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Thermal Physics - Thermometers
Types of Thermometers: Liquid in Glass, Laboratory, Clinical, Thermocouple and the advantages and disadvantages of each.
Author:Caleb Fortune
Edu Level: CSEC
Date: Dec 20, 2024
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THE THERMOMETER
A thermometer is an instrument that is used for measuring and indicating a change in temperature.
THE LIQUID IN GLASS THERMOMETER
A liquid-in-glass thermometer is a type of thermometer that uses the expansion of a liquid (often mercury or alcohol) inside a glass tube to measure temperature.
Below is a diagram of an average liquid in glass thermometer:
From imgur: https://imgur.com/a/M2djyGV
CHARACTERISTICS OF A GOOD LIQUID IN GLASS THERMOMETER
A good thermometer has the following:
- A suitable thermometric liquid (a liquid that can change its volume in accordance with a change in temperature)
- A thin bulb to allow a quick response to a change in temperature.
- A thin capillary tube to increase the sensitivity of the thermometer.
- A thick glass bore to allow magnification of the scale for easier reading and for increased durability of the thermometer.
THERMOMETRIC LIQUIDS USED IN LIQUID IN GLASS THERMOMETERS:
Liquid in glass thermometers use the following liquids:\
-
Alcohol
-
Mercury
ADVANTAGES AND DISADVANTAGES OF USING EACH THERMOMETRIC LIQUID
There are many advantages and disadvantages of using a certain thermometric liquid over another.
ALCOHOL THERMOMETER
| Advantages | Disadvantages |
|---|---|
| The expansion of alcohol is uniform. | Alcohol is colourless, so it must be dyed in order to observe a temperature change. |
| Alcohol is less dangerous to handle than mercury. | Alcohol wets the walls of the thermometer which can negatively impact the readings. |
| The cost of an alcohol thermometer is cheaper than a mercury thermometer. | Alcohol is less durable than mercury because it evaporates at a faster rate. |
| Alcohol can measure lower temperatures since the freezing point of alcohol is low, (-115°C). | Alcohol cannot be used to measure high temperatures since its boiling point is 78°C. |
| Alcohol is more sensitive than mercury and expands six times faster than mercury. |
MERCURY THERMOMETER
| Advantages | Disadvantages |
|---|---|
| The expansion of mercury is uniform since it is a metal that conducts heat. | Mercury cannot measure temperatures below its freezing point of -39°C. |
| Mercury can measure a wide range of temperatures since it has a high melting point (357°C) and a relatively low freezing point (-39°C). | Mercury is not as sensitive as alcohol; so it does not respond to temperature changes as quickly. |
| Unlike alcohol, mercury does not wet the walls of the thermometer. | Mercury is more expensive than alcohol. |
| Mercury can measure temperatures precisely. | Mercury is poisonous, making it dangerous to handle if the thermometer is broken. |
| Mercury does not need to be dyed because it is silver in colour. | The silver column of mercury in the capillary tube may be difficult to read. |
THE LABORATORY THERMOMETER
A laboratory thermometer is an instrument designed to measure temperature in scientific and industrial applications. It is typically used in experiments, chemical reactions, and industrial processes where accurate temperature readings are crucial.
Below is a diagram of a laboratory thermometer:
From imgur: https://imgur.com/a/nBC0ueB
THE CLINICAL THERMOMETER
A clinical thermometer is a specialized thermometer designed for measuring the body temperature of humans and animals. It is commonly used in medical settings and homes to monitor health conditions such as fever.
Below is a diagram of a clinical thermometer:
From imgur:https://imgur.com/a/mHoAOpw
SIMILARITIES AND DIFFERENCES BETWEEN A LABORATORY AND A CLINICAL THERMOMETER
| Similarities | Differences |
|---|---|
| Both the laboratory and clinical thermometer use the Celsius scale on the glass tube. | The laboratory thermometer is used to measure temperature changes in experiments performed in a laboratory. On the other hand, the clinical thermometer is used to measure the temperature of the human body. |
| Both thermometers use mercury as a thermometric liquid. | Unlike the laboratory thermometer, the clinical thermometer has a constriction in the capillary tube to prevent the immediate backflow of mercury into the bulb. |
| Both thermometers consist of a long and narrow, uniform glass tube. | The laboratory thermometer has a temperature range from -10°C to 110°C, whereas the clinical temperature has a temperature range from 35°C to 42°C. |
| Both thermometers have a bulb at one end, containing the thermometric liquid. |
THE THERMOCOUPLE THERMOMETER
A thermocouple thermometer is a temperature-measuring device that uses the principle of thermoelectricity to measure temperature. It is used primarily in industrial, scientific, and commercial applications due to its durability, wide temperature range, and quick response.
Below is a simplified diagram of a thermocouple thermometer:
From imgur: https://imgur.com/a/J9s570A
A thermocouple consists of two different metal wires joined at one end (the junction).When the junction is exposed to a temperature difference, a voltage (called the thermoelectric EMF) is generated.This voltage is directly proportional to the temperature difference between the hot junction and a reference point.
The generated voltage is measured and converted into a temperature reading using a calibration table or a digital interface.
MAIN PARTS OF THE THERMOCOUPLE THERMOMETER
-
Hot Junction: The point where the two metal wires are joined and exposed to the temperature being measured.
-
Cold Junction: The other end of the thermocouple, typically maintained at a known reference temperature.
-
Metal Wires: Made of specific metals or alloys chosen based on the desired temperature range and sensitivity.
-
Display Unit:Connected to the thermocouple to show the temperature reading.
ADVANTAGES AND DISADVANTAGES OF USING A THERMOCOUPLE THERMOMETER
| Advantages | Disadvantages |
|---|---|
| Wide temperature range, from -200°C to 1800°C. | A thermocouple requires calibration often to ensure precise measurements. |
| A thermocouple thermometer is very durable and reliable as it is able to withstand extreme conditions. | A thermocouple thermometer is susceptible to errors from electromagnetic interference. |
| A thermocouple can be integrated into automated systems to constantly monitor temperature changes. | A thermocouple is less accurate at lower temperatures than other thermometers. |
| A thermocouple thermometer is very sensitive and responds rapidly to temperature changes. | The output voltage of a thermocouple is very small and may require amplification. |