Thermometer pyrometer

Temperature is measured by such instruments as thermometers, pyrometers, thermocouples, etc., and by scales such as centigrade (Celsius), Fahrenheit, Rankine, Reaumur, and absolute (Kelvin). 

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. 

The other limit is the problem of temperature measurements. Classical temperature sensors could be avoided in relation to power level. Hence, temperature measurements will be distorted by strong electric currents induced inside the metallic wires insuring connection of temperature sensor. The technological solution is the optical fiber thermometers [35-39]. However, measurements are limited below 250 °C. For higher values, surface temperature can be estimated by infrared camera or pyrometer [38, 40], However, due to volumic character of microwave heating, surface temperatures are often inferior to core temperatures. 

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When very low temperatures, under —40° Fahr., have to bo estimated, a mercurial thermometer cannot be employed, since this metal solidities at that point in such cases, alcohol colored by some matter is used in the bulb of the instrument. On the other hand, mercury boils at about 600° Fahr., and, therefore, when, very high temperatures are to he estimated, a different instrument, termed the pyrometer, is employed, which Will be described in the sequel. 

The Pyrometer,—When the temperature is very high," such as that at which the more infusible metals malt, or of a reverberatory or wind furnace, the common thermometer is unavailable, and other instruments for estimating the heat have been invented, which aie called pyro-... 

The Broadband Radiation Thermometer (Total Radiation Pyrometer)... 

Fiber-optic thermometers can be applied up to 300°C, but are too fragile for real industrial applications. In turn, optical pyrometers and thermocouples can be used, but pyrometers measure only surface temperatures which in fact can be lower than the interior temperatures in reaction mixtures. Application of thermocouples which in case of microwaves are metallic probes, screened against microwaves, can result in arcing between the thermocouple shield and the cavity walls leading to failures in thermocouple performance. 

Look up and then describe the physical principles on which the operation of the following thermometers is based (a) semiconductors, (b) paramagnetic salts, (c) optical pyrometers, (d) thermocouples. 

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). 

No thermometer or pyrometer uses this principle of temperature measurement but scales defined by other means are supposedly in agreement with the thermodynamic scale or differ from this scale by small, known amounts. 

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... 

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. 

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