Temperature measurement thermistors

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.)... 

Thermistors are temperature-dependent resistances, normally constructed from metal oxides. The resistance change with temperature is high compared with the metallic resistances, and is usually negative the resistance decreases with temperature increase. The temperature characteristics are highly nonlinear. Such thermistors, having a negative temperature coefficient, are called NTC thermistors. Some thermistors have a positive temperature coefficient (PTC), but they are not in common use for temperature measurement. 

For isothermal measurements, it is advisable to use a furnace of low thermal capacity unless suitable arrangements can be made to transport the sample into a preheated zone. The Curie point method [132] of temperature calibration is ideally suited for microbalance studies with a small furnace. A unijunction transistor relaxation oscillator, with a thermistor as the resistive part with completion of the circuit through the balance suspension, has been suggested for temperature measurements within the limited range 298—433 K [133]. 

In clinical settings core temperature measurements, including pulmonary artery and esophagus measurements, are often required. In 1959 Benzinger [1] first proposed the human tympanic membrane as the ideal site for core temperature measurements. The tympanic membrane is ideal, because it is located near the carotid artery and shares its blood supply with the hypothalamus, which controls body temperature. First temperature measurements in the ear were performed with thermistor sensors in direct contact with the tympanic membrane. The invasiveness of this method limited its use mainly to anaesthetized patients. 

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... 

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. 

Metal Oxide-Polymer Thermistors. The variation of electrical properties with temperature heretofore described can be used to tremendous advantage. These so-called thermoelectric effects are commonly used in the operation of electronic temperature measuring devices such as thermocouples, thermistors, and resistance-temperature detectors (RTDs). A thermocouple consists of two dissimilar metals joined at one end. As one end of the thermocouple is heated or cooled, electrons diffuse toward... 

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. 

Thus, the sensitivity of a thermistor is a quadratic function of operating temperature. The optimum sensitivity is typically 4% °C 1. Because of their nonlinear response, thermistors are sometimes linearized by placing a resistor of similar nominal value in parallel with the thermistor, with a resulting loss of sensitivity. This is not generally necessary for thermochemical sensors, particularly for the push-pull applications, because the temperature range involved is small. For direct temperature measurement, the detection limit of 10 4oC can be achieved with a conventional Wheatstone bridge. 

One source of error in this method (ASTM D-3286) is in temperature measurement. If a mercury-in-glass thermometer is used, it must be calibrated accurately and consistent readings must be made. Many calorimeters are equipped with digital thermometers with thermistor probes and microprocessors to control the firing and record the temperatures at prescribed intervals. This alleviates most of the human error in recording the temperature changes. 

Although the temperature measurement ranges of thermistors are fairly wide, -200 to 300°C (-300 to 600°F), their spans can be rather narrow 1°C (2°F). These are high-resistance sensors 5,000-1,000,000 (1, with 250 ft/degree sensitivity and about 2% of span error. 

Figure 5. Temperature measuring circuit (thermistor bridge) and adiabatic cell for enthalpimetric analysis...

An osmometer is an instrument which measures the osmolality of a solution, usually by determining the freezing point depression of the solution in relation to pure water, a technique known as cryoscopic osmometry. A small amount of sample is cooled rapidly and then brought to the freezing point (Fig. 6.1), which is measured by a temperature-sensitive thermistor probe calibrated in mosmol kg . An alternative method is used in vapour pressure osmometry, which measures the relative decrease in the vapour pressure produced in the gas phase when a small sample of the solution is equilibrated within a chamber. 

Figure 1. On the basis of this observation, and in the absence of skin (< 1 mm) temperature measurements, sea surface radiance was calculated from 2.5-m temperatures and used in the algorithm to correct satellite temperatures to sea surface temperatures. This approach worked well in this study because the relative distribution of 2.5-m temperatures was correlated with the relative distribution of skin temperatures. This conlcusion was supported in two ways (1) the mean difference between surface temperature measured in samples collected in a bucket and the thermistor at 2.5 m was 0.1 °C, and (2) when 2.5-m temperatures were compared with cor-...

The earlier investigations employed several different types of plexiglas constructions containing the immobilized enzyme column. These devices were thermostated in a water bath, and the temperature at the point of exit from the column was monitored with a thermistor connected to a commercial Wheatstone bridge. The latter was constructed for general temperature measurements and osmometry. Later, we developed more sensitive instruments for temperature monitoring indigenously the water bath was replaced by a carefully temperature-controlled metal block, which contained the enzyme column. The enzyme thermistor concept has been patented in several major countries. 

In addition to reaction chambers and delivery systems, a number of supervising and sensor systems are of utmost importance for control and safety reasons. Sensors in automated workstations include measurement of temperature (thermocouple, thermistor, semiconductor), pressure, liquid flow and gas or liquid level. To monitor the presence or absence of vessels or devices, systems like capacitance, inductivity, ultrasonic monitors, magnetic sensors or optical sensors (reflective, beam interruption, color) can be integrated in automated workstations. 

In conclusion, the last 10 years has seen a greatly increased interest in temperature measurement, particularly in the use of thermistors in everything from clinical thermometers, thermal dilution catheters, water-bath regulators, hand-held digital thermometers, and, finally, an absolute temperature standard. We can look forward in the next 10 years to bringing temperature standardization to all areas of chemistry, biochemistry and clinical chemistry, as well as to the pasteurization and virus kill point for vaccines. For the present recommendations in biochemistry, see Expert Panel on Enzymes (1975). 

Many different thermometers have been used in temperature measurements. The most common types are those in which the temperature-dependent measured variable is (1) volume or length of a system, as with liquid-in-glass thermometers, (2) electrical resistance (platinum and other resistance thermometers, including thermistors), (3) electromotive force (EMF)—particularly as used in thermocouples, and (4) radiation emitted by a surface, as in various types of pyrometers that are used primarily with high-temperature systems. These thermometers as well as some others will be described below. 

Resistive materials used in thermometry include platinum, copper, nickel, rhodium-iron, and certain semiconductors known as thermistors. Sensors made from platinum wires are called platinum resistance thermometers (PRTs) and, though expensive, are widely used. They have excellent stability and the potential for high-precision measurement. The temperature range of operation is from -260 to 1000°C. Other resistance thermometers are less expensive than PRTs and are useful in certain situations. Copper has a fairly linear resistance-temperature relationship, but its upper temperature limit is only about 150°C, and because of its low resistance, special measurements may be required. Nickel has an upper temperature limit of about 300°C, but it oxidizes easily at high temperature and is quite nonlinear. Rhodium-iron resistors are used in cryogenic temperature measurements below the range of platinum resistors [11]. Generally, these materials (except thermistors) have a positive temperature coefficient of resistance—the resistance increases with temperature. 

Specification for Thermistor Sensors for Clinical Laboratory Temperature Measurements, ASTM Standard E879-93,1993. 

Figure 12. Comparison of temperatures derived from noble gas concentrations (NOT) with temperatures measured using thermistors (Aeschbach-Hertig et al. 1999b). Shown are several examples from surface waters including Lake Baikal and the Caspian Sea. In almost all cases the NOT agree very well with the data from temperature sensors.

Water temperature is measured during water sampling by a precision mercury thermometer with scale divisions 0.1-0.5°C, or an electric thermometer with a resistance or thermistor sensor. For the temperature measurement at different water depths, special submersible thermometers are used [13, 14]. 

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