Temperature measurement pressure-compensated

Differential pressure meters are widely used. Temperature, pressure, and density affect gas density and readings of differential pressure meters. For that reason, many commercial flowmeters that are based on measurement of differential pressure often have integral temperature and absolute pressure measurements in addition to differential pressure. They also frequently have automatic temperature and pressure compensation. 

The greatest impact of the Clark oxygen electrodes has been in medicine and physiology. (A schematic diagram of a catheter-size Clark electrode is shown in Fig. 7.7.) On the other hand, a temperature- and pressure-compensated Clark electrode for oceanographic measurements up to 600 ft has also been developed (Fatt, 1976). The normal temperature coefficient of the Clark electrode is 2%/°C... 

Commercial applications of UTT measurement started in 1976 however, the method has not found widespread use. An important application in extrusion is the noncontacting temperature measurement of heat-sensitive materials. Protruding temperature sensors can easily cause degradation with such materials. The UTT measurement, pressure compensated, can be used for stock temperature control in a similar fashion as with the conventional melt temperature sensor (see Section 4.6). 

Provide a linear flow measurement for steam flow control. Also provide temperature and pressure compensation. 

Compensation of the measured value for conditions within the instrument, such as compensating the output of a pressure transmitter for the temperature within the transmitter. Smart transmitters are much less affected by temperature and pressure variations than conventional transmitters. 

Intelligent transmitters have two major components (1) a sensor module which comprises the process connections and sensor assembly, and (2) a two-compartment electronics housing with a terminal block and an electronics module that contains signal conditioning circuits and a microprocessor. Figure 6.9 illustrates how the primary output signal is compensated for errors caused in pressure-sensor temperature. An internal sensor measures the temperature of the pressure sensor. This measurement is fed into the microprocessor where the primary measurement signal is appropriately corrected. This temperature measurement is also transmitted to receivers over the communications network. 

The partial derivatives are usually assumed to be constants that are evaluated at the steadystate operating level from the vapor-liquid equilibrium data. Thus, pressure and temperature on a tray can be measured, as shown in Fig. 8.3c, and a composition signal or pressure-compensated temperature signal generated and controlled. 

The hot-wire anemometer can, with suitable cahbration, accurately measure velocities from about 0.15 m/s (0.5 fl/s) to supersonic velocities and detect velocity fluctuations with frequencies up to 200,000 Hz. Eairly rugged, inexpensive units can be built for the measurement of mean velocities in the range of 0.15 to 30 m/s (about 0.5 to 100 ft/s). More elaborate, compensated units are commercially available for use in unsteady flow and turbulence measurements. In cahbrating a hotwire anemometer, it is preferable to use the same gas, temperature, and pressure as will be encountered in the intended apphcation. In this case the quantity I RJAt can be plotted against /v, where I = hot-wire current, = hot-wire resistance. At = difference between the wire temperature and the gas bulk temperature, and V = mean local velocity. A procedure is given by Wasan and Raid [Am. Inst. Chem. Eng. J., 17, 729-731 (1971)] for use when it is impractical to calibrate with the same gas composition or conditions of temperature and pressure. Andrews, Rradley, and Hundy [Int. J. Heat Mass Transfer, 15, 1765-1786 (1972)] give a cahbration correlation for measurement... 

In the case of gases, properties may be tabulated til terms of their existence at 0°C and 760 mm pressure, To determine the volume of a gas at some different temperature and pressure, corrections derived from known relationships (Charles , Amonton s. Gay-Lussac s, and other laws) must be applied as appropriate. In tile case of pH values given at some measured value (standard for comparison), the same situation applies. Commonly, lists of pH values are based upon measurements taken at 25°C. The pH of pure water at 22°C is 7.00 at 25,JC, 6.998 and at 100°C. 6.13. Modern pH instruments compensate for temperature differences through application of the Nernst equation. 

Figure 14 illustrates a typical steam generator level detection arrangement. The AP detector measures actual differential pressure. A separate pressure detector measures the pressure of the saturated steam. Since saturation pressure is proportional to saturation temperature, a pressure signal can be used to correct the differential pressure for density. An electronic circuit uses the pressure signal to compensate for the difference in density between the reference leg water and the steam generator fluid. 

