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Nitty-gritty: The highs and lows of temperature
Temperature is the macroscopic manifestation of the energy level of atoms and molecules in a substance, or substances existing in the same thermal equilibrium. Higher the molecular energy level, the more the molecular activity and greater the volume.
This implies that a reduction in temperature would result in a decrease in the energy level of the substance, and vice versa. If the temperature keeps going down it would lead to a situation where all particles stop their movements. This point is termed the absolute zero, the lowest temperature theoretically attainable at which the kinetic energy of atoms and molecules is minimal.
The concept of temperature measurement belongs to the modern age. Although people in the past knew of the difference between ãhotä and ãcoldä (hot and cold), they never felt the need to find out thermal pressure with precision. Scientists had no method to quantify the correct intensity of heat in the thermodynamic system.
One of the first attempts to create a standard temperature scale was made around 170 AD, when Cladius Galen, a Greek physician and writer, suggested a “neutral” parameter made up of equal quantities of boiling water and ice. This scale had four points each for hot and cold.
In 1610, Italian astronomer Galileo Galilee invented a device called thermoscope, or water thermoscope. This contraption consisted of a tubular body of glass extending downwards into a container (probably a sealed jar) of coloured water. When the jar was heated, the air inside expanded and pushed the liquid up in the tube. The water level in the tube could be compared at intervals as indicator for change in temperature, with the addition or removal of heat. However, the thermoscope lacked an easy way to measure temperature directly.
Although, in the past, people could tell the difference between hot and cold, measuring temperature is still a relatively new practice
Measurement of temperature is not only important for matters pertaining to meteorology, psychometry or pathobiology, but it has great significance in the manufacturing or process industry. It plays a substantial role as a touchstone in highlighting the condition of a product or piece of machinery.
Accurate temperature monitoring improves product quality and increases productivity. Also, the downtime requirement of the machinery is comparatively lowered, since the manufacturing processes can proceed without interruption and under optimal conditions.
The indication of the correct temperature is required in any open loop control system — a system of operation where the equipment is controlled manually. If we are handling a closed loop or feedback control system, we need to ensure corrective measures automatically in case of any fluctuation in temperature. This situation warrants an actual change in temperature with a minimum delay.
Measurement of temperature is contingent on the fact that all the objects such as equipment or the controlled medium (if air temperature is to be controlled, then air is the controlled medium) and temperature sensors exchange energy with their surroundings. There are three ways to exchange energy: conduction, convection, and radiation. For good temperature measurement, the temperature sensor and its surroundings are in thermal equilibrium. Hence, there would be a minimum of heat exchange in case of change in the situation.
To ensure observance of the parameter as stipulated above, the temperature sensing system should be accurate, highly sensitive, and must respond quickly.
Temperature measuring devi- ces can be divided into four basic categories. Each of them adheres to a unique principle.
Mechanical devices
As mentioned earlier a change in temperature results in a change in the molecular or atomic movements leading to changes in the volume. This volumetric change depends upon the co-efficient of thermal expansion of that particular substance.
This expansion can be formed as some sort of mechanical motion, since the more the change in temperature, the more the expansion. Mechanical thermometers can be used for measuring temperatures of liquids, solids, or even gases in the case of sensitive material.
Thermojunction voltage
A thermocouple, also called a sensing junction temperature measuring device, is simply two metal wires joined together. When two dissimilar metals are connected, a small voltage called thermojunction voltage is generated.
This is called the Peltier Effect (liberation or absorption of heat at a junction of two dissimilar metals through which an electric current is passing).
Furthermore, with the variation in temperature at the junction, voltage changes. This is called the Thompson Effect. Both effects as a whole can be used to measure temperature. The combined effect is known as the thermojunction effect or the Seebeck Effect.
Electrical resistance
Every material can be classified as a conductor, an insulator, or a semi-conductor. These three distinct classes of material stem from a difference in the structure of the allowed electron energy levels.
A change in temperature causes a change in the electrical resistance of a material. The change in resistance is measured to judge the temperature variation. Basically, there are two types of thermoresistive measuring devices: resistance temperature detectors and thermistors.
A resistance temperature detector (RTD) is either a long, small diameter metal wire wound on a coil, or an etched grid on a substrate, a surface on which an organism grows or is attached, much like a strain gauge.
Most resistive elements that are sensitive to temperature are good conductors of electricity, like nickel, copper, platinum and silver. Platinum is the most commonly used element for such applications.
A typical electrical circuit designed to measure temperature with RTDs actually measures a change in resistance of the RTD, which is then used to compute a change in temperature. The resistance of an RTD increases with increasing temperature, just as the resistance of a strain gauge increases with increasing strain.
Pyrometry
Radiative properties of an object change with changes in temperature. Therefore, radiative properties are measured to indicate the temperature of the object. Also known as radiative pyrometry, this device has a few advantages. It measures without a physical contact with the object of which temperature is to be measured. It can also be used in very high temperature situations.
The writer cocographer@ yahoo.com is a senior instructor at a technical college
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