Temperature Measurement – Instrument and Scale

In this post we will learn about temperature and instrument used for measuring it.

When we put our hand in hot water, we feel hot. When we touch the ice, we feel cold. But in both situations, we cannot tell how hot or how cold, water or ice is. Further, it is not advisable to use your hand as an instrument for finding the same. Thus, to measure the degree of hotness or coldness we carry out temperature measurement.

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For example,

When we put our hand in hot water, we feel hot because the temperature of hot water is more than our body temperature. Similarly, if we touch the ice, we feel cold because the temperature of ice is less than that of our hand. It is the difference in temperature that makes the feeling of hotness or coldness.

Definition of temperature for kids –

The temperature is the measure of the degree of hotness or coldness of a body or environment. The standard international unit or SI unit of temperature is Kelvin (K).

THERMOMETER

For the measuring temperature, we use an instrument/device called a thermometer. There are various types of thermometers. However, the glass thermometer is one of the most common instrument used for measuring temperature.

This thermometer consists of a very fine glass tube having a very small bore called capillary tube. At one end of the tube is provided a very thin glass bulb. The bulb contains mercury. The other end of the capillary tube is sealed, after removing the air from it. To protect the capillary tube, we enclose it in a thick glass tube called a stem. The stem bears graduated marking called degrees. The shape of the stem is triangular so that markings on the body get magnified when we try to see them.

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Advantages of using mercury in a thermometer –

• It is opaque and shining. Hence, it is easily visible through the glass.
• It does not stick or wet the glass.
• Mercury has a low freezing point (-39^oC), and a high boiling point (357^oC). Hence, we can use it over a wide range of temperature.
• It is a good conductor of heat, so it quickly absorbs the temperature of the object.
• Mercury has a high coefficient of cubical thermal expansion. Hence, it helps in the measurement of even a slight change in temperature of a substance or a body.

THERMOMETRIC SCALES

To avoid wrong temperature measurement due to variation in marking on a different thermometer, we use standard thermometric scales. The most popular scales used are – Celsius, Fahrenheit, and Kelvin. For carrying out markings on a thermometer, we usually take the melting point of ice as a lower fixed point and the boiling point of water as a higher fixed point, respectively.

Video courtesy: Region 10 ESC

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Celsius or Centigrade Scale

The temperature of a given body can be less than zero degree Celsius. To express such temperatures we place a minus sign before the number. For example, -10oC or -100oC. Conversely, the temperature of bodies like flame or boiling oil can be above 100oC.

The Celsius scale is a commonly used scale for the measurement of the temperature. It is named after Swedish scientist Anders Celsius. The lowest point on this scale is zero and we express it as 0^oC (zero degree Celsius). It corresponds to the melting point of the pure ice.  Likewise, the highest point on this scale is hundred and we express it as 100^oC (100 degree Celsius). It corresponds to the boiling point of pure water. For carrying out markings, we divide the length between these two points in 100 parts or divisions. Therefore, we also call this scale as the centigrade scale which means 100 degrees.

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Fahrenheit Scale

The Fahrenheit scale is named after the German scientist G.D. Fahrenheit. We mostly use this scale for temperature measurement in clinical application. The lowest point on this scale is 32^oF (0^oC), which corresponds to the melting point of the pure ice. Likewise, the highest point on this scale is 212^oF (100^oC), which corresponds to the boiling point of pure water. This scale distributes the two reference temperatures into 180 degrees.

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Kelvin or Standard International (SI) Scale

The standard international (SI) scale used for the measurement of temperature is the Kelvin scale. The Kelvin scale derives its name from Lord Kelvin, a famous Irish mathematician and physicist.

The temperature of bodies can be below zero degree Celsius. However, the question arises what is the last limit of temperature. Lord Kelvin theoretically calculated that the lowest possible temperature is 273.15 degrees below the zero degree Celsius. He named this last limit of temperature, that is -273.15^oC as absolute zero. At absolute zero motion of all atomic and subatomic particles – electrons, protons, and neutrons within a body cease. Thus, it is not possible to achieve the temperature of absolute zero even with the best research tools available to modern science.

A unit on the Kelvin scale is the same size as a Celsius degree and is called Kelvin abbreviated as K. Therefore, to obtain Kelvin as a unit of temperature, we have to simply add 273 to the given value of temperature in degree Celsius. On this scale -273^oC = 0 K, similarly, 0^oC = 273 K and 100^oC = 373 K. It is important to note that we do not express Kelvin in degrees.

