thermometers

A thermometer is a device that measures temperature or temperature gradient, using a variety of different principles. A thermometer has two important elements – the temperature sensor in which some physical change occurs with physical temperature e.g. the bulb on a mercury thermometer and spring or some other means of converting this physical change into a value e.g. the scale on a mercury thermometer.

There are different types of thermometers.

1. The liquid in glass thermometers

The liquid in glass thermometer utilizes the variation in the volume of a liquid in temperature. They use the fact that most fluids expand on heating. The fluid is contained in a sealed glass bulb, and its expansion is measured using a scale etched in the stem of the thermometer. As we know that the thermometer does not expand then as the physical property it utilizes the variation of length of liquid with temperature.

The liquids commonly used in the liquid-in-glass thermometers are Mercury and Alcohol. Based on the liquid used, they are of two types: mercury-in-glass thermometers and alcohol-in-glass thermometers.

 The liquid in glass thermometer consists of two basic parts:

• The bulb: It acts as a reservoir holding the liquid whose volume changes with temperature. The bulb also acts as a sensor or gauge which is inserted in the body whose temperature is to be measured.
• The stem: It contains the scale that is measuring the temperature and a capillary through which the liquid can accordingly expand and contract.

Advantages:

• They are cheap to manufacture
• They are easy to carry and handle

Disadvantages:

• They tend to have high heat capacities and are not sensitive enough, that is they cannot measure rapid temperature changes.

1.1. The mercury in glass thermometer

These were invented by a German physicist Daniel Gabriel Fahrenheit.

This thermometer consists of mercury in a glass tube. The calibrated marks on the tube allow the temperature to be read by the length of the mercury within the tube. The length of the mercury within the tube varies according to the temperature. To increase the sensitivity, there is usually a bulb of mercury at the end of the thermometer which contains most of the mercury; expansion and contraction of this volume of mercury are them amplified in the much narrower bore of the tube. The space above the mercury may be filled with nitrogen or it may be a vacuum.

Mercury-in-glass thermometer covers a wide temperature range from - 38 °C to 356 °C, although the introduction of a gas into the instrument can increase the range to 600 °C or beyond.

Advantages of a mercury-in-glass thermometer

• Mercury is a naturally opaque liquid (silver). This means that it can be directly utilized in its pure form.
• Mercury does not wet glass. When it moves up and down in the capillary strong cohesive properties of mercury do not allow it to leave any traces on inside the capillary.
• Mercury is a liquid metal. As a metal, it has high conducive properties that allow it to be sensitive than the alcohol-in-glass thermometer.

Disadvantages of a mercury-in-glass thermometer

• Mercury poses a potential toxic hazard if the glass container is ruptured.

1.2. The alcohol-in-glass thermometer

As a liquid, it utilizes ethyl alcohol, toluene and technical pentane, which can be used down to -200 °C. Its range is -200°C to 80°C, though range tends to be highly dependent on the type of alcohol used.

Advantage: Its biggest advantage is that it can measure very low temperatures.

Disadvantage: As alcohol is transparent, it requires a dye to make it visible. Dyes tend to add impurities that may not have the same temperature range as the alcohol. This makes reading difficult especially at the limits of each liquid. Also, alcohol wets glass.

2. The Resistance Thermometer

The resistance thermometer or resistance temperature detector (RTD) uses the resistance of an electrical conductor for measuring the temperature. The resistance of the conductor varies with time. This property of the conductor is used for measuring the temperature. The main function of the RTD is to give a positive change in resistance with temperature.

The metal has a high-temperature coefficient that means their temperature increases with the increase in temperature. The carbon and germanium have low-temperature coefficient which shows that their resistance is inversely proportional to temperature.

The resistance thermometer uses a sensitive element made of extremely pure metals like platinum, copper or nickel. The resistance of the metal is directly proportional to the temperature. Mostly, platinum is used in a resistance thermometer. The platinum has high stability, and it can withstand high temperature.

Gold and silver are not used for RTD because they have low resistivity. Tungsten has high resistivity, but it is extremely brittle Copper is used for making the RTD element because it has low resistivity and also it is less expensive. The only disadvantage of the copper is that it has low linearity. The maximum temperature of the copper is about 120oC.

The RTD material is made of platinum, nickel or alloys of nickel. The nickel wires are used for a limited temperature range, but they are quite nonlinear.

The following are the requirements of the conductor used in the RTDs

• The resistivity of the material is high so that the minimum volume of the conductor is used for construction

• The change in resistance of the material concerning temperature should be as high as possible.

• The resistance of the material depends on the temperature

The resistance thermometer is placed inside the protective tube for providing the protection against damage. The resistive element is formed by placing the platinum wire on the ceramic bobbin. This resistance element is placed inside the tube which is made up of stainless steel or copper steel.

The lead wire is used for connecting the resistance element with the external lead. The lead wire is covered by the insulated tube which protects it from short circuit. The ceramic material is used as an insulator for high-temperature material and for low-temperature fiber or glass is used.

Resistance thermometers are slowly replacing thermocouples in much lower temperature industrial applications (below 600 °C). Resistance thermometers come in a number of construction forms and offer greater stability, accuracy, and repeatability. The resistance tends to be almost linear with temperature. 

