Pressure, measurement instrumentation devices

Industrial and Control Instruments. Mercury is used in many industrial and medical instmments to measure or control reactions and equipment functions, including thermometers, manometers (flow meters), barometers and other pressure-sensing devices, gauges, valves, seals, and navigational devices (see Pressure measurements Process control Temperature measurement). Whereas mercury fever thermometers are being replaced by... 

These techniques have been described previously in the pressure measurement section. Usually, one of the flow-measuring devices and the required instrumentation is incorporated as a part of the plant piping. The choice of technique depends on the allowable pressure drop, flow type, accuracy required, and cost. 

Most thermometry using the KTTS direcdy requires a thermodynamic instrument for interpolation. The vapor pressure of an ideal gas is a thermodynamic function, and a common device for realizing the KTTS is the helium gas thermometer. The transfer function of this thermometer may be chosen as the change in pressure with change in temperature at constant volume, or the change in volume with change in temperature at constant pressure. It is easier to measure pressure accurately than volume thus, constant volume gas thermometry is the usual choice (see PRESSURE MEASUREMENT). 

Pressure-Measuring Device. Accurate pressure measurements are made with a manometer if needed. In most instruments the carrier gas, hydrogen, and air (for flame ionization detector) are provided with separate gauges. 

The LAUDA measuring instruments program covers systems for measuring the surface and inteifacial tension of liquids. Available devices include ring plate tensiometers (automated and nonautornated), drop volume tensiometers, bubble pressure tensiometers, and film balances. 

Mercury compounds continue to have numerous commercial uses. Besides its use as a preservative, mercury is used in the manufacture of many fechnical and medical instruments including blood pressure measurement devices, manometers, thermometers. 

Mercury compounds continue to have numerous commercial uses. Besides its use as a preservative, mercury is used in the manufacture of many technical and medical instruments including blood pressure measurement devices, manometers, thermometers, and barometers. Mercury is also used in production of certain types of fluorescent lamps and in the chloralkali industry, where chlorine and caustic soda are produced using brine electrolysis in mercury cells. Metallic mercury is used in the production of precious metals such as gold and silver. As part of the production process, metallic mercury can be used to concentrate gold from... 

Several terms for pressure measuring devices are used interchangeably including transmitters, transducers, gauges, sensors, and manometers. More precisely, a gauge is a self-contained device that converts a force from the process to a mechanical motion of needle or other type of pointer. A manometer is a term reserved for an instrument that measures the hydrostatic head of a liquid and generally operates near atmospheric pressure. A transducer or transmitter combines the sensor with a power supply and a converter— generally mechanical-to-electrical or mechanical-to-pneumatic. The sensor... 

Experimentation and novel measurement techniques are cmcial for the further development of microfluidic devices. Pressure is one basic parameter involved in microflmdic experiments. However, it is not realistic to apply the conventional pressure measurement techniques to microsystems, since the characteristic dimension of these measurement instruments is already comparable with the size of the microdevices. Therefore, novel pressure measurement methods are needed for pressure measurement at the microscale. 

In Figure 2.6, the movement of solvent is detected by the movement of a bubble in a very narrow tube attached to the solvent compartment of the osmometer. When the photocell detects movement of the bubble, an external head of solvent is established by the raising of the solvent reservoir. In Figure 2.7, a capacitance device is used to detect solvent movement across the membrane. The measured signal is used to generate an appropriate external solvent head equivalent to the osmotic pressure. Modern instrumentation, once set up properly, can reduce the measurement time to a matter of minutes thus reducing the errors which are found if the membranes are anything less than truly semi-permeable. 

Portable pressure measurement device (PPMD) is a lightweight carry-on type instrument comprising digital pressure modules (DPM) packed together with a power supply in a tamper-indicating enclosure (Landat et aL 1997). The PPMD can be connected to the level, density, and reference probes of a tank in parallel to an operator s own equipment. The sensor data are used to independently verily the volume of solutions in various storage and process tanks. 

Volumetric measurements of gas adsorption equilibria reduce, if the mass of the sorbent sample used has been determined, to measurements of pressures and temperatures in gas phases. For this a variety of high precision measuring instruments operating in a fairly wide range and partly also in corrosive environment are available today. Of course these instmments prior to measurement have to be calibrated with meticulous care which may be laborious and even cumbersome. For pressure measuring devices, calibration with pressure maintaining valves of Desgranges Huot has proved to be successful. 

Manufacturers of measurement devices always state the accuracy of the instrument. However, these statements always specify specific or reference conditions at which the measurement device will perform with the stated accuracy, with temperature and pressure most often appearing in the reference conditions. When the measurement device is apphedat other conditions, the accuracy is affected. Manufacturers usually also provide some statements on how accuracy is affected when the conditions of use deviate from the referenced conditions in the statement of accuracy. Although appropriate cahbration procedures can minimize some of these effects, rarely can they be totally eliminated. It is easily possible for such effects to cause a measurement device with a stated accuracy of 0.25 percent of span at reference conditions to ultimately provide measured values with accuracies of 1 percent or less. Microprocessor-based measurement devices usually provide better accuracy than the traditional electronic measurement devices. 

The orifice, the venturi, and the nozzle are instruments for the measurement of duct or pipe flow rate. A constriction, throttling the flow, is placed in the duct, and the resulting differential pressure developed across the constriction is measured. It is the difference in the geometric shape that characterizes the three devices see Fig. 12.22. 

Any obstruction inserted into a duct or pipe that creates a measurable pressure difference can be used as a flow meter. The three basic standardized flow measurement devices presented above are perhaps more suitable for laboratory work than installation as permanent ductwork instruments in ventilation applications. They are sensitive to flow disturbances, relatively expensive, require considerable space, and have a narrow measurement range and a high permanent pressure loss. For these reasons, numerous attempts have been made to develop instruments without these drawbacks. Some of them, like the... 

The simplest calibration procedure for a gas flow-measuring device is to connect it in series with a reference meter and allow the same flow to pass th tough both instruments. This requires a reference instrument of better metrological quality than the calibrated instrument. One fact to consider when applying this method is that the mass flow rate in the system containing both instruments is constant (assuming no leakage), but the volume flow rate is not. The volume flow rate depends on the fluid density and the density depends on the pressure and the temperature. The correct way to calibrate is to compare either the measured mass... 

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