical property can indicate temperature
- Describe fixed points and liquid-in-glass thermometer action
- Use sensitivity, range and linearity to compare thermometers
- Explain when a thermocouple is useful
A property that changes with temperature
A thermometer uses a physical property that changes predictably with temperature. Examples include the length or volume of a liquid column, electrical resistance, or the e.m.f. produced by a thermocouple. Calibration connects property readings to known temperatures.
Fixed points provide repeatable references. For the Celsius scale, the melting point of pure ice and the boiling point of pure water at standard atmospheric pressure are the familiar reference points.
Liquid-in-glass thermometers
Heating makes the liquid expand more than the glass reservoir and tube, so the liquid column rises. Cooling makes it fall. A narrow uniform bore makes small temperature changes easier to read and keeps the scale approximately linear across its working range.
Sensitivity describes how much the indicated property changes for a small temperature change. Range is the lowest to highest temperature that can be measured. Linearity describes whether equal temperature changes produce equal scale intervals.
Thermocouples
A thermocouple joins two different metals. A temperature difference between the junctions produces a small e.m.f. This makes a thermocouple useful for high or rapidly changing temperatures, where a liquid-in-glass thermometer may be unsuitable. It still needs calibration and a suitable measuring circuit.
Sources for this lesson
Pause and explain
Which term means the temperature span an instrument can measure?
Worked example
A liquid thermometer has a column length of 4.0 cm at 0 °C and 24.0 cm at 100 °C. Assuming a linear scale, what temperature corresponds to 13.0 cm?
Show the worked solution
- The 100 °C interval spans 20.0 cm.
- 13.0 cm is 9.0 cm above the 0 °C reference.
- The fraction of the interval is 9.0 / 20.0 = 0.45.
Answer 45 °C. Under the stated linear assumption, the column is 45% of the way from the 0 °C fixed point to the 100 °C fixed point.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A thermometer's liquid rises from 4.0 cm at 0 °C to 24.0 cm at 100 °C. If the scale is linear, estimate the temperature when the column is 10.0 cm.
Give me a hint
Compare the rise from the lower fixed point with the full column-length interval.
Compare my reasoning
- The full 0–100 °C interval is 20.0 cm.
- The column has risen 6.0 cm above the 0 °C reference.
- 6.0 / 20.0 = 0.30, so the temperature is 30% of 100 °C.
30 °C. The column moved 6.0 cm out of a 20.0 cm calibrated interval, so it represents 0.30 × 100 °C.
Look for these in your work
- Uses the 0 °C reference
- Finds the fraction of the full interval
- States the result in °C
Thermometer A covers 0–100 °C and moves 2 mm per degree. Thermometer B covers 0–200 °C and moves 1 mm per degree. Compare their sensitivity and range.
Give me a hint
Sensitivity is scale movement per degree; range is the lowest-to-highest interval.
Compare my reasoning
- Compare the movement for a 1 °C change.
- Compare the endpoints of each measuring interval.
- State the trade-off without claiming either instrument is universally better.
A is more sensitive because it moves 2 mm per degree rather than 1 mm per degree. B has the wider range, 0–200 °C rather than 0–100 °C. Sensitivity and range are different properties.
Look for these in your work
- Identifies A as more sensitive
- Identifies B as having the wider range
- Explains the two terms separately
A furnace temperature changes quickly and may exceed the safe range of a glass thermometer. Choose an instrument and explain its operating idea and one limitation.
Give me a hint
Compare liquid expansion with the e.m.f. produced by two different metals.
Compare my reasoning
- Select an instrument suited to high and rapidly changing temperatures.
- Describe the temperature-related property it uses.
- Include calibration or measurement as a limitation.
A thermocouple is suitable because a temperature difference between junctions of two different metals produces an e.m.f. that can be read by a meter. The e.m.f. must be calibrated to temperature, and a suitable meter and junction arrangement are needed.
Look for these in your work
- Chooses a thermocouple
- Explains the e.m.f. at the junctions
- Mentions calibration or a measurement limitation
Common mistakes
- Do not assume a thermometer is linear unless the scale or data support it.
- Sensitivity and range describe different features.
- A thermocouple measures a temperature-related e.m.f.; it does not directly display temperature without calibration and a meter.
What can you explain now?
Which thermometer is usually more suitable for a rapidly changing furnace temperature: liquid-in-glass or a thermocouple?
Compare with the explanation
A thermocouple, because it responds quickly and can measure high temperatures.
A thermocouple uses the e.m.f. from two different metals at different temperatures and can be read with an appropriate calibrated meter.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall one fixed point, one temperature-sensitive property and the meaning of sensitivity, range and linearity.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.