Other Types of Thermal Detectors

Several other thermal effects have received some attention but have not received widespread use. Insofar as at frequencies less than 100 Hz the performance of some of the existing detectors approaches close to the ideal limit there seems little advantage in developing a new type of detector. There is still a large discrepancy between the performance of an ideal detector and the best achievable even with a pyroelectric detector at higher frequencies so that new developments here would be worthwhile. 

An interesting device (a solid state analogue of the Golay cell ) was developed by Jones and Richards [3.59] who used an optical lever to amplify the linear expansion of a constantan strip. With an area of 5 x 0.2 mm the noise equivalent power was 10 WHz corresponding to a D of 10 °. The response time was about 0.1 s. The D achieved places the performance within a factor of 2 of the ideal limit. 

The temperature variation of the dielectric constant has been considered several times (Moon and Steinhardt [3.60], Hanel [3.61], Maserjian [3.62]) but 

Some study has been given to the magnetic analogue of the pyroelectric effect—the pyromagnetic effect Walser et al. [3.63]). At low frequencies this could probably achieve a similar performance to the pyroelectric detector but the requirements for the magnetic Held are likely to make higher frequency operation more difficult. 

The Nernst effect in suitable semiconductors (InSb-NiSb mixed crystals, Paul and Weiss [3.64] CdjAsj-InAs, Goldsmid et al. [3.65] Bi-Sb, Washwellet al. [3.66]) has also been studied. This has produced a fairly fast rather insensitive detector which could perhaps be developed further. 

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The bolometer is another type of thermal detector that can offer extreme sensitivity for specialized applications, t This is essentially a resistance thermometer, usually with a platinum, nickel, carbon, or germanium element, although a semiconductor thermistor can also be used. Typically, two elements are used in a bridge circuit with one exposed to radiation and the other kept dark as a reference. The germanium bolometer provides exceptional... 

Newton used a liquid in glass-thermometer to study heat radiation. Rumford and Leslie used a difierential gas thermometer. Herschel reverted to the liquid thermometer, but this was soon replaced by the thermopile (Melloni [3.4]). Some time later (Langley [3.5]) the first bolometers were used. More recently the use of the gas thermometer, in the shape of the Golay [3.6] and Luft cells has been reintroduced and is now widely used in spectrometers. Another type of thermal detector now widely used is that utilizing the pyroelectric effect. In addition to these, several other detection processes have been suggested, including thermal expansion and changed dielectric properties with temperature. 

Most laminar flowmeters (also called capillary flowmeters) measure very low flow rates of gases in applications where other types of meters either give marginal performance or cannot be used at all. Commercial units use either capillary elements or matrix shapes (Figure 3.67). The pressure drop generated by these elements is in linear relationship with the process flow. When higher accuracy and rangeability is desired, thermal instead of d/p detectors are used with the laminar flow element. 

Two other types of element-specific detector for nitrogen currently in use coupled to SFCs are the nitrogen phosphorus detector (NPD) and the thermal energy analyzer (TEA). The NPD uses a hot, catalytically active solid surface immersed in a layer of dissociated H2 and O2 to form electronegative N and P ions which are detected on a nearby electrode [2]. NPD has been shown to have broad application in SFC, especially in the agrochemical industry [3]. The TEA, as described by Fine et al. [4], uses low-temperature pyrolysis, followed by ozone-induced chemiluminescence, for the detection of compounds containing NO2 groups. The TEA has been used for the determination of tobacco-specific nitrosamines and explosives [5]. Both of these detectors require spedlic standards of the analytes of interest for quantitation... 

Thermal conductivity detectors have been discussed in detail by Ingraham (107), who also described their application to thermodynamic and kinetic measurements. In this same book. Lodding (4) describes the gas density detector as well as several ionization detectors, such as the argon ionization detector, the electron capture detector, and others. Flame ionization detectors have been described in detail by Brody and Chaney (108) and Johnson (109). The latter also discusses other types of detectors. Malone and McFad-den (110) described many different types of special identification detectors, such as those listed in Table 8.3. Numerous texts on gas chromatography describe a wide variety of detectors, many of them useful in EGD and EGA. 

Heat is the most obvious choice of a characteristic by which a fire can be automatically recognized. In the section on fire suppression systems, the fusible links in the sprinkler heads represented one type of heat detector. Alloys have been developed that will have reproducible melting points. When the temperature at the detector site exceeds the melting point of the alloy, contacts are allowed to move so that the device can either make or break a circuit, just as with a manual alarm system. There are plastics which can perform in the same manner. Fixed temperature systems are very stable and not prone to false alarms, but are relatively slow to respond. There are several other versions of these fixed temperature detectors, including bimetalhc strips, where the differential rate of expansion of two different metals causes the strip to flex or bend to either make or break the contact. Others depend upon the thermal erqransion of hquids. 

Two other types of element-specific detector for nitrogen currently in use coupled to SFCs are the nitrogen phosphoms detector (NPD) and the thermal energy... 

The Universal Fast Breeder Counter (UFBC) is a thermal neutron coincidence counter designed to assay FBR fuel assemblies and other types of plutonium fuel (Menlove et al. 1984). UFBC has 7.0% efficiency using 12 He tubes surrounded by polyethylene and a thin cadmium sleeve. The uniform counting (flat response) region in the detector head (141 cm high and 30.5 cm in diameter) is 105 cm. UFBC can measure assemblies with plutonium loadings of up to 16 kg (24% Pu). 

In this type of detectors material properties are modified by heating, typically independently on the detected radiation wavelength, and this is used to generate electric output or some other type of readout signal. Table 1.3 lists some basic types of thermal IR photodetectors arranged according their detection mechanisms. 

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