Temperature-measuring devices

Reactivity is measured by placing a standard quantity, 100 mL, of isopropyl alcohol in a 500- or 1000-mL Dewar flask equipped with a stirrer and a temperature-measuring device. The temperature of the alcohol is adjusted to 30°C. Thirty-six grams of the sample are added and the temperature is observed as a function of time from the addition until a maximum is reached. Reactivity is defined as the temperature rise divided by the time interval to reach this maximum. Other alcohols may also be used for measuring reactivity (30). 

Fig. 10. The Groth ZG 5 centrifuge. R, rotor R, stationary shaft T, Teflon seal K, K, chambers for gas scoops S, S2, scoops V, gas supply M, manometer Z, Z2, tapping points for enriched and depleted gas P, P2, vacuum chambers E, electromagnet for eddy current heating Tb, Tb2, temperature measuring devices K, cooling coil and D, D2, labyrinth seals.

Time constants. Where there is a capacity and a throughput, the measurement device will exhibit a time constant. For example, any temperature measurement device has a thermal capacity (mass times heat capacity) and a heat flow term (heat transfer coefficient and area). Both the temperature measurement device and its associated thermowell will exhibit behavior typical of time constants. 

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. 

For example, a temperature-measuring device, having its sensor placed in a protecting rube, is a system of second order. For such a system no single rime constant exists in the same way as a first-order system. The behavior of such a system is often given by a response time. Another concept is to give the apparent time constant t, which can be constructed by placing a line in the inflection point of the step response curve see Fig. 12.14. 

One day the temperature-measuring device gave a false indication of high temperature. The air valve closed, and the drain valve opened. The temperature indication fell, perhaps because the reactor was now empty. The drain valve stayed open, but the air valve reopened, and a flammable mixture was formed in the reactor. Fortunately it did not ignite. 

A temperature measuring device that reports data back to a recorder. Threshold effect ... 

The simple feedback control system below consists of a continuous-flow stirred tank, a temperature measurement device, a controller and a heater. 

Melting point instruments consist of a resistance heater to increase temperature, a sample holder and a temperature-measuring device. In its simplest form, an oil bath can be heated with a Bunsen burner, while a capillary tube with a few milligrams of sample is attached to a thermometer with the sample next to the mercury bulb. 

Laboratory furnaces. Several types of furnaces are used in the laboratory these are often available as commercial rigs, generally equipped with more or less sophisticated temperature measurement and control devices. As an alternative, a lab-made or commercial furnace and its temperature measuring devices may be connected to a multi-channel data acquisition/actuator/switch unit, to be programmed by a personal computer, in order to plan and carry out thermal treatments, to collect and retrieve measured thermal data, etc. 

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... 

A resistance thermometer is a temperature measuring device based on the principle that the resistance of metals proportionally increases as the temperature of a metal increases. 

A gas-free solution was prepared by mixing methanol, water, and salt, each of which was separately degassed or desorbed. The methanol and water were each boiled in a flask with a reflux condenser under a reduced pressure to remove dissolved gases in the solvent. Then they were introduced into constant-temperature measuring devices out of contact with air. The amounts of both solvents were adjusted to accommodate a desired composition of the mixture. 

Two main principles of temperature measurement use thermocouples and the so-called resistance thermometer. In chemical plants both methods were applied because they are easy to fit and to maintain.The accuracy of the measurement is influenced by, for example, radiation, which must be taken into account. Thermocouples can be inserted into the pressure system using special sealing techniques, or they may be mounted within a protective tube which is introduced into the pressurized volume. Thermocouple-wires are usually protected with an isulating input in closed-end capillaries with outer diameters of at least 0.5 mm. Thermocouples are technically well tested for pressures up to 6 kbar and temperatures to approx. 800°C. Above these ranges the exact measurement is negatively influenced by several parameters, and the deviations must be taken into account. The accuracy of the temperature measurement devices is normally better than 1 °C. 

The same featuresas mentioned before can be discussed for the resistance thermometers. The outer minimum diameter of these sensors is approx. 4 mm [9], A typical high-pressure temperature measurement device is presented in Fig. 4.4-1. 

The temperature measurement devices which do not contact the hot surfaces, for example, optical -, radiation pyrometers, and infrared techniques, are not typical for high-pressure application. 

Temperature measurements ranging from 760 to 1760°C are made using iron—constantan or chromel—alumel thermocouples and optical or surface pyrometers. Temperature measuring devices are placed in multiple locations and protected to allow replacement without incinerator shutdown (see Temperature measurement). 

Thermocouples are the most commonly used temperature measuring device in elevated temperature thermal analysis. Thermocouples are made up of two dissimilar metals. If the welded junctions between the two materials are at different temperatures, a current through the loop is generated. This phenomenon can be explained by visualizing electrons in a solid as analogous to a gas in a tube (Figure 2.3). 

The linear drop and exponential recovery shape of these transformations also appear in power-compensated DSC traces, but for different reasons. The temperature measuring device (RTD) measures its own temperature, which is influenced by all substances in the chamber, the housing, the sample crucible, as well as the melting sample. The device adds power to the sample side as needed to compensate for the cooling effect on the chamber due to sample melting. This energy requirement increases lineaxly since the setpoint sample temperature increases linearly. When melting is over, the need for extra heat flow to the sample chamber side drops exponentially as the chamber temperature quickly catches up to the setpoint. 

By diluting a reactive sample powder with an inert powder (e.g. AI2O3), the heat capacity of the sample can be made to match more closely that of the reference, hence baseline float can be dampened. This is a useful technique for reactions of significant thermal effect, e.g. combustion, since sample dilution diminishes the intensity of the differential temperature signal. Since the diluent adds a thermal resistance between the reaction zones and the temperature measuring device, the onset of reactions will shift to higher temperatures. 

Table 2.31 Ranges of Common Temperature Measuring Devices 1...

By hooking the other end of joined dissimilar wires up to a voltmeter and measuring the emf (output), it is possible to determine temperature. Such a temperature-measuring device is called a thermocouple. 

Equation (3.62) may be used for a thermocouple in a fluid stream that is at a different temperature from that of the plate supporting the thermocouple. This is a useful correction for temperature measurement devices employed by experimental engineers. 

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