Thermometer principle

Accurate temperature measurements in real-life situations are difficult to make using the KTTS. Most easily used thermometers are not thermodynamic that is, they do not operate on principles of the first and second laws. Most practicable thermometers depend upon some principle that is a repeatable and single-valued analogue of temperature, and they are used as interpolation devices of practical and utilitarian temperature scales which are themselves... 

Whereas it is no longer an iaterpolation standard of the scale, the thermoelectric principle is one of the most common ways to transduce temperature, although it is challenged ia some disciplines by small iadustrial platinum resistance thermometers (PRTs) and thermistors. Thermocouple junctions can be made very small and ia almost infinite variety, and for base metal thermocouples the component materials are very cheap. Properties of various types of working thermocouple are shown in Table 3 additional properties are given in Reference 5. 

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. 

Thermochemistry, 254, 507 Thermodynamic potentials, 99 Thermo-electric circuit, 450 inversion, 451 theories, 453 Thermometers, 3, 140, 166 Thermometry, 1, 353 Thomsen-Berthelot principle, 257, 506... 

The Zeroth Law of Thermodynamics An extension of the principle of thermal equilibrium is known as the zeroth law of thermodynamics, which states that two systems in thermal equilibrium with a third system are in equilibrium with each other. In other words, if 7), T2, and 77, are the temperatures of three systems, with 7j - T2 and T2 = T2, then 7j = 7Y This statement, which seems almost trivial, serves as the basis of all temperature measurement. Thermometers, which are used to measure temperature, measure their own temperature. We are justified in saying that the temperature T3 of a thermometer is the same as the temperature 7j of a system if the thermometer and system are in thermal equilibrium. 

Rodebush has also implied that the accuracy with which very low temperatures can be measured is restricted by the uncertainty principle and by the nature of the substance under investigation. However, the accuracy of a temperature measurement is not limited in a serious way by the uncertainty principle for energy, inasmuch as the relation between the uncertainty in temperature and the length of time involved in the measurement depends on the size of the thermometer, and the uncertainty in temperature can be made arbitrarily small by sufficiently increasing the size of the thermometer we assume as the temperature of the substance the temperature of the surrounding thermostat with which it is in either stable or metastable equilibrium, provided that thermal equilibrium effective for the time of the investigation is reached. 

Further, we examined the Heck reaction between w-butyl acrylate and 4-bromobenzotrifluoride 5 in the presence of 2 mol% Pd clusters in a singlevessel monomode m/w oven fitted with an infrared thermometer. 100% conversion with quantitative yield to the cinnamate was obtained after 5 min irradiation at 75 W/240 °C. We then repeated the reaction under conventional heating at 240 °C. After 3.5 min a black tarry gel formed. Extraction followed by GC analysis showed only cinnamate, but the tarry material (probably acrylate polymers/oligomers) could not be analysed. These experiments show that when clusters are present different results are obtained depending whether m/w heating or conventional heating is used. In principle, this could be the result of hot spots created on the metal clusters. 

Capacitance measurements are quite simple. A typical drawback is the need of coaxial cables that introduce a thermal load which is not negligible in low-power refrigerators. On the other hand, capacitance bridges null the cable capacitance. Multiplexing is more difficult than for resistance thermometers. In principle, capacitors have low loss due to Joule heating. This is not always true losses can be important, especially at very low temperatures. Dielectric constant thermometers have a high sensitivity capacitance differences of the order of 10-19F can be measured. 

The CBT is in principle a primary thermometer, but its absolute accuracy is at present too low for metrological applications [115]. 

Measuring body temperature is important for the detection of disease and assessment of the response to treatments. The first thermometer was developed by Galileo in 1603. Thermometers for measuring body temperature have been in use since about 1870. The first measurements taken were axillary, and later oral and rectal measuring methods were introduced. The working principle of those thermometers, the expansion of matter by temperature increase, is still used for body temperature measurement in mercury-in-glass thermometers. Electronic thermo-... 

Total immersion thermometers, 24 464 Total internal reflection principle,... 

Of course, if not accompanied by a full understanding of the working principles of these devices, the status of their development, and their potential, the results of this test would be misleading when deciding on the choice of the right type of sensor for further development. It has been shown by later development, as discussed, that the lifetime-based scheme is much to be favored in terms of performance and cost. This was the reason that a major manufacturer, Luxtron, substituted the lifetime-based thermometer for the early, intensity-based one in its commercial production/5 ... 

The underlying principle of this method of measuring temperature is that the resistance of a thin piece of metal increases as the temperature increases. This makes an extremely sensitive thermometer yet it is fragile and has a slow response time. 

