Resistance detectors

Temperature control is normally carried out using thermocouples in a stainless steel pocket. The type of thermocouple used is either a platinum resistance detector (RTD) or a thermocouple using two dissimilar metals that produce a voltage (EMF). The indicators for these thermocouples must match the probe type and grade. The positioning of the probes is very important as well as any lag (delay) in the system. The output from the probe is connected to the indicator and/or controllers. Most indicators have at least a set point with an on/off output. The more advanced units will allow anticipated switching, more than one set point, temperature ramping between temperatures, time, and hold facilities. Thermocouple break and over-temperature alarm outputs are also commonly provided features. 

Figure 7.21 The Blankenship and Borkowski nomogram that relates resistivity, detector thickness, and detector bias. The detector capacitance as a function of detector thickness is also given.

The lack of sensitivity of solid-state detectors to lower energy particles is due to the dead layer of the detector. In the high resistivity detectors, tltis dead layer is due to the thickness of the electrode which can be on the order of 1 fim. Charge carriers generated by particles that do not have sufficient energy to penetrate significantly beyond this silicon layer are trapped there. 

Considering modulated radiation of frequency a> (chopper frequency), calling the total capacity (detector and preamplifier) C, the total resistance (detector resistance and amplifier input resistance in parallel) R, and the change in polarization dP/dT = a,the voltage change per temperature change becomes... 

A PC detector has the simplest possible structure it is a homogenous semiconductor with two electrical contacts. PC detectors are relatively poor conductors whose conductivity is improved in the presence of photons. We could call these resistive detectors, but it is easiest to derive the necessary equations if we work with the formulas for conductivity, so photoconductor is descriptive and appropriate. 

Azimuthal resistivity (depth of investigation Gamma ray 12in. orless) detector... 

Thermal Conductivity Detector One of the earliest gas chromatography detectors, which is still widely used, is based on the mobile phase s thermal conductivity (Figure 12.21). As the mobile phase exits the column, it passes over a tungsten-rhenium wire filament. The filament s electrical resistance depends on its temperature, which, in turn, depends on the thermal conductivity of the mobile phase. Because of its high thermal conductivity, helium is the mobile phase of choice when using a thermal conductivity detector (TCD). 

Thermocouples, bolometers and pyroelectric and semiconductor detectors are also used. The first three are basically resistance thermometers. A semiconductor detector counts photons falling on it by measuring the change in conductivity due to electrons being excited from fhe valence band info fhe conduction band. 

There are important figures of merit (5) that describe the performance of a photodetector. These are responsivity, noise, noise equivalent power, detectivity, and response time (2,6). However, there are several related parameters of measurement, eg, temperature of operation, bias power, spectral response, background photon flux, noise spectra, impedance, and linearity. Operational concerns include detector-element size, uniformity of response, array density, reflabiUty, cooling time, radiation tolerance, vibration and shock resistance, shelf life, availabiUty of arrays, and cost. 

Fig. 12. CdS film detector in a package showing interdigitation to reduce resistance. See text.

Fig. 16. Resistance area (R ) product for HgCdTe photodiodes cooled to 77 K. The soHd line represents the theoretical limit, the dashed lines (—) and (- -) high and low performance, respectively. Dark current caused by defects lowers R and detector sensitivity. In the high performance range dark...

The fire death rate in the United States is decreasing, dropping from a rate of 76 per million in the 1940s, when most constmction and decorative products were made of natural materials, to 29 per million in the 1980s, by which time, PVC had replaced natural materials in numerous appHcations (189). This downward trend can be attributed in large part to improved building codes and the broader use of sprinkler systems and smoke detectors. However, the increased use of more fire-resistant materials, such as PVC, deserves part of the credit for this improvement. 

Thermal Methods Level-measuring systems may be based on the difference in thermal characteristics oetween the fluids, such as temperature or thermal conductivity. A fixed-point level sensor based on the difference in thermal conductivity between two fluids consists of an electrically heated thermistor inserted into the vessel. The temperature of the thermistor and consequently its electrical resistance increase as the thermal conductivity of the fluid in which it is immersed decreases. Since the thermal conductivity of liquids is markedly higher than that of vapors, such a device can be used as a point level detector for liquid-vapor interface. 

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