Thermometer thermistor

In this section, the design and operation of familiar liquid thermometers, thermocouples, platinum resistance thermometers, thermistors, and optical pyrometers are discussed in detail. Briefer descriptions are also given of a variety of special thermometric devices such as quartz thermometers, germanium resistance thermometers, and sihcon-diode thermometers. 

The most important method of achieving temperature control is to use a sensitive thermometer that generates an electrical signal, such as a resistance thermometer, thermistor, or thermocouple. Comparison of this signal with a reference signal that establishes the set point provides an error signal to a feedback circuit that controls the power to the heater. Thns at r temperature deviation from the desired value detected by the sensor will be corrected antomatically. 

Thermistors are resistance thermometers, where the temperature-sensible element is the semiconductor, and are made of a mixture of different metal oxides. The large resistance of the thermistor enables us to lower substantially its dimensions in comparison to the resistance thermometer. Thermistors are very sensible and give a fast response, which is very suitable for use in small calorimeters. 

Other parameters pH, temperature, turbidity, conductivity Potentiometry. Digital thermometer. Thermistor. Thermocouple. Turbidlmetry. Nephelometry. Conduct imetry. 

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. 

An adiabatic calorimeter is designed to have negUgible heat flow to or from its surroundings. The calorimeter contains the phase of interest, kept at either constant volume or constant pressure, and also an electric heater and a temperature-measuring device such as a platinum resistance thermometer, thermistor, or quartz crystal oscillator. The contents may be stirred to ensure temperature uniformity. 

Other potential applications are resistance thermometers, thermistors, photoconductors, photodiodes, photoelements, solar batteries, electrical reproduction of information, electroluminescence, electrostatic storage batteries, image storage, and catalysis in chemical and biochemical systems [5]. 

Temperature measurement thermometers, thermistors, thermocouples, radiation pyrometers and bimetallic strips. 

Among nonmetallic resistance thermometers, an important class is that of thermistors, or temperature-sensitive semiconductkig ceramics (5). The variety of available sizes, shapes, and performance characteristics is very large. One manufacturer Hsts ki the catalog a choice of characteristics ranging from 100 Q at 25°C to 1 M Q at 25°C. 

Whereas it is no longer an iaterpolation standard of the scale, the thermoelectric principle is one of the most common ways to transduce temperature, although it is challenged ia some disciplines by small iadustrial platinum resistance thermometers (PRTs) and thermistors. Thermocouple junctions can be made very small and ia almost infinite variety, and for base metal thermocouples the component materials are very cheap. Properties of various types of working thermocouple are shown in Table 3 additional properties are given in Reference 5. 

In addition to the mercuiy-in-glass thermometer, other temperature-sensing elements may be used for psychrometers. These include resistance thermometers, thermocouples, bimetal thermometers, and thermistors. 

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

The pH (or pI) term of the Nemst equation contains the electrode slope factor as a linear temperature relationship. This means that a pH determination requires the instantaneous input, either manual or automatic, of the prevailing temperature value into the potentiometer. In the manual procedure the temperature compensation knob is previously set on the actual value. In the automatic procedure the adjustment is permanently achieved in direct connection with a temperature probe immersed in the solution close to the indicator electrode the probe usually consists of a Pt or Ni resistance thermometer or a thermistor normally based on an NTC resistor. An interesting development in 1980 was the Orion Model 611 pH meter, in which the pH electrode itself is used to sense the solution temperature (see below). 

For example, if we want a thermometer overheating AT/T less than 1%, with a contact surface A = 10 4m2, the power P(T) to be supplied to the thermistor must be ... 

The noise thermometer is based on the temperature dependence of the mean square noise voltage V2 developed in a thermistor (Nyquist theorem, 1928) ... 

Up to now, in the formulation of a bolometer model, only the heat capacity of itinerant carriers was considered [57], However, our measurements show that, even at 24 mK, the presence of a spurious heat capacity in the thermometer increases the expected value of the pulse rise time. We expect that the spurious contribution in Fig. 12.17 increases down to the temperature of the Schottky peak at T = k.E/khT about 10 mK. Since gc decreases at low temperatures, the total effect on pulse rise time and pulse amplitude can be dramatic at lowest temperatures. In reality, the measured rise time of CUORICINO pulses is about three times longer than that obtained from a model which neglects the spurious heat capacity of the thermistor. For the same reason, also the pulse amplitude is by a factor two smaller than the expected value (see Section 15.3.2). 

