Resistance thermometer, metallic

If the temperature range of interest is large, say 1 to 400 K, then diode thermometers are recommended. Diodes have other advantages compared to resistance thermometers. By contrast, diode thermometers are veiy much smaller and faster. Bv selection of diodes all from the same melt, they may be made interchangeable. That is, one diode has the same cahbration cui ve as another, which is not always the case with either semiconductor or metallic-resistance thermometers. It is well known, however, that diode thermometers may rectify an ac field, and thus may impose a dc noise on the diode output. Adequate shielding is required. 

Temperature Ihe temperature in a bioreactor is an important parameter in any bioprocess, because all microorganisms and enzymes have an optimal temperature at which they function most efficiently. For example, optimal temperature for cell growth is 37 °C for Escherichia coli and 30 °C for Saccharomyces sp, respectively. Although there are many types of devices for temperature measurements, metal-resistance thermometers or thermistor thermometers are used most often for bioprocess instrumentation. The data of temperature is sufficiently reliable and mainly used for the temperature control of bioreactors and for the estimation of the heat generation in a large-scale aerobic fermentor such as in yeast production or in industrial beer fermentation. 

Most low-temperature engineering temperature measurements are made with metallic resistance thermometers, nonmetallic resistance thermometers, or thermocouples. In the selection of a thermometer for a specific application one must consider such factors as absolute accuracy, reproducibility, sensitivity, heat capacity, self-heating, heat conduction, stability, simplicity and convenience of operation, ruggedness, and cost. Other characteristics may be of importance in certain applications. 

Metal resistance thermometer Coil of fine platinum wire Low High -100 to 700... 

There are different sensors of temperature [9,19,20], but three find particularly wide application to biomedical problems. Table 2.3 summarizes the properties of various temperature sensors, and these three, including metallic resistance thermometers, thermistors, and thermocouples, are described in the following sections. 

Semiconductors thus have over a wide temperature range the opposite dependence of resistance on temperature to that of metals. Typically, one may have as many at 10 charge carriers per cubic centimeter at room temperature, and specific resistances may vary from 10 to 10ncm. Mathematically Eq. (3) expresses the change of resistance with temperature. Three constants, Roo, B, and 0, are characteristic for a particular semiconductor. The temperature coefficient of the resistivity may be ten times that of a typical metal resistance thermometer. 

For most purposes, the platinum resistance thermometer is still the first choice among metallic resistance thermometers. For measurements extending either over a broad temperature range from 1 to 300 K or for a narrower temperature range below 30 K, semiconductor and diode thermometers are the principal competitors to PRTs and are often to be preferred. 

For temperatures above about 20 K, the metallic resistance thermometers are more sensitive than the nonmetallic resistance thermometers. Temperatures above 20 K can be measured routinely with an industrial-type platinum resistance thermometer with an accuracy of better than 100 mK with time responses somewhat better than 1 s. Accuracy at the millidegree level requires a precision capsule type platinum resistance thermometer and careful calibration. 

Why it is difficult to use a pure metal resistance thermometer at very low temperatures (below approximately 15 K) ... 

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