Filled-bulb temperature sensors are also widely used. An inert gas is enclosed in a constant-volume system. Changes in process temperature cause the pressure exerted by the gas to change. Resistance thermometers arc used where accurate temperature or diflcrcntial-temperature measurement is required. They use the principle that the electrical resistance of wire changes with temperature. 

If larger displacements are required, an arrangement as shown in Fig. 9.7, the bimorph, can be applied. The principle is similar to the bimetal thermometer. Two thin plates of piezoelectric material are glued together. By applying a voltage, one plate expands and the other one contracts. The composite flexes. 

A resistance thermometer is a temperature measuring device based on the principle that the resistance of metals proportionally increases as the temperature of a metal increases. 

Non-stoichiometry, which originates from various kinds of lattice defect, can be derived from the phase rule. As an introduction, let us consider a trial experiment to understand non-stoichiometry (this experiment is, in principle, analogous to the one described in Section 1.4.8). Figure 1.1 shows a reaction vessel equipped with a vacuum pump, pressure gauge for oxygen gas, pressure controller for oxygen gas, thermometer, and chemical balance. The temperature of the vessel is controlled by an outer-furnace and the vessel has a special window for in-situ X-ray diffraction. A quantity of metal powder... 

The mother wishes to demonstrate this wonderful principle to her husband. She weighs the sawdust, measures the sinkwater, and reads the thermometer to evaluate the successive terms in (S2.13-5), then announces proudly that the result is once again 37 blocks But, asks the husband, what exactly are the blocks you are talking about ... 

An alternative method of calorimetry that gives less accurate results, but is simpler in concept, uses only a single insulated container and a thermometer. Temperature changes in the calorimeter are brought about by adding hot (or cold) objects of known weight and temperature. Calculations are based on the principle that the heat lost by the added hot object is equal to that gained by the water in the calorimeter and the calorimeter walls. This simple approach is illustrated in the next two problems. 

Two main principles of temperature measurement use thermocouples and the so-called resistance thermometer. In chemical plants both methods were applied because they are easy to fit and to maintain.The accuracy of the measurement is influenced by, for example, radiation, which must be taken into account. Thermocouples can be inserted into the pressure system using special sealing techniques, or they may be mounted within a protective tube which is introduced into the pressurized volume. Thermocouple-wires are usually protected with an isulating input in closed-end capillaries with outer diameters of at least 0.5 mm. Thermocouples are technically well tested for pressures up to 6 kbar and temperatures to approx. 800°C. Above these ranges the exact measurement is negatively influenced by several parameters, and the deviations must be taken into account. The accuracy of the temperature measurement devices is normally better than 1 °C. 

The actual temperature of the solution in the vicinity of the working electrode should be measured. It is not wise to assume that the solution temperature equals that of the bath, particularly when using a circulating refrigerated bath, because the temperature of the coolant will rise on passage from the bath to the cell. Low-temperature thermometers can be used in principle, but the almost universal choice is a thermocouple. Many commercial units are available with digital output and control features. The thermocouple junction can be coated with Teflon and inserted directly into the cell. 

A This thermometer works on the principle that the liquids in the different glass bulbs have slightly different densities, causing them to rise or sink as the density of the fluid in the column varies with temperature. 

The useful screw-thread connector is becoming widely available. The simplest is the screw-capped adapter (Fig. 2.32) where the screw-capped joint is associated with a ground glass cone. The figure shows the silicone rubber ring and PTFE washer, and the adapter is useful for the insertion of gas tubes, thermometers or stirrer shafts. The screw-thread connection principle has been applied to the connection of water hoses to condensers and vacuum lines to Buchner flasks, to securing simple sealing septa to flasks, etc. 

Reconstruction of faunal records in deep-sea sediments and oxygen isotope measurements have been yielding useful information of the ocean s paleotemperature. However, the lack of these tools in the case of continents has hampered the estimation of paleotemperature in continents. The latter estimate has mainly been made on somehow indirect approach such as pollen data, periglacial feature, and soil carbonate, all of which suffer from considerable uncertainty stemming from the assumption that must be made to convert the observation to temperature. The advantage of the noble gas thermometer, as compared to the other paleotemperature methods, is that it is based on the relatively simple physical principle that directly relates noble gas concentration to the ambient temperature (see Stute Schlosser, 1993, for a recent review of the noble gas paleotemperature method). However, in actual practice, we need some cautions. 

It is believed that Galileo invented the liquid-in-glass thermometer around 1592. Thomas Seebeck discovered the principle behind the TC—the existence of the thermoelectric current—in 1821. The same year Sir Humphry Davy noted the temperature dependence of metals, but C. H. Meyers did not build the RTD until 1932. Today, some 20 different types of temperature sensors are available, and Table 3.160 lists the temperature ranges and accuracies of a number of them. 

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