Thermistor constant

A plot of the conductivity of the thermistor material MgGaMn04 is given in the following table. Estimate the value of the thermistor constant (3. 

NTC thermistor usually consists of transition metals (Cu, Fe, Co, Ni, etc.) spinel manganites. The conductivity is due to the transfer of electrons between Mn + and Mn" + ions. The resistance and thermistor constant is dependent on the composition, purity, cation distribution, and crystal structures. 

NTC thermistors are used as temperature compensation, temperature sensing, and surge current suppression devices. All of these applications are based on the resistance-temperature characteristics of NTC thermistors. Although various thermistor constant B and resistivity are required for many applications, these values are obtained within the limits as illustrated in Figure 2.1.7, because the thermistor constant B is dependent on the resistivity. 

FIGURE 2.1.7 Relationship between thermistor constant and resistivity. 

Temperature (°C) 0.2 °C (thermistor thermometer) 0.5 °C (liquid-in-glass thermometer) Constant for 3 consecutive readings 0.2 °C... 

As a first example, let us consider a metallic thermistor inserted in fig. 3, whose resistance is, in a first approximation, expressed as R(T)=Ro(l+aT). R(T) is the resistance of a PTC thermistor at a given temperature T, Ro is the resistance at To, and I represents a suitable DC (or AC current), while A is the constant gain of a low noise amplifier, operating in a suitable bandwidth. Let us suppose that the injected current I does not induce, through the heating process, a detectable change of the resistance value. 

Figure 11.1a shows a scheme of a widely used reaction vessel for isoperibol titration calorimetry [211]. It consists of a silvered glass Dewar A, which can be adjusted to a lid B supporting a stirrer C, a resistance D for electrical calibration, a thermistor E for temperature measurement, and a Teflon tube F for titrant delivery. The assembled Dewar and lid set-up is immersed in a constant... 

T = temperature in Kelvin, R = thermistor resistance, A, B and C fitting constants)... 

The thermocouple utilizes the Seebeck effect. Copper and constantan are the two metals most commonly used and produce an essentially linear curve of voltage against temperature. One of the junctions must either be kept at a constant temperature or have its temperature measured separately (by using a sensitive thermistor) so that the temperature at the sensing junction can be calculated according to the potential produced. Each metal can be made into fine wires that come into contact at their ends so that a very small device can be made. 

Next the time to reach the maximum signal and the relative peak area were determined as a function of the duration of the shutter opening (Figure 6). Relative peak area is almost constant after 1 s, whereas the time to reach the maximum amplitude becomes constant beyond 6 s. These findings are consistent with the presence of barriers to heat transfer in the DSC itself and between sample pan and the thermistor, which delay the transfer of the heat rather than changing the total amount of heat detected. 

Experimentally, AT is determined for approx, five different polymer concentrations. After several minutes, a constant temperature difference AT of the two drops is reached which is proportional to their initial difference in vapor pressure and thus proportional to the number of dissolved macromolecules in the solution drop. AT can then be determined by measuring the difference in electric resistance of the two thermistors. Then, ATIKc is plotted vs. c (thus the power law series is broken after the linear term in c) and the plotted values are extrapolated to c 0. Mj, is finally calculated from they axis intercept. 

Vapor Pressure Osmometry. VPO is a very practical method for determining Mn values in a wide range of solvents and temperatures. Recently, results obtained with classical pendant-drop instruments showed a significant dependence of the calibration constant upon the molecular weight of the standards (8,9). On the other hand, with an apparatus equipped with thermistors allowing the volume of the drops to be kept constant, this anomaly is not observed (10,11). 

Parylenes are superior candidates for dielectrics in high quality capacitors. Their dielectric constant and loss remain constant over a wide temperature range. The thermistor sensing probe of a disposable bathythermograph is coated with parylene. This instrument is used to chart the ocean water temperature as a function of depth. 

Flow-through conductivity sensors suitable for insertion in pipelines (see Fig. 6.47a) are now available for use at temperatures up to 480 K. and pressures up to 1700 kN/m2(64). As conductivity is temperature sensitive, a thermistor is usually included in the detector circuit as part of a temperature compensator. Screw-in cells (Fig. 6.476) will withstand higher pressures. More recently, electrodeless methods of measuring conductivity have become available. In this case the solution is placed between two energised toroids. The output voltage of the instrument (from the output toroid circuit) is proportional to the conductivity of the solution provided that the input voltage remains constant. This type of conductivity meter can be used under much more severe conditions, e.g. with highly corrosive or dirty systems 43 . 

Since the thermal events observed calorimetrically contain both chemical and nonchemical components, all extraneous thermal effects must be subtracted from this composite of thermal events to obtain the relevant chemical reaction heat. Nonchemical thermal effects result from stirring, thermistor heating, heat transfer between the reaction vessel and the constant-temperature bath, and titrant/titrate temperature mismatch. Chemical thermal effects result from evaporation, dilution of the reactants, and chemical reaction heat. Details of the data reduction and correction for extraneous heat effects are described by Winnike (1989). 

Thermal sensor array systems for medical-analytical purposes were developed early [7]. Temperature distributions and blood perfusion in the brain were measured with thin film thermistors exhibiting a temperature resolution of 0.1 mK with a time constant of few milliseconds. 

Thermistors have the desirable characteristics of small size, narrow spans, fast response (their time constant can be under 1 second), and a very high sensitivity. They do not need a cold-junction compensation, errors due to contact or lead-wire resistance are insignificant, and they are well suited for remote temperature sensing. They are inexpensive, their stability increases with age, and they are the most sensitive differential temperature detectors available. 

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