Question

Working of laboratory thermometer

Answer 1

Laboratory thermometers are used to measure temperatures or temperature changes with a high degree of precision. They are made of metal or glass and strengthened through thermal tempering or annealing.



The way a laboratory thermometer works depends upon its type. They are generally a liquid-in-glass device, a bimetallic strip, an electronic thermistor thermometer, or infrared (IR) device.

Answer 2

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Structure:

The thermometer is made relatively small so that it is portable and cheap.
The liquid is contained in a thin-walled glass bulb. The bulb is made relatively larger than its bore to contain more of the liquid, so as to improve sensitivity.
The narrow bore of the capillary tube is uniform. The round glass stem around the capillary tube is made thick. It acts as a magnifying glass.

Sensitivity:

Small expansion of the liquid in the liquid bulb will cause a big change in the length of the liquid thread in the capillary tube as it is made narrow. The narrower the bore, the higher the sensitivity.

Range:

The range is limited by the freezing and boiling points of liquid.
For mercury thermometer: -39 to 357°C
For alcohol-in-glass thermometer: -115 to 78°C
In colder countries, most of its liquid-in-glass thermometers use alcohol and not mercury.
The range can be increased by lengthening the bore.
Range is the converse of sensitivity, i.e., the longer the range, the lower is its sensitivity. Factors that increase range would at the same time reduce its sensitivity.

Linearity:

Mercury expands quite uniformly over a good range of temperatures.
Alcohol expands non-linearly over different range of temperatures.

Responsiveness:

As the liquid is contained in a thin-walled, small glass bulb, it will be more responsive (faster response) to heat.
Mercury reacts quickly to the temperature changes whereas alcohol reacts slowly.
It takes several tens of seconds to record one reading.

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