In a cryogenic experiment, one or several detectors are used for a definite goal for which they have been optimized. For example, in CUORE experiment described in Section 16.5, the sensors are the Ge thermistors, i.e. thermometers used in a small temperature range (around 10 mK). One detector is a bolometer made up of an absorber and a Ge sensor. The experiment is the array of 1000 bolometers arranged in anticoincidence circuits for the detection of the neutrinoless double-beta decay. Note that the sensors, if calibrated, could be used, as well, as very low-temperature thermometers. Also the array of bolometers can be considered a single large detector and used for different purposes as the detection of solar axions or dark matter. 

Given this situation, it is not surprising that, over the last few years, products based on platinum thin film technology have been finding their way into the home. With the growing use of electronic control systems in the new generation of domestic appliances, platinum temperature sensors have been more widely used in ovens where they have replaced electromechanical regulators such as capillary tubes, solid expansion thermometers and NTC thermistors. Typical sensor applications in the food preparation sector are shown in Fig. 5.3. 

Temperature Recording. Use an accurate temperature-sensing device, such as a clinical thermometer or thermistor or similar probe, that has been calibrated to ensure an accuracy of 0.1° and has been tested to determine that a maximum reading is reached in less than 5 min. Insert the temperature-sensing probe into the rectum of the test rabbit to a depth of not less than 7.5 cm and, after a period of time not less than that previously determined as sufficient, record the rabbit s temperature. 

S. R. Gunn. On the Calculation of the Corrected Temperature Rise in Isoperibol Calorimetry. Modifications of the Dickinson Extrapolation Method of Treatment of Thermistor-Thermometer Resistance Values. J. Chem. Thermodynamics 1971, 3, 19-34. 

New types of thermometers include sensitive and reliable thermistors and quartz crystal thermometers. 

The change in resistivity with temperature is used in thermistors, which can be used as thermometers and in fire alarm circuits. 

Bead thermistors are formed by placing two wires, commonly of platinum, in dose proximity and paralld to each other and bridging them with a drop of slurry, which is then sintered into a hard bead and encapsulated in protective glass. Such thermistors are quite stable, approaching, over narrow temperature limits, the stability of industrial metallic thermometers. However, the resistance tolerance may vary from unit to unit by as much as 20%, and matching or interchangeability is usually achieved by selection. Beads can be made quite small, which may allow application in, eg, temperature probes mounted in intravenous needles. 

However, it is now recommended that thermometers be calibrated in °C. More recently, thermistors have been used instead of mercury thermometers. Cryoscopes based on dew point depression have also been approved for use. These latter instruments also use thermistors and are based on changes in osmotic pressure. Thermistor cryoscopes are now used more widely than Hortvet instruments. 

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. 

Thermistor and resistance thermometer elements Thermoelectric-bolometric sensors Semiconductor-based elements... 

The calorimetric thermometer measures temperature changes within the calorimeter bucket. It must be able to provide excellent resolution and repeatability. High single-point accuracy is not required since it is the change in temperature that is important in fuel calorimetry. Mercurial thermometers, platinum resistance thermometers, quartz oscillators, and thermistor systems have all been successfully used as calorimelric thermometers. 

There are two principal classes of this type of sensor, viz. (i) resistance thermometers (resistance temperature detectors)—which are constructed from normal metallic conducting materials, and (ii) thermistors—which are bulk semiconductor sensors. 

Fig. 1.28. Cryoscopy cell, (a) Typical dimensions of the lower section are 260-mm height and 20-mm diameter. The thermistor well is about 27 mm. (b) Cooling apparatus for the cryoscopy cell. The cryoscope slips into the jacketed tube. The jacket is evacuated or partially evacuated to control the cooling rate. Electrical readout is achieved with a digital multimeter or commercial electronics for thermistor thermometers.

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