电子信息类专业英语,http:/www.cmpbook.com,机械工业出版社同名教材配套电子教案,温丹丽 高源 制作,Reading Material Temperature measurement,Text 1. Thermocouples Thermoelectricity（热电，温差电）was discovered by Seebeck in 1821. A thermocouple consists basically of two dissimilar metals, such as iron and constantan wires, joined to produce a thermal electro-motive force（热电动势）when the junctions are at different temperature. The measuring, or hot, junction is inserted into the medium where the temperature is to be measured. The reference, or cold, junction is the open end that is normally connected to the measuring instrument terminals. The e.m.f. of a thermocouple increases as the difference in junction temperature increase. Therefore, a sensitive instrument, capable of measuring e.m.f. can be calibrated and used to read temperature directly.,机械工业出版社 http:/www.cmpbook.com,Reading Material Temperature measurement,To make accurate temperature measurements with thermocouples, the reference junction temperature must remain constant. If it varies, suitable compensation for these variations must be provided. Should there be an uncompensated variation in the reference junction temperature; there will be a corresponding change in millivolt a resultant error in temperature measurement.,Reading Material Temperature measurement,When used in the laboratory and for other checking purposes, the thermocouple reference junction can be placed in a vacuum bottle filled with shaved ice saturated with water. This method provides close temperature control (within a fraction of a degree) and permit accurate reading. To ensure accurate readings, most thermocouples are now installed with instruments that provide automatic reference junction compensation（补偿）. In most instruments, this is accomplished by passing current through a temperature-responsive resistor, which measures the variations in reference temperature and automatically provides the necessary compensating e.m.f. by means of the voltage drop produced across it.,Reading Material Temperature measurement,2. Resistance Temperature Detectors（RTDs） Resistance thermometry is based on the change of electrical conductivity with the temperature. Therefore, a coil of wire can act as a temperature sensor（传感器）, with a direct relationship established between resistance and temperature. Standard curves are available, with certified accuracy within 0.1 or . Platinum RTDs used as laboratory standards can be obtained with tolerances （容限）well within this limit, and are capable of precise temperature measurement up to 1650是 or 900. If a RTD is adjusted to conform to its curve, it may be interchanged with other RTDs calibrated（标定，校准） according to the some curve.,Reading Material Temperature measurement,One of the most popular circuits used with RTDs is Wheatstone bridge（惠斯登电桥）. Fig.5-1 shows a simplified circuit diagram of Wheatston Bridge. The resistance temperature detector (RTD) is one arm of a Wheatstone bridge excited by a DC power supply. Point A and Point B form the input to the amplifier. When the temperature changes, the resistance of the RTD will change. This unbalances the bridge and creates an error signal between points A and B, which is amplified by the amplifier.,Reading Material Temperature measurement,With all resistance-temperature measurements, the use of three-conductor RTD cable is recommended. The effect of ambient（周围的）temperature variations on the cable is thereby minimized. One of the conductors is common to both sides of the bridge while the other two connect one to each side of the bridge. Any change in cable temperature will be cancelled as both sides of the bridge are changed a like amount. In practice, a RTD may be used with as much as 500 feet of three-conductor cable without creating a perceptible（可察觉的）error.,Class is over!,Have a rest!,追 求,机械工业出版社 http:/www.cmpbook.com,