1 January 2017

A thermometric property is a physical property that changes in a known way with temperature, and can therefore be used to measure temperature. The substance used as a thermometer must have a property that varies proportionally / linearly with temperature. [ In other words: When objects are heated or cooled, their temperatures change, along with some of their properties, these properties are known as Thermometric Properties. ] Two commonly used thermometric properties are resistance and length. 2. Water is generally used to determine the fixed point on a thermometer.

The fixed points are freezing point (32oF/0oC) and boiling point (212oF/100oC). Upper fixed point is the temperature of pure water boiling at normal atmospheric pressure. Lower fixed point is the temperature of a mixture of pure ice and pure water at normal atmospheric pressure. Generally the upper and lower fixed points of a thermometer are determined with considering the freezing point and boiling point of water.

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In Celsius thermometer, lower fixed point is 0 ? C and upper fixed point is 100 ? C, in Fahrenheit thermometer lower fixed point is determined as 32.

They need no power source. Disadvantages: If it so happens that the thermometer falls and breaks, a spill of mercury in a doctor’s office, school or home environment poses significant health hazards. Exposure could result in serious damage to kidneys, lungs, brain, heart and immune system. Digital Thermometers Digital thermometers use thermocouples or thermistors to sense the change in temperature and display the temperature on a digital display. There are a number of different types of digital thermometers therefore the temperature ranges vary. Advantages: Digital thermometers are easy to use.

When using a digital thermometer you won’t be exposed to mercury. Disadvantages: Dead batteries are a drawback of digital thermometers. It is hard to determine how much power is left in a digital thermometer’s battery, which in turn makes it difficult to predict when it will die. Because they are specialized batteries, it is hard to locate replacements. And close-to-dying batteries will provide inaccurate readings. Outdoor Thermometers Outdoor thermometers are used to measure the temperature of the surrounding air. Temperature ranges from -58of/-50oc to 158of/ 70oC. Advantages: Low battery consumption. High stability and accuracy.

Disadvantages: If an object is considerably close to the outdoor thermometer it can influence the actual temperature of the surrounding air. Thermocouple Thermometer A thermocouple thermometer is a device for measuring temperature consisting of a pair of wires of different metals or semiconductors joined at both ends. One junction is at the temperature to be measured, the second at a fixed temperature. The electromotive force generated depends upon the temperature difference. Temperature ranges from 200oC to 2,600oC. Advantages: It is very responsive to rapidly changing temperatures due to its low thermal capacity.

The low thermal capacity is due to its low mass and metals are good conductors of electricity. As the output is an electrical impulse it can be connected to a suitable electrical equipment for checking rapid or sudden temperature changes. Disadvantages: It is extremely expensive. Generates low signal which is non-linear Must be assembled with care to avoid considerable noise levels. II. Thermal Expansion Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation.

Materials which contract with increasing temperature are rare; this effect is limited in size, and only occurs within limited temperature ranges. The degree of expansion divided by the change in temperature is called the material’s coefficient of thermal expansion and generally varies with temperature. 1. Thermal Expansion of Solids Heating a solid makes it particles vibrate vigorously. As these vibrations become larger the molecules are pushed further apart and the solid expands slightly in al directions. Thermal Expansion of Liquids Like solid liquids also expand on heating.

Since liquids do not have a definite shape they can have only volume expansion. Since the container containing liquid also expands with heat therefore Real Expansion = Apparent Expansion + Expansion in a liquid of liquid of Container . When a liquid is heated in a container, the container expands first and therefore the level of liquid decreases since the expansion of liquid is more than that of solid, therefore after sometime the liquid expands and its level increases. The lava lamp above illustrates the differential thermal expansion in liquids. Thermal Expansion of Gases A gas expands when heated and condenses when cooled.

Charles’ Law predicts a four-fold decrease in the volume of a gas that is cooled from room temperature to the temperature of liquid nitrogen (77 K). If the gas inside the balloon condenses into a volatile liquid at or around 77 K, the volume change will be even larger. Removing the balloon exposes the cold balloon to higher temperatures. The resulting thermal expansion fills the balloon to its original size again. Liquid nitrogen or dry-ice/acetone is poured over a balloon. 2. Thermal Expansion – Real-life applications Engine Coolant Another example of thermal expansion on the part of a liquid can be found inside the car’s radiator.

If the radiator is “topped off” with coolant on a cold day, an increase in temperature could very well cause the coolant to expand until it overflows. In the past, this produced a problem for car owners, because car engines released the excess volume of coolant onto the ground, requiring periodic replacement of the fluid. Later-model cars, however, have an overflow container to collect fluid released as a result of volume expansion. As the engine cools down again, the container returns the excess fluid to the radiator, thus, “recycling” it. This means that newer cars are much less prone to overheating as older cars.

Combined with improvements in radiator fluid mixtures, which act as antifreeze in cold weather and coolant in hot, the “recycling” process has led to a significant decrease in breakdowns related to thermal expansion. Building Settlement All pipes and vessels expand and contract in direct proportion to a temperature change. Whenever pipes are routed across structural movement joints in buildings, roads, bridges etc. , they will be subjected to displacements. Expansion & building settlement products are installed to control this thermally induced expansion and contraction, and pipe movements caused by structural settlement.

Pipe Solutions are able to offer technical advice or a full design, for any expansion or building movement problem. Expansion Joints are used to physically compensate for the movements of the pipework. They can be designed to suit almost any combination of working temperature, pressure and movement capability. 3. The Design of a Bimetallic Strip A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases brass instead of copper.

The strips are joined together throughout their length by riveting, brazing or welding. The different expansions force the flat strip to bend one way if heated, and in the opposite direction if cooled below its initial temperature. The metal with the higher coefficient of thermal expansion is on the outer side of the curve when the strip is heated and on the inner side when cooled. The sideways displacement of the strip is much larger than the small lengthways expansion in either of the two metals. This effect is used in a range of mechanical and electrical devices.

In some applications the bimetal strip is used in the flat form. In others, it is wrapped into a coil for compactness. The greater length of the coiled version gives improved sensitivity. The operation of the bimetallic strip relies on the different expansions rates of the two metals to temperature change ( the different coefficients of thermal expansion of the metals). One of the many uses for bimetallic strips is in electrical breakers where excessive current through the strip heats it and bends it to trip the switch to interrupt the current. Bimettalic strips are also used as thermostats.

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