Temperature and its Measurement · Lesson 2 of 5
Measuring Body Temperature
“Use a digital clinical thermometer thoughtfully and make sense of body readings and temperature scales.”
• Describe how to use and clean a digital clinical thermometer safely. • Interpret 37.0 °C as an average reference rather than every person’s exact reading. • Compare contact and non-contact thermometers and recognise the limits of older mercury instruments. • Read and write Celsius, Fahrenheit and kelvin correctly, and convert Celsius to kelvin.
From a warm forehead to a reading
A clinical thermometer is designed to measure body temperature. A digital model uses a battery, a heat sensor, and a display that shows a number. When its tip is placed correctly in contact with the body, it gives a reading more useful than guessing from touch. Some infrared thermometers can take a reading without touching a person, which helps reduce the chance of spreading disease.
Older clinical thermometers sometimes contained mercury. Mercury is toxic and troublesome to dispose of if glass breaks; a digital thermometer avoids that particular hazard and usually makes the number easier to read. This is a reason to handle instruments carefully and to follow their instructions, not to open or dismantle them.
Make a careful and hygienic measurement
With consent and adult guidance where needed, first read the thermometer’s manual. Wash your hands and clean the tip with soap and water, keeping the display and other digital parts out of water. Reset the device as directed. For a thermometer meant for oral use, place its tip under the tongue, close the mouth, and wait for the beep or light before removing it to read the display. Clean and dry the tip after use. Never hold it by the sensing tip while taking a reading.
| Person | Temperature (°C) | Context or time |
|---|---|---|
| Self | Record your reading | Record when measured |
| Friend 1 | Record with permission | Record when measured |
| Friend 2 | Record with permission | Record when measured |
The chapter suggests recording up to ten people’s readings and comparing them. A longer personal record taken at different times and on different days can show variation. Record the time and what you had been doing; that helps you reason about a change rather than assuming every difference means illness.
Problem
One healthy student reads 36.8 °C and another reads 37.1 °C. Must one of the readings be wrong?
- 1.The widely used 37.0 °C reference is an average of many healthy people, not a fixed number for every person.
- 2.Readings can differ with age, time of day and activity, as well as between people.
- 3.Check that both measurements followed the thermometer instructions; a small difference alone does not prove an error or illness.
Understand the reference value
A healthy human body temperature is often taken as 37.0 °C, but individual healthy readings can be slightly higher or lower. Age, time of day, and activity can affect the reading. The chapter states that human body temperature does not normally fall below about 35 °C or rise above about 42 °C; it does not ask students to diagnose a person from one number.
For small children or older people, a suitable digital thermometer may be used in the armpit. The chapter notes that this reading is about 0.5 °C to 1 °C below the actual body temperature. Therefore, an armpit number must not be compared thoughtlessly with an oral reading. Follow the device instructions for where and how it should be placed.
A single reading should be considered with the measuring site, instrument instructions, and the person’s situation. A pulse rate alone is also an unreliable sign of fever because other circumstances can change it.
Three ways to label temperature
The Celsius scale uses degree Celsius, written °C. Many clinical thermometers use this scale. The Fahrenheit scale uses degree Fahrenheit, written °F; the chapter gives 37.0 °C and 98.6 °F as two ways to state the same reference temperature. Scientific work also uses the Kelvin scale, whose unit is kelvin, written K. Kelvin is the SI unit of temperature.
| Scale | Unit and symbol | Useful chapter connection |
|---|---|---|
| Celsius | degree Celsius (°C) | 37.0 °C reference body temperature |
| Fahrenheit | degree Fahrenheit (°F) | 98.6 °F equals 37.0 °C |
| Kelvin | kelvin (K) | SI unit; no degree sign |
Write a space between the number and unit, such as 37 °C or 300 K. Use the degree sign with °C and °F, never with K. The scale names begin with capital letters; the word kelvin, when written out as a unit, begins with a lower-case letter. Different scales give different numbers to the same physical temperature, so “101 degrees” is incomplete without the scale.
Celsius and kelvin readings are linked by adding 273.15 to the Celsius number. This changes how the same temperature is written; it does not warm the object. Kelvin values in this chapter are measured without a degree sign.
Problem
Write 37.0 °C in kelvin.
- 1.Identify the Celsius number: 37.0.
- 2.Add 273.15: 37.0 + 273.15 = 310.15.
- 3.Write the result as 310.15 K, with no degree sign.
Problem
One display says 37.0 °C and another says 98.6 °F. Are they necessarily reporting different temperatures?
- 1.First check the unit symbol on each display: one uses Celsius and the other Fahrenheit.
- 2.The chapter gives 37.0 °C = 98.6 °F.
- 3.Thus these two readings can describe the same temperature despite their different numbers.
A temperature close to −273.15 °C corresponds to 0 K and is called absolute zero, the lowest temperature according to the chapter’s discussion. The core of the Sun, by contrast, can reach about 15 million degrees Celsius. These extremes show why the scale and the size of a number must always be read together.
Quiz
A healthy person reads 36.9 °C. Which interpretation best fits the chapter?
Which temperature is equivalent to 37.0 °C in the chapter?
How should the unit kelvin be abbreviated?
Why is cleaning a shared clinical thermometer tip before and after use important?
What is 0 °C in kelvin according to the chapter’s conversion?
Why might an armpit reading be lower than a reading used as the actual body temperature?
Practice Problems
- Write the steps for taking and recording an oral reading with a suitable digital thermometer, including cleaning precautions.
- Explain why the 37.0 °C reference does not mean every healthy person has exactly that reading.
- A thermometer reads 25 °C. Convert the number to kelvin and write its unit correctly.
- Compare 37.0 °C and 98.6 °F. Why are the numbers different if the temperature is the same?
- Explain why “my temperature is 101 degrees” leaves out essential information.
- Describe one benefit of a non-contact infrared thermometer and one reason older mercury thermometers are being replaced.
Key Takeaways
• A digital clinical thermometer measures body temperature when used according to its instructions. • 37.0 °C is an average reference, so healthy body readings can vary. • 37.0 °C and 98.6 °F describe the same reference temperature. • Kelvin is the SI unit; K has no degree sign, and kelvin = Celsius + 273.15. • Record the unit, measurement site and context when interpreting a reading.