Resistance measurements, thermometer

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. 

R = resistance, T = thermometer, ETS = measurement with electric temperature sensors Note The original tables in EN 50019 and IEC 60079-7 indicate the limiting temperatures for insulated windings (and not the limiting overtemperatures). To enable an easier comparison, these values are transformed to overtemperatures taking into account an ambient temperature of +40°C. 

If only two lead wires are carried from the thermometer coil to the indicator, the resistance measured is the sum of the resistances of the coil, the platinum or gold lead wires to the head of the thermometer, and the copper lead wires from this point to the indicator. The resistance of the platinum or gold lead wires will depend upon the form of temperature gradient along the thermometer from the bulb to the head and upon the depth of immersion. Hence, this variable resistance is introduced into the temperature measurement, and changes in the resistance of the lead wires will be interpreted as changes in the temperature of the thermometer coil. 

The ceramic material obtained by this method (sample C) is compared to those obtained by solid state reactions using either a carbonate (A) or nitrate (B) as the source of barium. Micrographs for the three samples are shown in Figure 1. Note that sample C is homogeneous with a particle size of about 1 micron whereas about 100 and 20 micron particle sizes are observed for samples A and B respectively. Resistivity measurements on the same set of samples, were performed using a standard four-probe method with silver paint contacts in an exchange gas cryostat with a Si-diode thermometer. A Tc of 91K with a transition width of 0.5K is observed for sample C whereas samples B and A exhibit widths of 2 and IK respectively. The Meissner effect was observed to be 35, 65 and 50% for samples A, B and C respectively. 

The most common are thermocouples, resistance bulb thermometers, and thermistors. All provide measurement in terms of electrical signals. Independently of their constructional differences, their basic dynamic behavior can be examined in terms of the temperature profiles in Figure 13.6a and b. The temperature-sensing element is always inside a thermowell (Figure 13.7). In the first case (Figure 13.6a) we assume... 

The principle of resistance measurement involves either a dc Wheatstone bridge, as shown in the bottom sketch of Fig. 3.4, or a potentiometric arrangement in which the voltage drop over a standard resistor in series with the thermometer is determined. The potentiometer is described in Fig. 3.5 as part of the discussion of thermocouples. The calculation of Eqs. (4) to (7) shows how the lead resistances Rq and Rj can be eliminated in precision thermometry by performing two measurements (a and b) with reversed leads coimected to the bridge circuit. The measured resistances are represented by the unknown resistance by R. ... 

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. 

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. 

The temperature gradient in the direction of flow can be measured directly with Pt-resistance thermometers, but it is difficult and expensive. When this is small, it is better to calculate from the material balance and thermochemical properties. 

Temperature gradient normal to flow. In exothermic reactions, the heat generation rate is q=(-AHr)r. This must be removed to maintain steady-state. For endothermic reactions this much heat must be added. Here the equations deal with exothermic reactions as examples. A criterion can be derived for the temperature difference needed for heat transfer from the catalyst particles to the reacting, flowing fluid. For this, inside heat balance can be measured (Berty 1974) directly, with Pt resistance thermometers. Since this is expensive and complicated, here again the heat generation rate is calculated from the rate of reaction that is derived from the outside material balance, and multiplied by the heat of reaction. 

The availability of large quantities of liquid helium as well as an excellent support staff led to the undertaking of many experiments at 8 K (the boiling temperature of helium) as well as the lower temperatures obtained by pumping. One subset was the measurement of the resistivity (conductivity) of metals, since this property was useful as a secondaiy thermometer. Although a linear decline was obseived, various speculations were made as to what the result would be when zero absolute temperature was reached. In April 1911 came the surprising discoveiy that the resistivity in mercury disappeared. At first the sur-... 

The impedance of the transducer is important if it provides an output signal to an electronic device (an amplifier, for example) and the impedance of the two must be matched for accurate measurement. Some transducers (thermocouples, for example) generate their output by internal mechanisms (i.e. they are self-excited). Others such as resistance thermometers need an external source and an appropriate type must be available. Transducers used in the measurement of the more common physical quantities are discussed below. 

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