Thermometry sensor

The resistance thermometry is based on the temperature dependence of the electric resistance of metals, semiconductors and other resistive materials. This is the most diffused type of low-temperature thermometry sensors are usually commercial low-cost components. At very low temperatures, however, several drawbacks take place such as the low thermal conductivity in the bulk of the resistance and at the contact surface, the heating due to RF pick up and overheating (see Section 9.6.3)... 

Fig. 9.20. Normalized conductance G/GT of a coulomb blockade thermometry sensor versus bias voltage V.

Similar to the catalyst of the catalytic thermometry sensor, the catalytic activity of the CTL-based sensor depends not only on the kind of catalyst material and the surface-to-volume ratio of the powder but also on the preparation procedure of the powder. In considering these conditions, a detailed comparison of the CTL catalytic activity has not been reported so far. The present authors and coworkers observed the CTL emission by ethanol vapor on y-aluminum oxide, barium sulfate, calcium carbonate, and zirconium oxide at a few hundred degrees. On the other hand, CTL emission is not observed during the catalytic oxidation on metal and semiconductive materials, e.g., tin oxide, zinc oxide, and copper oxide. 

A variety of fiber optic thermometry systems using fluorescence sensors have been discussed or become available over the past years. Most of the earliest systems are based on the temperature-dependent fluorescence intensity of appropriate materials. One such example of an early commercial system is the Luxtron model 1000, shown in Figure 11.2, which utilized europium-activated lanthanum and gadolinium... 

Perhaps the first detailed discussion of such a technique in fluorescent thermometry (shown in Figure 11.10) was given by Zhang et al. in their work(36) based on both mathematical analysis and experimental simulation. Examples of the electronic design of the corresponding system and the application of the technique in a ruby fluorescence-based fiber-optic sensor system are also listed. This shows that there is no difference in the measurement sensitivity between a system using square-wave modulation and one using sinusoidal modulation. However, the former performs a little better in terms of the measurement resolution. 

J. P. Dakin, D. J. Pratt, G. W. Bibby, and J. N. Ross, Distributed anti-Stokes ratio thermometry, 3rd Intern, Conf. on Optical Fiber Sensors, Postdeadline Session, San Diego, February 1985. 

Phosphor-based temperature sensors are adaptable to the needs for a wide variety of situations and are based on fluorescence material properties. The thermal dependence of fluorescence may be exploited to provide a noncontact, emissivity-independent optical alternative to other more conventional techniques, such as those employing pyrometry, thermocouples, or thermistors. In fact, there are certain situations where fluorescence-based thermometry is the only useful approach, such as in devices like turbines or engines. 

Resistive materials used in thermometry include platinum, copper, nickel, rhodium-iron, and certain semiconductors known as thermistors. Sensors made from platinum wires are called platinum resistance thermometers (PRTs) and, though expensive, are widely used. They have excellent stability and the potential for high-precision measurement. The temperature range of operation is from -260 to 1000°C. Other resistance thermometers are less expensive than PRTs and are useful in certain situations. Copper has a fairly linear resistance-temperature relationship, but its upper temperature limit is only about 150°C, and because of its low resistance, special measurements may be required. Nickel has an upper temperature limit of about 300°C, but it oxidizes easily at high temperature and is quite nonlinear. Rhodium-iron resistors are used in cryogenic temperature measurements below the range of platinum resistors [11]. Generally, these materials (except thermistors) have a positive temperature coefficient of resistance—the resistance increases with temperature. 

Resistance Measurement. The common methods of resistance measurement in resistance thermometry are the bridge method and the potentiometric method. Basically, the bridge method uses the resistance sensor together with a variable resistor and two fixed resistors to form the four legs of a conventional Wheatstone bridge circuit. On the other hand, the potentiometric method, also called a half bridge, connects the resistance sensor in series with a known resistor. 

The detection methods used include spectrophotometry, chemiluminescence, fluorescence, amperometry, conductometry, thermometry and potentiometry with ion-selective electrodes or gas sensors. We have focused our attention only on the electrochemical detectors. Some examples of applications of reactor biosensors with the specification of enzyme used, reactor type and detection system are summarized in Table 5. 

Infrared thermometers are used where nonintru-sive sensors are needed or where other electromagnetic fields might interfere with thermocouples. Infrared thermometry is used to capture the change in temperature due to the change in skin friction between laminar and turbulent regions of the flow in the boundary layer over the skin of the space shuttle to determine if there are regions where the flow has separated because of missing tiles or protuberances. 

Almost all catalytic gas transducers currently used employ resistance thermometry as the temperature sensor. This section will deal only with this type of transducer, although the general principles are applicable to all types of transducer. 

In the fiber-optic thermometry probe technique, a temperahue sensor, consisting of a small amount of a temperature-sensitive material (manganese-activated magnesium fluorogermanate), is mounted on the end of a probe and is placed on the surface of the device under test (DUT). A filtered xenon flash lamp provides a blue-violet light to excite the phosphor on the probe to fluoresce. When excited by this wavelength of light, the phosphor in the sensor exhibits a deep red fluorescence. 

Plasma/electroluminescent display Thermometry Higher resolution, lower power requirement, low voltage operation Noncontact temperature sensing based on the change in the excitation and emission spectra, decay Hfetime and intensity of luminescence with temperature. Unlike infiaied temperature sensors, the measurements are not influenced by the infrared absorption by glass, water, gas, biological cells, and other materials... 

The form of SThM most relevant to the subject of this discussion is carried out using near-field electrical resistance thermometry, and this method has been adopted in the work reported in this chapter. This is because miniaturized resistive probes have the considerable advantage that they can be used both in passive mode as a thermometer and as an active heat source. This enables local thermal analysis (L-TA see text below) as well as SThM to be carried out. At present the most common type of resistive probe available is the Wollaston or Wollaston Wire probe, developed by Dinwiddle et al. (1994) and first used by Balk et al. (1995) and Hammiche et al. (19%a) The construction details of this probe are illustrated in Fig. 7.3. A loop of 75-pm-diameter coaxial bimetallic Wollaston wire is bent into a sharp V-shaped loop. The wire consists of a central 5-pm-diameter platinum/10% rhodium alloy core surrounded by silver. The loop is stabilized with a small bead of epoxy resin deposited approximately 500 pm from its apex. The probe tip or sensor is made... 

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