Flow measurements mechanical gauges

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. 

The die which can be center- or end-fed converts the melt from a circular cross section to a imiformly thick melt curtain of the required width. The thickness of the film is continuously measured across the web after the stenter process, giving a thickness or gauge profile. This profile data is used to make fine adjustments to flow profile at the die either through thermoviscous heating or by actuation of mechanical bolts (which physically modulate the die gap profile to achieve uniform film thickness profile). Combinations of thermoviscous and mechanical modulation are also employed in some cases. 

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