Resistance pyrometer

The electrical resistance pyrometer depends on the principle that the electrical resistance of a conductor changes with temperature the instrument is designed to measure these changes and is calibrated in... 

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 vaiy with temperature in a simple, reproducible manner and thus can be cahbrated against known references (sometimes called secondaiy thermometers). The three dominant measurement devices used in automatic control are thermocouples, resistance thermometers, and pyrometers and are applicable over different temperature regimes. 

Feuer-gefahrlichkeit, /. (in)fiammability. -gradmesser, m. pyrometer, -gradmessung, /. pyrometry. -gnibe, /. ash pit. -baken, m. fire hook, poker, -hsut, /. (fire-resistant) lining, liner, -holz, n. firewood, feuerig, a. = feurig. 

The ITS-90 scale extends from 0.65 K to the highest temperature measurable with the Planck radiation law (—6000 K). Several defining ranges and subranges are used, and some of these overlap. Below —25 K, the measurements are based on vapor pressure or gas thermometry. Between 13.8 K and 1235 K, Tg is determined with a platinum resistance thermometer, and this is by far the most important standard thermometer used in physical chemistry. Above 1235 K, an optical pyrometer is the standard measrrremerrt instmment. The procedtrres used for different ranges are sttmmarized below. 

In this section, the design and operation of familiar liquid thermometers, thermocouples, platinum resistance thermometers, thermistors, and optical pyrometers are discussed in detail. Briefer descriptions are also given of a variety of special thermometric devices such as quartz thermometers, germanium resistance thermometers, and sihcon-diode thermometers. 

The temperature of a substance in a particular state of aggregation (solid, liquid, or gas) is a measure of the average kinetic energy possessed by the substance molecules. Since this energy cannot be measured directly, the temperature must be determined indirectly by measuring some physical property of the substance whose value depends on temperature in a known manner. Such properties and the temperature-measuring devices based on them include electrical resistance of a conductor (resistance thermometer), voltage at the junction of two dissimilar metals (thermocouple), spectra of emitted radiation (pyrometer), and volume of a fixed mass of fluid (thermometer). 

In this range (100 to 550°) the domains of pyrometry and thermometry overlap somewhat, for high-temperature resistance thermometers and thermocouples generally classed as pyrometers are often used below 550°C. These instruments will not be... 

Resistance of Indicating Instruments.— When operated at the highest safe working temperatures most base-metal couples develop a maximum electromotive force of less than 50 to 70 millivolts and the LeChatelier couple an electromotive force of about 16 millivolts. In order to measure such small electromotive forces accurately a very sensitive indicator or millivoltmeter is required. On the other hand the instrument must be robust and able to withstand rough handling to which it is necessarily more or less subjected. These conditions of mechanical robustness and of high accuracy as a pyrometer indicator are difficult to satisfy. 

Recording Pyrometry.—The pyrometers which can be made to record automatically fall under the following classifications (1) Gas, saturated vapor, and liquid thermometers (2) resistance thermometers (3) thermoelectric pyrometers (4) radiation pyrometers. 

FIGURE 5.34. Schematic view of an indirect resistance-heated sintering fiimace [5.1] (1) furnace assembly with dome, (2) bottom heat shield, (3) top heat shield, (5) heating elements, (6) element support posts, (7) element clamping posts, (8) heat shields, (9) pyrometer window, (10) cooling inlet, (11) cooling outlet. 

---