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OTHER INSTRUMENTS USED FOR MEASURING TEMPERATURE

Thermometers or the instrument used for the measuring temperature are of various types:

Laboratory Thermometers

This instrument is a type of glass thermometer with marking on its stem between  -10^oC to 110^oC.  The distance between two consecutive markings represents one degree Celsius. We use it for measurement of temperature other than that of the human body.

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Clinical Thermometer

The temperature of a healthy person is 37-degree Celsius. If the temperature is above 37-degree Celsius than the person is having a fever. However, if the temperature is less than 37-degree Celsius, the person is weak and his body is not generating enough heat energy. We use this instrument to measure the temperature of the human body. It measures temperature in a short range of 35^oC to 42^oC. This is because the temperature of human-body does not vary beyond these limits. Our normal body temperature is 37^oC (or 98.6^oF). It has a triangular stem in which runs a very fine capillary tube. The tube is provided with a kink near the bulb. The kink allows the mercury to push its way in the capillary tube but does not allow it to flow back unless the thermometer is jerked.

The clinical thermometer should never be washed with hot water or placed in the sun as it is likely to break.

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Six’s Thermometer

We use this instrument for the measurement of maximum and minimum temperature attained during the day. It derives its name from its inventor James Six, in 1972.

It has a U-shaped tube with two separate temperature readings, one for the maximum temperature and the other for the minimum. The bulbs are fixed at the top of each arm. The bulb at the top of the minimum reading scale contains alcohol. The other bulb contains a vacuum or low-pressure alcohol vapors. Mercury present in U-tube separates alcohol and vacuum from each other.

It is the alcohol, which measures the temperature and the mercury indicates temperature reading on both the scales. When the temperature rises, the alcohol in the left bulb expands and pushes the mercury down, with the result that the mercury rises into the other arm and we can read the corresponding high temperature on the scale below the right bulb.

Similarly, when the temperature falls, the alcohol in left bulb contracts and to fill the gap, the mercury below it moves towards the left bulb and we can read the corresponding low temperature on the scale below the left bulb.

As the mercury moves towards the left bulb or the right bulb, it pushes two small steel markers inside the tube. These markers record the farthest point reached by the mercury in each arm of the tube. The markers can be reset for reuse by using a small magnet which can drag the markers along the tube so that they can again rest on the surface of the mercury.

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Thermocouples

A thermocouple is an instrument which uses electricity for the measurement of temperature. It works on the principal, called Seebeck Effect. Seebeck effect states that – when a temperature difference is maintained at two junctions formed from two dissimilar electricity conducting metallic wires then electric current flows through the wires. It further states that voltage or electrical potential difference across the two junctions is directly proportional to the temperature difference. Thus, if we know the temperature at one junction than we can find the temperature at another junction by measuring the electric voltage across the circuit.

Depending on the types of materials used for the construction of thermocouples, they can be used for measuring a wide range of temperature from -270^oC to 1300^oC.

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Thermistors

A thermistor is an instrument which uses electricity for the measurement of temperature. It is a resistor, whose resistance changes with variation in temperature. The flow of the same value of electric current, change in resistance will result in a proportional change in voltage or electrical potential. Thus, we can carry out temperature measurement by measuring the change in electric potential across the circuit.

Thermistors are of two types – Negative temperature coefficient (NTC) and Positive temperature coefficient (PTC). NTC thermistors are those whose resistance decreases with the increase in temperature, whereas PTC thermistors are those whose resistance increases with the increase in temperature.

A thermistor is generally of ceramic or polymer material, therefore operating temperature range of a thermistor is −55 °C to +150 °C. However, some glass-body thermistors have a maximal operating temperature of +300 °C.

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Resistance Temperature Detectors (RTDs)

Resistance temperature detectors (RTDs) or resistance thermometers are type of thermistors.  Thermistors differ from RTD in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges.

RTDs are sensors used to measure temperature. Many RTD elements consist of a length of fine wire wrapped around a ceramic or glass core. The RTD wire is a pure material, typically platinum, nickel, or copper. The material has an accurate resistance/temperature relationship which is used to indicate temperature. As RTD elements are fragile, they are often housed in protective probes.

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Pyrometer or Infrared Thermometers

A pyrometer is a type of remote-sensing thermometer or instrument used to measure the temperature of a surface from a distance. It determines the temperature of a surface from the amount of the thermal radiation it emits, a process known as pyrometry. The picture depicts a sailor checking the temperature of a ventilation system using a pyrometer. Image courtesy Wikipedia.

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