Mounting mass-flow sensors

Flow Low mass flow indicated. Mass flow error. Transmitter zero shift. Measurement is high. Measurement error. Liquid droplets in gas. Static pressure change in gas. Free water in fluid. Pulsation in flow. Non-standard pipe runs. Install demister upstream heat gas upstream of sensor. Add pressure recording pen. Mount transmitter above taps. Add process pulsation damper. Estimate limits of error. 

In addition, the analyzer can accept analog signals From other field-mounted analyzers or sensors such as flowmeters and pressure transducers. The signal can be sealed, digitized, and incorporated into special calculations to determine mass flow, therms per day, reactor yields, and so on. 

To characterize the sensors, they were placed inside a measurement chamber, which was itself mounted in a temperature-controlled oven (Fig. 3). The atmosphere in the measurement chamber could be controlled via gas-mixing equipment involving mass-flow controllers. The chamber was first heated up to 270 °C in a nitrogen (Nj) atmosphere. When the steady state was reached, selected amounts of propane, propene and water were added to the nitrogen carrier gas. The overall gas-flow was held constant at 500 cmVmin. The analytes were prethinned to 800 ppm in nitrogen carrier gas. A bubbler was used for the humidification of the gas. 

Parameters such as impeller speed and shaft power (in a stirred bioreactor) and fluid velocity are indicators of the degree of mixing and thus play an important role in the control of mass transfer. Impeller speed is easily monitored with a tachometer (electronic or mechanical) [39], but the measurement of shaft power input is not as straightforward. The most common method utilizes a torsion dynamometer attached to the impeller drive however, this technique includes losses due to friction in the drive shaft. Better data can be obtained from balanced strain gauges mounted on the impeller [37]. On-line measurement of the liquid velocity in a flowing or stirred system can be obtained by a heat-pulse method in which a resistance thermometer is used to measure a brief temperature increase caused by an upstream pair of electrodes [43]. Use of this sensor system has been limited to laboratory applications. 

Various shear stress measurement techniques have been proposed in the literature. Some of the principal measurement techniques are Stanton tube, Preston tube, electrochemical technique, velocity measurements, thermal method, floating element sensors, sublayer fence, oil-film interferometry and shear stress sensitive liquid crystal. The Stanton tube is a rectangular shaped pitot tube located very close to the boundary wall and the mean velocity measured from this pitot tube pressure difference is directly related to the shear stress. The Preston tube is similar to the concept of the Stanton tube using a pitot static tube close to the surface and the difference between the stagnation pressure at the center of the tube from the static pressure is related to the shear stress. The electrochemical or mass transfer probe is flush mounted with the wall and the concentration at the wall element is maintained constant. The measurement of mass transfer rate between the fluid and the wall element is used for determination of the wall shear stress. One of the limitations of the mass transfer probe is that at very high flow rates, the mass transfer rate becomes large and it may not be possible to maintain the wall concentration constant. A detailed discussion on the above three techniques can be found in Hanratty and Campbell [1]. These shear stress measurement techniques are not ideal MEMS-based techniques. 

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