Advantages

• Depending on the metal being used resistance thermometers are able to cover extensive temperature ranges. Maximum values are generally related to the melting points of the metal used. 
• Variation of resistance with temperature is stable over an extensive temperature range. 
• Very accurate 

Disadvantages:

• Compared to liquid in glass thermometers, they tend to be expensive. 
• Require other equipment to measure temperature
• They exhibit high heat capacities thus they are not sensitive to temperature change meaning that they cannot be used to measure rapid temperature changes

3. Thermocouples

Thermocouples are sensors composed of two metals that generate electromotive forces (EMFs) or voltages when there are temperature differences between them. The amount of voltage produced is dependent on these differences. Thermocouples operate based on the principle of the Seebeck effect.

The Seeback effect was discovered by German physician turned physicist Thomas Johann Seebeck. He found that when he produced a series of circuits by forming a junction of two different metals, with one metal at a higher temperature than the other, that he was able to generate a voltage. The larger the difference, the higher the voltage, and he found that the results were independent of the shape of the metal.

A thermocouple is composed of a junction formed by two metal alloys. One portion of the junction is placed on a source whose temperature is to be measured, while the other end is maintained at a constant reference temperature in accordance with the zeroth law of thermodynamics. Older thermocouples use ice baths as their temperature source, but modern day ones use a solid state temperature sensor.

Thermocouples are valuable in science and engineering due to their accuracy, fast reaction time, small size, and ability to measure extreme temperatures. The latter ability is based on the metal combinations used; a nickel-nickel combination can measure -50 °C to 1410 °C, while a rhenium-rhenium combination can measure 0 °C to 2315 °C. The most common combinations are iron-constantan, copper-constantan, and chromel-alumel. The disadvantages of thermocouples are that the signals produced may not be non-linear, and thus they need to be calibrated carefully.

4. Gas thermometer

A gas thermometer measures temperature by the variation in volume or pressure of a gas. Gas thermometers work best at very low temperatures.

There are two main types of gas thermometer – one operating at constant volume and the other at constant pressure.

• Constant volume gas thermometer: It usually consists of a bulb filled with a fixed amount of a dilute gas which is attached to a mercury manometer. The manometer is used to measure variation in pressure. This thermometer functions by Charles’ Law which states that when the temperature of an ideal gas increases, there is a corresponding increase in pressure. Also, when the temperature decreases, the pressure too decreases correspondingly. This is how constant volume gas thermometer traces the increase in temperature with the change in pressure while the volume remains constant. These types of thermometers are used in calibration of various other thermometers.
• Constant pressure gas thermometer: The constant pressure gas thermometer is based on the relationship between the variations in volume with the corresponding variation in the temperature while the pressure of the process remains constant. The constant pressure gas thermometer functions on Gay-Lussac’s Law or pressure law which states that for a given mass and constant volume of an ideal gas, the pressure exerted on the sides of its container is directly proportional to its absolute temperature. This consists of a long, thin glass tube with a known constant mass of air and mercury sealed inside by a rubber stopper. Such processes are called as the isobaric process. The tube, air, and mercury are then surrounded by a larger, open tube through which liquid or gas can freely pass. On changing the temperature of the inner tube, the air either expands or contracts, moving the mercury along the tube. Measuring the amount of movement of the mercury gives us a relationship to how much it has expanded at certain temperatures.

5. Pyrometer

A pyrometer is a type of thermometer used to measure high temperatures. It is used for measuring temperature without any physical contact. It is used for measuring body temperature by measuring its electromagnetic radiation.

Its principle depends upon the relationship between the temperature of a hot body and electromagnetic radiation emitted by the body. When a body is heated it emits thermal energy known as heat radiation. It is a technique for determining body temperature by measuring its electromagnetic radiation.

Optical pyrometer - The optical pyrometer is a non-contact type temperature measuring device. It works on the principle of matching the brightness of an object to the brightness of the filament which is placed inside the pyrometer. The optical pyrometer is used for measuring the temperature of the furnaces, molten metals, and other overheated material or liquids. It is not possible to measure the temperature of the highly heated body with the help of the contact type instrument. Hence the non-contact pyrometer is used for measuring their temperature.

Advantages of an optical pyrometer

• The optical pyrometer has high accuracy
• The temperature is measured without contacting the heated body. Because of this property, the pyrometer is used for the number of applications

Disadvantages of an optical pyrometer

• The working of the pyrometer depends on the intensity of light emitted by the heated body. Thereby, the pyrometer is used for measuring the temperature having a temperature more than 700 °C. The accuracy of the pyrometer depends on the adjustment of the filament current. Also, the pyrometer is not used for measuring the temperature of clean gases.

Difference between clinical and laboratory thermometer

Clinical Thermometer	Laboratory Thermometer
Clinical thermometer is scaled from 35°C to 42°C or from 94°F to 108°F.	Laboratory thermometer is generally scaled from -10°C to 110°C.
Mercury level does not fall on its own, as there is a kink near the bulb to prevent the fall of mercury level.	Mercury level falls on its own as no kink is present.
Temperature can be read after removing the thermometer from armpit or mouth.	Temperature is read while keeping the thermometer in the source of temperature, e.g. a liquid or any other thing.
To lower the mercury level jerks are given.	No need to give a jerk to lower the mercury level.
It is used for taking body temperature.	It is used to take the temperature in the laboratory.