Temperature measurement resistance measurements

Table 6.20 Limiting temperatures for insulated windings (for stalled motors) (referring to an ambient temperature of +40°C, and to the end of time fE). Method of temperature measurement resistance...

Determining the time constant of the sensor is usually more difficult than estimating the repeatability. To determine the time constant of the sensor system, one needs to know the actual process measurement. Consider a temperature measurement. A measurement of the actual process temperature is required to estimate the time constant of a sensor. Instead, the thermal resistance, which causes the excessive thermal lag of the temperature sensor, can be evaluated. The location of the thermowell should be checked to ensure that it extends far enough into the line that the fluid velocity past the thermowell is sufficient the possibility of buildup of insulating material on the outside of the thermowell should be assessed, and the thermal contact between the end of the temperature probe and the thermowell walls should be evaluated. In this manner, an indirect estimate of the responsiveness of the temperature sensor can be developed. The velocity of a sample in the line, which delivers a sample from a process line to a GC, can indicate the transport delay associated with the sample system. A low velocity in the sample line from the process stream to a GC can result in excessive transport delay, which can greatly reduce controller effectiveness. 

Fig. 23. Cr-Cu-Fe. Composition of (Cu) solid solution in alloys quenched from aging temperature after resistivity measurements...

Some of the most interesting and esoteric recent research is on low-temperature electrical resistivity. Measurements are frequently coupled with NMR, EPR and other newer experimental techniques. In combination with the phase, structural, surface and electronic results discussed earlier, results of these studies should give the reader a more clearly defined picture of hydride properties and their temperature dependence in the MHj-MHj composition range. 

Knoop developed an accepted method of measuring abrasive hardness using a diamond indenter of pyramidal shape and forcing it into the material to be evaluated with a fixed, often 100-g, load. The depth of penetration is then determined from the length and width of the indentation produced. Unlike WoodeU s method, Knoop values are static and primarily measure resistance to plastic flow and surface deformation. Variables such as load, temperature, and environment, which affect determination of hardness by the Knoop procedure, have been examined in detail (9). 

K. T. Hartwig, "An Eddy-Current Decay Technique for Low Temperature Resistivity Measurements," in G. Birnbaum and G. Eree, eds.,ASTM STP 722, American Society for Testing and Materials, Philadelphia, Pa., 1981, pp. 157—172. 

Resistivity measurements of doped, alpha-siUcon carbide single crystals from —195 to 725°C showed a negative coefficient of resistivity below room temperature, which gradually changed to positive above room temperature (45). The temperature at which the changeover occurred increased as the ionization of the donor impurity increased. This is beUeved to be caused by a change in conduction mechanism. 

Measurement of the hotness or coldness of a body or fluid is commonplace in the process industries. Temperature-measuring devices utilize systems with properties that vaiy with temperature in a simple, reproducible manner and thus can be cahbrated against known references (sometimes called secondaiy thermometers). The three dominant measurement devices used in automatic control are thermocouples, resistance thermometers, and pyrometers and are applicable over different temperature regimes. 

As normally used in the process industries, the sensitivity and percentage of span accuracy of these thermometers are generally the equal of those of other temperature-measuring instruments. Sensitivity and absolute accuracy are not the equal of those of short-span electrical instruments used in connection with resistance-thermometer bulbs. Also, the maximum temperature is somewhat limited. 

Temperature The level of the temperature measurement (4 K, 20 K, 77 K, or higher) is the first issue to be considered. The second issue is the range needed (e.g., a few degrees around 90 K or 1 to 400 K). If the temperature level is that of air separation or liquefact-ing of natural gas (LNG), then the favorite choice is the platinum resistance thermometer (PRT). Platinum, as with all pure metals, has an electrical resistance that goes to zero as the absolute temperature decreases to zero. Accordingly, the lower useful limit of platinum is about 20 K, or liquid hydrogen temperatures. Below 20 K, semiconductor thermometers (germanium-, carbon-, or silicon-based) are preferred. Semiconductors have just the opposite resistance-temperature dependence of metals—their resistance increases as the temperature is lowered, as fewer valence electrons can be promoted into the conduction band at lower temperatures. Thus, semiconductors are usually chosen for temperatures from about 1 to 20 K. 

This is the preferred method. The temperature of the winding is determined by observing the increase in resistance of the winding with respect to the cold resistance measured. 

The resistance must be measured with extreme care and accuracy, since a small error in measuring the resistance will cause a much larger error in determining the temperature rise. When the temperature of the winding is to be determined by the resistance, the temperature of the winding before the test, measured either by thermometer or by ETD, may be considered as the cold temperature for the resistance measured. The machine must be left cold for at least 12 to 24 hours, depending upon the size of the machine, to obtain a stable reading. 

The resistance method gives an average temperature of the whole winding. Some parts will be hotter than others usually the end turns will be somewhat cooler than parts of the winding in the middle of the iron core. NEMA committee members have been collecting test data on many machines to determine the correlation between temperature measurements by detector and by resistance, and the standards are periodically updated to reflect any of the technology improvements. 

Song et al. [16] reported results relative to a four-point resistivity measurement on a large bundle of carbon nanotubes (60 um diameter and 350 tm in length between the two potential contacts). They explained their resistivity, magnetoresistance, and Hall effect results in terms of a conductor that could be modeled as a semimetal. Figures 4 (a) and (b) show the magnetic field dependence they observed on the high- and low-temperature MR, respectively. 

Data-based (DDC) or programmable (PLC) controllers with universal inputs and outputs can be used. It is essential that they are configured before use. In some cases the input may be used only for temperature measurement from special types of thermistors. (Thermistors are constructed from semiconductor materials where the resistance changes reversibly proportional to the temperature, i.e., a negative temperature coefficient.)... 

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