Resistance gauges can be made up as single elements to measuring strain in one direction only, or a combination of elements such as rosettes will permit simultaneous measurements in more than one direction. b) Unbounded strain gauges a typical unbounded-strain-gauge arrangement shows fine resistance wires stretched around supports in such a way that the deflection of the cantilever spring system changes the tension in the wires and thus alters the resistance of wire. Such an arrangement may be found in commercially available force, load, or pressure transducers. 5.3 Resistance Temperature Transducers The materials for these can be divided into two main groups: a) metals such as platinum, copper, tungsten, and nickel which exhibit and increase in resistance as the temperature rises; they have a positive temperature coefficient of resistance. b) Semiconductors, such as thermostats which use oxides of manganese, cobalt, chromium, or nickel. These exhibit large non-linear resistance changes with temperature variation and normally have a negative temperature coefficient of resistance. a) Metal resistance temperature transducers these depend, for many practical purpose and within a narrow temperature range, upon the relationship R1=R0*[1+a*(b1-b2)] where a coefficient of resistance in ℃-1, and R0 resistance in ohms at the reference temperature b0=0℃ at the reference temperature range ℃. The international practical temperature scale is based on the platinum resistance thermometer, which covers the temperature range -259.35℃ to 630.5℃. b) Thermostat resistance temperature transducers Thermostats are temperature-sensitive resistors which exhibit large non-liner resistance changes with temperature variation. In general, they have a negative temperature coefficient. For small temperature increments the variation in resistance is reasonably linear; but, if large temperature changes are experienced, special linear zing techniques is used in the measuring circuits to produce a linear relationship of resistance against temperature. Thermostats are normally made in the form of semiconductor discs enclosed in glass vitreous enamel. Since they can be made as small as 1mm, quite rapid response times are possible. 5.4 Photoconductive Cells The photoconductive cell, uses a light-sensitive semiconductor material. The resistance between the metal electrodes decreases as the intensity of the light striking the semiconductor increases. Common semiconductor materials used for photo-conductive cells are cadmium word文档 可自由复制编辑

supplied, lead supplied, and copper-doped germanium. The useful range of frequencies is determined by material used. Cadmium sapphire is mainly suitable for visible light, whereas lead supplied has its peak response in the infra-red region and is, therefore, most suitable for flame-failure detection and temperature measurement. 5.5 Photo emissive Cells When light strikes the cathode of the photo emissive cell are given sufficient energy to arrive the cathode. The positive anode attracts these electrons, producing a current which flows through resistor R and resulting in an output voltage V. Photo electrically generated voltage V=Purl Where Imp=photoelectric current(A),and photoelectric current Imp=Kat’s Where Kt=sensitivity (A/imp),and B=illumination input (lumen) Although the output voltage does give a good indication of the magnitude of illumination, the cells are more often used for counting or control purpose, where the light striking the cathode can be interrupted. 6、Capacitive Transducers The capacitance can thus made to vary by changing either the relative permittivity, the effective area, or the distance separating the plates. The characteristic curves indicate that variations of area and relative permittivity give a linear relationship only over a small range of spacing’s. Thus the sensitivity is high for small values of d. Unlike the potentiometer, the variable-distance capacitive transducer has an infinite resolution making it most suitable for measuring small increments of displacement or quantities which may be changed to produce a displacement. 7、Inductive Transducers The inductance can thus be made to vary by changing the reluctance of the inductive circuit. Measuring techniques used with capacitive and inductive transducers: a) A.C. excited bridges using differential capacitors inductors. b）A.C. potentiometer circuits for dynamic measurements. c ) D.C. circuits to give a voltage proportional to velocity for a capacitor. d） Frequency-modulation methods, where the change of C or L varies the frequency of an oscillation circuit. Important features of capacitive and inductive transducers are as follows: I) resolution infinite ii)accuracy+- 0.1% of full scale is quoted word文档 可自由复制编辑