Thermometers secondary

Miscellaneous. Ruthenium dioxide-based thick-film resistors have been used as secondary thermometers below I K (92). Ruthenium dioxide-coated anodes ate the most widely used anode for chlorine production (93). Ruthenium(IV) oxide and other compounds ate used in the electronics industry as resistor material in apphcations where thick-film technology is used to print electrical circuits (94) (see Electronic materials). Ruthenium electroplate has similar properties to those of rhodium, but is much less expensive. Electrolytes used for mthenium electroplating (95) include [Ru2Clg(OH2)2N] Na2[Ru(N02)4(N0)0H] [13859-66-0] and (NH 2P uds(NO)] [13820-58-1], Several photocatalytic cycles that generate... 

Secondary thermometers must be calibrated by means of a primary thermometer or at fixed points as discussed in Chapter 8. Secondary thermometers are often quite easy to use and more sensitive than primary thermometers. A typical example of secondary thermometer is the electrical resistance thermometer (see Section 9.6). 

The vapour pressure of a liquid depends on the kind of liquid and changes rapidly with temperature (see Section 2.4). Hence, there is the possibility of realizing convenient secondary thermometers, which were widely used before the advent of commercially calibrated resistance thermometers. 

Measurement of the hotness or coldness of a body or fluid is commonplace in the process industries. Temperature-measuring devices utilize systems with properties that vary with temperature in a simple, reproducible manner and thus can be calibrated against known references (sometimes called secondary thermometers). The three dominant measurement devices used in automatic control are thermocouples, resistance thermometers, and pyrometers, and they are applicable over different temperature regimes. 

In principle, any device that has one or more physical properties uniquely related to temperature in a reproducible way can be used as a thermometer. Such a device is usually classified as either a primary or secondary thermometer. If the relation between the temperature and the measured physical quantity is described by an exact physical law, the thermometer is referred to as a primary thermometer otherwise, it is called a secondary thermometer. Examples of primary thermometers include special low-pressure gas thermometers that behave according to the ideal gas law and some radiation-sensitive thermometers that are based upon the Planck radiation law. Resistance thermometers, thermocouples, and liquid-in-glass thermometers all belong to the category of secondary thermometers. Ideally, a primary thermometer is capable of measuring the thermodynamic temperature directly, whereas a secondary thermometer requires a calibration prior to use. Furthermore, even with an exact calibration at fixed points, temperatures measured by a secondary thermometer still do not quite match the thermodynamic temperature these readings are calculated from interpolation formulae, so there are differences between these readings and the true thermodynamic temperatures. Of course, the better the thermometer and its calibration, the smaller the deviation would be. 

Well-designed low-pressure gas thermometers can be used to determine (really approximate) the thermodynamic temperature. However, from a practical standpoint, where precision and simplicity in the implementation and transfer are the major considerations, secondary thermometers were chosen as the defining standard thermometers for a practical temperature scale. This scale was defined by the use of fixed reference points whose thermodynamic temperatures were determined from gas thermometer measurements. The International Committee of Weights and Measures (Comite International des Poids et Mesures, CIPM) is responsible for developing and maintaining the scale. 

Primary thermometer Secondary thermometer Defining standard thermometer... 

Almost all thermometers used in practical applications belong to the class of secondary thermometers. They can be categorized according to their temperature-dependent function, represented by the type of transducer or output they employ. They include systems whose temperature is indicated by ... 

FIGURE 16.4 Temperature ranges for commonly used secondary thermometers. 

An mtemationaUy agreed upon secondary, empirical temperature scale is described in Fig. A.8.1 It is called the International Temperature Scale of 1990, in short, the ITS 1990 (Tjo in kelvin, K tgo in degrees Celsius, °C). It makes use of the higher precision possible with secondary thermometers. The given fixed points have been agreed upon internationally to achieve as close a match to the kelvin scale as possible. The values for the fixed points are to be corrected when better absolute measurements become available. Earher international temperature scales were accepted in 1927 (ITS 27), 1948 (ITS and IPTS 48), and 1968 (IPTS) (IPTS is the abbreviation for International Practical Temperature Scale , the second version of the 1948 scale and the 1968 scale). 

Several years ago investigations of carbon and (As-doped) germanium resistors for use as precise secondary thermometers indicated appreciable effects of stray rf fields upon the measured dc resistance. The rf pickup was eliminated by shielding the dc measuring apparatus with a double wall screen cage. Recently, Corruccini [3] has found that the resistance change due to rf pickup is caused by ac joule heating. 

The national laboratories of several countries, including the National Institute of Standards and Technology in the United States, maintain stable secondary thermometers (e.g., platinum resistance thermometers and thermocouples) that have been calibrated according to the ITS-90. These secondary thermometers are used as working standards to calibrate other laboratory and commercial temperature-measuring devices. 

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