Expansion Thermometers

Given this situation, it is not surprising that, over the last few years, products based on platinum thin film technology have been finding their way into the home. With the growing use of electronic control systems in the new generation of domestic appliances, platinum temperature sensors have been more widely used in ovens where they have replaced electromechanical regulators such as capillary tubes, solid expansion thermometers and NTC thermistors. Typical sensor applications in the food preparation sector are shown in Fig. 5.3. 

When you mention thermometers, volumetric expansion thermometers are what typically come to mind (see Fig. 2.30). The material that expands within a volumetric expansion thermometer is typically mercury or (ethyl) alcohol. Another name for a volumetric expansion thermometer is a liquid-in-glass thermometer. The parts of a standard liquid-in-glass laboratory thermometer are as follows ... 

The typical pressure expansion thermometer is a volume of gas that maintains the same number of molecules throughout a test. Because the volume, number of molecules, and gas constant are all constant, any drop in temperature will subsequently cause a drop in pressure. Likewise, any rise in temperature will cause a rise in pressure. 

Gas Thermometers. These are expansion thermometers that depend on the coefficient of thermal expansion. They use, for example, helium gas and have helped to establish the thermodynamic temperature scale, and also for measurements at very low temperatures. 

Changes in dimension. With such devices, a change in physical dimension occurs with a change in temperature. In this category are liquid-in-glass or other fluid-expansion thermometers, bimetallic strips, and others. 

An expansion thermometer—with graduations of 1 °C—and two thermocouples—lypes T and K—are used to observe the evolution of temperature in a heated water bath. The temperature is read directly off the expansion thermometer. The reference temperature for the two thermocouples is 25 °C and the data is summarized in Table Q5.17. Note that the thermocouples precision is 0.05 mV. C. Neagoe... 

Temperature Thermistor Thermocouple Resistance thermometer Expansion thermometer Optical pyrometer... 

Probe Thermometers. Volume expansion thermometers use the expansion of liquids with rising temperature through a narrow tube. The expansion coefficient, defined as the increase in volume per unit volume per unit rise in temperature, is 0.00018 per Kelvin for mercury and 0.00109 per Kelvin for ethyl alcohol colored with dye. Calculating temperature from the actual random thermal motion velocity of every molecule, or the energy contained in a vibrational excitation of every molecule, is impractical. So temperature is measured indirectly in most applications. Different metals expand to different extents when their temperature rises. This difference is used to measure the bending of two strips of metal attached to one another in outdoor thermometers. Thermocouples use the Seebeck or thermoelectric effect discovered by Cerman physicist Thomas Johann Seebeck, in which a voltage difference is produced between two junctions between wires of... 

Temperature measurement is used in a multitude of applications. Volume expansion thermometers work in the range from about 250 to 475 K, but each thermometer is usually designed for a much narrower range for specific purposes. Examples include the measurement of human body temperature, the atmospheric temperature, and the temperature in ovens used for cooking. 

The most widely used thermometer in polymer technology is the resistance temperature detector (RTD). For higher temperatures or with point measurements, the thermocouple is often preferred. Mechanical contact thermometers (expansion thermometers, rod thermometers, or bimetallic thermometers) are nearly always used only for local measurements. For optical thermometers, radiation pyrometers are used. 

If, on the other hand, the thermometer has previously been used at some temperature below the freezing-point of benzene, when the bulb is originally placed in the beaker of water at 7-8 C., the mercury will rise in the capillary and ultimately collect in the upper part of the reservoir at a. When the expansion is complete, again tap the thermometer sharply at R so that this excess of mercury drops down into b, and then as before check the success of the setting by placing the thermometer m some partly frozen benzene. In either case, if the adjustment is not complete, repeat the operations, making a further small adjustment, until a satisfactory result is obtained. 

Two convenient forms of bath are shown ui Fig. 11,10, 2, a and 6. The former consists of a long-necked, round-bottomed flask (a longnecked Kjeldahl flask of 100 ml. capacity is quite satisfactory) supported by means of a clamp near the upper part of the neck. The thermometer is fltted through a cork, a section of the cork being cut away (see inset) so that the thermometer scale is visible and also to allow free expansion of the air in the apparatus. The bulb is about three-quarters filled with... 

Fig. II, 56, 6 is a simple distillation head when this is fitted into a flask with a ground glass socket, the assembly is virtually a distillation flask. The bottom cone is usually 19, 24 or 29 the side cone is generafly B19 but may be 24 the thermometer socket is 14. For many purposes, a thermometer is fitted into a one-hole rubber stopper of correct taper and then inserted into the 14 socket the area of rubber which is exposed to the action of the organic vapour is relatively so small that the amount of contamination thus introduced is negligible. If, however, all rubber stoppers must be absent because of the highly corrosive character of the vapour, a thermometer with a 14 cone is employed. It is important to have the thermometer of the same glass as the distillation head, otherwise difficulties may arise owing to the different expansion coefficients of the two kinds of glass.

Other nonelectrical thermometers are bimetal, fiHed-system, and pyrometric cone thermometers. In bimetal thermometers, two strips of metal of differing expansion characteristics are welded together face-to-face. If one end of such a strip is fixed, the strip bends in response to temperature change as the... 

Full consideration of the thermal expansion of the tubes is necessary, with adequate provision for expansion and contraction. It is a wise policy to fit thermometer wells in the pipes near the inlets and outlets of all airheating batteries, as these provide a useful means of checking the coil performance. 

Instruments based on the contact principle can further be divided into two classes mechanical thermometers and electrical thermometers. Mechanical thermometers are based on the thermal expansion of a gas, a liquid, or a solid material. They are simple, robust, and do not normally require power to operate. Electrical resistance thermometers utilize the connection between the electrical resistance and the sensor temperature. Thermocouples are based on the phenomenon, where a temperature-dependent voltage is created in a circuit of two different metals. Semiconductor thermometers have a diode or transistor probe, or a more advanced integrated circuit, where the voltage of the semiconductor junctions is temperature dependent. All electrical meters are easy to incorporate with modern data acquisition systems. A summary of contact thermometer properties is shown in Table 12.3. 

Liquid-in-glass thermometers measure the thermal expansion of a liquid, which is placed in a solid container, on a length scale. The mercury thermometer is one example of liquid thermometers. Alcohol is also used with this type of instrument. The temperature range is -80 to a-330 °C depending on the liquid. The quality, stability, and accuracy vary considerably. The advantages are a simple construction and low price. A disadvantage is that they are not compatible for connection to monitoring systems. 

In filled thermometers the thermal expansion of a gas or a liquid is transmitted through a thin capillary tube to a bellows or helix, where the deformation indicates the temperature. The temperature range of filled thermometers is very wide, approximately -200 to +700 °C. They are extremely robust but are not very high in accuracy. The application is mainly for process instrumentation and as stand-alone control devices. 

Bimetal thermometer A thermometer that uses two dissimilar bars of metals (with different rates of linear thermal expansion) riveted together. A variation in temperature produces a bending moment on the bar, which is magnified by a lever to record temperature on a dial. 

On account of the low condensing temperatures of hydrogen and helium, thermometers contaihing them are particularly useful at very low temperatures the regularity of expansion of gases makes them very suitable for thermometric purposes. 

With the same apparatus as in example (1), the absolute temperature of the gas in its initial state was Ti. The tap was then opened so that the gas rapidly expanded to atmospheric pressure, and the temperature, determined immediately after expansion by a platinum resistance thermometer, or a thermo-element, in the centre of the vessel, was T2. Show that ... 

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