The head flow meter actually measures volume flow rate rather than mass flow rate. Mass flow rate is easily calculated or computed from volumetric flow rate by knowing or sensing temperature and/or pressure. Temperature and pressure affect the density of the fluid and, therefore, the mass of fluid flowing past a certain point. If the volumetric flow rate signal is compensated for changes in temperature and/or pressure, a true mass flow rate signal can be obtained. In Thermodynamics it is described that temperature and density are inversely proportional, while pressure and density are directly proportional. To show the relationship between temperature or pressure, the mass flow rate equation is often written as either Equation 4-1 or 4-2. 

As the previous equations demonstrate, temperature and pressure values can be used to electronically compensate flow for changes in density. A simple mass flow detection system is illustrated by Figure 9 where measurements of temperature and pressure are made with commonly used instruments. 

By measuring temperature and pressure, a computerized system can be used to electronically compensate a steam or gas flow indication for changes in fluid density. 

As previously discussed, the density of the fluid whose flow is to be measured can have a large effect on flow sensing instrumentation. The effect of density is most important when the flow sensing instrumentation is measuring gas flows, such as steam. Since the density of a gas is directly affected by temperature and pressure, any changes in either of these parameters will have a direct effect on the measured flow. Therefore, any changes in fluid temperature or pressure must be compensated for to achieve an accurate measurement of flow. 

Most distillation columns are operated under constant pressure, because at constant pressure temperature measurement is an indirect indication of composition. When the column pressure is allowed to float, the composition must be measured by analyzers or by pressure-compensated thermometers. The primary advantage of floating pressure control is that one can operate at minimum pressure, and this reduces the required heat input needed at the reboiler. Other advantages of operating at lower temperatures include increased reboiler capacity and reduced reboiler fouling. 

Experimental proof of control of the mask temperature with the chiller in a Gen 2 OVPD module under process conditions (showerhead heated to 325 °C) was achieved by in situ temperature measurement, as shown in Fig. 9.4. The experiments were performed at atmospheric pressure and at a deposition pressure of 0.9 mbar typical for OVPD, and for chiller temperatures between 5 and 30 °C. The mask temperature can be linearly controlled by the chiller temperature. The observed AT of 6.5 degrees is in good agreement with modeling prediction of 3 degrees in Fig. 9.3. In addition, measurements during a typical OVPD deposition time of 2 to 6 min confirmed there is no temperature drift under process conditions over time. The data prove that heat conductance and radiation is perfectly compensated by the chiller capacity. 

Barolo et al. (1998) developed a mathematical model of a pilot-plant MVC column. The model was validated using experimental data on a highly non-ideal mixture (ethanol-water). The pilot plant and some of the operating constraints are described in Table 4.13. The column is equipped with a steam-heated thermosiphon reboiler, and a water-cooled total condenser (with subcooling of the condensate). Electropneumatic valves are installed in the process and steam lines. All flows are measured on a volumetric basis the steam flow measurement is pressure- and temperature-compensated, so that a mass flow measurement is available indirectly. Temperature measurements from several trays along the column are also available. The plant is interfaced to a personal computer, which performs data acquisition and logging, control routine calculation, and direct valve manipulation. 

Pressure variations can be compensated for by measuring both temperature and pressure at a tray location and computing a pseudocomposition signal. This computed composition signal (pressure-compensated temperature) can then be controlled ... 

For high temperature measurements Hincke used a silica U-tube sealed at both ends at one end was a bulb A containing the substance (Fig. 5.VIII J). The manometric liquid was fused bismuth. The tube was supported at the bulb end by a fused-on hook, engaging with a hook on the wall of the heatr ing chamber, and a fused-on hook on the other side was connected by a wire with a balance-pan outside the furnace. The pressure of the vapour drove the liquid into the right-hand -side of the tube, and from the increase in weight the displacement of liquid, and hence the vapour pressure, could be calculated. Jenkins used a fused metal (Cd, Zn) as the manometric. liquid in one arm of a V-tube, and compensated, the vapour pressure by nitrogen gas. 

Rolla,2 in low temperature measurements, enclosed the substance in a vacuous thin metal box, acting as an aneroid barometer, and compensated by an external measured air pressure, the movement of the top of the box being observed by an optical method. 

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