Physical properties, measurement temperature-measuring devices

In tile next section we cover the major types of measurement devices used in the process industries, principally the big five measurements temperature, flow rate, pressure, level, and composition, along with online physical property measurement techniques. Table 8-8 summarizes the different options under each of the principal measurements. 

Bulk property detectors function by measuring some bulk physical property of the mobile phase, e.g., thermal conductivity or refractive index. As a bulk property is being measured, the detector responses are very susceptible to changes in the mobile phase composition or temperature these devices cannot be used for gradient elution in LC. They are also very sensitive to the operating conditions of the chromatograph (pressure, flow-rate) [31]. Detectors such as TCD, while approaching universality in detection, suffer from limited sensitivity and inability to characterise eluate species. 

When we measure the temperature of a body, we are depending on the heat of the body to be transferred to (or from) our measuring device. Once the heat has been taken to (or from) our measuring device, any physical changes in that device are interpreted as a temperature change. The process where we analyze the effects caused by a property to determine the amount of that property is known as inferred measurement. For temperature we have a variety of physical properties from which to infer the amount of energy (heat) that a given object has. 

The temperature of a substance in a particular state of aggregation (solid, liquid, or gas) is a measure of the average kinetic energy possessed by the substance molecules. Since this energy cannot be measured directly, the temperature must be determined indirectly by measuring some physical property of the substance whose value depends on temperature in a known manner. Such properties and the temperature-measuring devices based on them include electrical resistance of a conductor (resistance thermometer), voltage at the junction of two dissimilar metals (thermocouple), spectra of emitted radiation (pyrometer), and volume of a fixed mass of fluid (thermometer). 

Any device or system that has one or more physical properties (e.g., electrical resistance, electrical potential, length, pressure at constant volume, or volume at constant pressure) that vary monotonically and repro-ducibly with temperature may be used to measure temperature. The science of the measurement of temperature is called thermometry. In the past, the measurement of high temperature was known as pyrometry but now that term usually refers to radiation thermometry at any temperature. Although the accuracy of a measurement refers to the difference between the measured value and the true value of the quantity being measured, and the precision of measurement refers to the degree of agreement among repeated measurements of the same quantity, it follows that a set of measurements of the same quantity, it follows that a set of measurements may be very precise but terribly inaccurate. Since in many instances the word accuracy is used when inaccuracy is meant and the word precision is used when imprecision is meant, perhaps it would be better always to refer to uncertainties of measurement, statistical and systematic, rather than to accuracy and precision. 

Temperature is measmed with a thermometer, a device in which a physical property of some component of the device changes when the device is put in thermal contact with a sample. That property may be the volume of a hquid (as in a mercury-in-glass thermometer) or an electrical property such as resistance. Electronic probes based on resistance changes in a semiconductor material are also used to measure temperature. 

In principle, any device that has one or more physical properties uniquely related to temperature in a reproducible way can be used as a thermometer. Such a device is usually classified as either a primary or secondary thermometer. If the relation between the temperature and the measured physical quantity is described by an exact physical law, the thermometer is referred to as a primary thermometer otherwise, it is called a secondary thermometer. Examples of primary thermometers include special low-pressure gas thermometers that behave according to the ideal gas law and some radiation-sensitive thermometers that are based upon the Planck radiation law. Resistance thermometers, thermocouples, and liquid-in-glass thermometers all belong to the category of secondary thermometers. Ideally, a primary thermometer is capable of measuring the thermodynamic temperature directly, whereas a secondary thermometer requires a calibration prior to use. Furthermore, even with an exact calibration at fixed points, temperatures measured by a secondary thermometer still do not quite match the thermodynamic temperature these readings are calculated from interpolation formulae, so there are differences between these readings and the true thermodynamic temperatures. Of course, the better the thermometer and its calibration, the smaller the deviation would be. 

Plate efficiencies and HETP values are complex functions of measurable physical properties temperature, pressure, composition, density, viscosity, diflusivity, and surface tension measurable hydrodynamic factors pressure drop and liquid and vapor flow rates plus factors that cannot be predicted or measured accurately foaming tendency, liquid and gas turbulence, bubble and droplet sizes, flow oscillations, emulsification, contact time, froth formation, and others. Values for plate efficiency, HETP, or HTU, particularly those that purport to compare various devices, are usually taken over a limited range of concentration and liquid-to-vapor ratios. The crossovers in Fig. 2.5 and the rather strange behavior of the ethyl alcohol-water system, Fig. 2.6, demonstrate the critical need for test data under expected operating conditions. ... 

Forty years ago these computed variables were calculated using pneumatic devices. Today they are much more easily done in the digital control computer. Much more complex types of computed variables can now be calculated. Several variables of a process can be measured, and all the other variables can be calculated from a rigorous model of the process. For example, the nearness to flooding in distillation columns can be calculated from heat input, feed flow rate, and temperature and pressure data. Another application is the calculation of product purities in a distillation column from measurements of several tray temperatures and flow rates by the use of mass and energy balances, physical property data, and vapor-liquid equilibrium information. Successful applications have been reported in the control of polymerization reactors. 

Hot-wire anemometers ( micro/nano anemometers) have been developed for a wide spectrum of applications from experimental fluid mechanics to aerospace engineering to measure physical parameters such as temperature, flow rates, and shear stress. The advent of microelectromechan-ical systems (MEMS) and nanoscale thermal sensors has provided an entry point to microfluidics, biomedical sciences, and micro-circulation in cardiovascular medicine. These MEMS and nanoscale devices are fabricated with semiconductor-based sensing elements which harbor the physical property of a resistor and have the dimension of one-tenth of a strand of hair. On the basis of the heat transfer principle, these resistant elements are heated by the Joule effect due to the passage of electric current. As the... 

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