Methods of Temperature Measurement

Temperature can be measured with resistive temperature sensors, thermocouple temperature sensors, and radiation pyrometers. There are two types of resistive temperature sensors the conductive type and the semiconductor type. Both operate on the principle that the resistance of sensor material changes with temperature. 

The conductive-type temperature sensor (RTD) uses a metal element to measure temperature. The resistance of most metals increases with temperature thus, by measuring resistance, one can determine the temperature. Platinum is used where very precise measurements are required and where high temperatures are involved. Platinum is available in highly purified condition it is mechanically and electrically stable and corrosion-resistant. Most of the RTD sensors have a wound wire configuration although for some applications, metal-film elements are used. 

The semiconductor type sensor utilizes the fact that the resistance of a semiconductor decreases with temperature. The most common type of semiconductor temperature sensor is the thermistor shown in Fig. 4.9. 

Because of their small size, thermistors can be used where other temperature sensors cannot be used. A typical resistance temperature (RT) curve is generally nonlinear this is one of the drawbacks of thermistors. However, techniques to deal with the thermistor non-linearity are now well established [14] thus, the non-linearity is not a major problem. Other disadvantages are the low operating currents ( 100 pA) and the tendency to drift over time. An advantage is their quick response time. 

Thermocouple (TC) temperature sensors are also known as thermoelectric transducers a basic TC circuit is shown in Fig. 4.10. 

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T. D. McGee. Principles and Methods of Temperature Measurement. New York John Wiley Sons, 1988. 

One worker (B9) employed a tube of carbon as the heat source. Electric current was passed through this tube, and boiling occurred outside. A thermocouple was in contact with the inside surface. This is a reliable method of temperature measurement, for the air around the wires is at the same temperature as the inner surface. 

The methods of temperature measurement of graphite filaments are also subject to criticism. Temperature must be measured by an optical pyrometer. Duval (19) admits a possible error of 50° C. due to uncertainty in the calculated emissive power of a dull graphite surface (60). Furthermore, the temperature range of investigation cannot be extended far below 1000° C. without making arbitrary extrapolations of temperature vs. voltage curves. 

We will begin by reviewing methods of temperature measurement, furnace design, and temperature control. The instruments, how they work, what they measure, potential pit-... 

The most common method of temperature measurement is with a liquid-in-glass thermometer. This method depends on the expansion of fluids when they are... 

Table 6.20 Limiting temperatures for insulated windings (for stalled motors) (referring to an ambient temperature of +40°C, and to the end of time fE). Method of temperature measurement resistance...

Kinetic rate data may depend on the test equipment used and the method of temperature measurement. 

The result as it stands is perhaps not as certain as we might wish, because the method of temperature measurement was only checked... 

The measurement of temperature is one of the most common physical measurements routinely made. It is so common that it is often overlooked as a variable when complex biochemical reactions are being studied. This is unfortunate, because an error in the temperature of a reaction may produce a large error in the results that becomes apparent when the results are compared with those of known standard reactions. For example, if the rate of reaction of an unknown enzyme is being studied at a temperature that is different by 0.1°C from the temperature at which the standard reaction was measured, an error as large as 2-5% in the observed rate of reaction can occur. The experimental data would not correlate then with the known enzyme reaction rates. Such errors lead to confusion in determining mechanisms and to the large variations that occur even in normal values from one clinical laboratory to another. This article seeks to bring the importance of accurate temperature measurements to the attention of biomedical scientists. We will identify the latest methods of temperature measurement and control as well as new temperature fixed-point standards that are or will shortly become available. 

Thomas, D. L. "Problems in Applying the Line Reversal Method of Temperature Measurement to Flames." Combustion and Flame 12 (1968) 541M9. 

Here we would like to remark that correct and/or complete description of the setup used, the method of temperature measurement, the reaction size, and the energy input are especially important in a microwave procedure [20, 76). 

First report on spectroscopic methods of temperature measurements. lUPAC, Rep. of the committee on standards and measurements in plasma chemistry, (ed. P. Fauchais), Limoges Univers., april 1980... 

Two noninvasive methods of temperature measurement based on the electroosmotic mobility and conductance are discussed in further detail below. The linear relationships between both the electroosmotic mobility and the conductance versus PIL are illustrated in Figure 18.7. 

Typical operating data for the refrigerator are shown in Table II. The discrepancy between the output calculated from the temperature rise of the gas stream, and the measured output from the wattmeter has not been resolved. Repeated checks on the method of temperature measurement have not revealed any appreciable errors, nor do heat leaks into the system seem adequate to explain this. The actual average value of Cp in the 14° to 20 °K range may be less than assumed. 

McGee TD (1988) Principles and methods of temperature measurement. Wiley, New YotIc... 

The most common method of temperature measurement is contact thermometry, as demonstfated in Fig. 4.1. One brings a thermometer, C, a system with a known thermal property, into intimate contact with the to be measured system, A. Next, thermal equilibration is awaited. When reached, the temperatures of A and C are equal. The use of C as a contact thermometer is based on the fact that if the two systems A and B are in thermal equilibrium with C they must also be in thermal equilibrium with each other. This statement is sometimes called the zeroth law of thermodynamics. It permits to use B with a known temperature to calibrate C, and then use C for measurement of the temperature of system A. A calibration with B can be made at a fixed temperature of a phase transition without degree of freedom, as given by the phase rule of Sect. 2.5.7. Less common are methods of temperature measurement without a separate thermometer system. They make use of the sample itself. For example, the temperature of the sample can be determined from its length, the speed of sound within the sample, or the frequency of light emitted. 

Temperature measurement occurs at various locations of the extruder along the extruder barrel, in the polymer melt, and at the extrudate once it has emerged from the die. The choice of the type of temperature measurement will depend on what is being measured and where. First, the methods of temperature measurement will be reviewed. 

The liquid crystal microthermography method of temperature measurement requires that the surface of the DLJT be exposed. Therefore, this technique cannot be used on sealed or encapsulated devices without deprocessing. 

The drop or mixing method is one of the oldest tools of the thermochemist. As no fundamental change has been made in recent years a detailed description is unnecessary. However, very high temperatures have now been studied by the application of modern methods of temperature measurement and heating. For example, Fredrickson et alJ have used... 

The discussion of some general principles of temperature measurements and thermometers is summarized in Fig. 3.3. The general method of temperature measurement is easily understood. One must first bring the thermometer, a system of known thermal properties, into intimate contact with the unknown system. For a mercury-in-glass thermometer this usually means complete immersion into the unknown system. Next, equilibration must be awaited. Finally, one must check whether a temperature correction is necessary... 

Commercial products are available that employ the last two methods of temperature measurement called Tempilsticks in the former case and Seger cones in the latter case. 

T.D. McGee Principles and Methods of Temperature Measurements , Wiley, New York 1988... 

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