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Detection of overflows with the highest possible measuring certainty

Detection of overflows with the highest possible measuring certainty

14 October 2012

SCHMIDT Technology: The flow sensors SS 20.400 and their ATEX model SS 23.400 from SCHMIDT Technology detect very small air flows and directions of flow in quick response. A certified high-precision adjustment and adjustment in the new vertical flow channel give users in cleanroom and industrial applications an even greater measuring certainty.

In cleanrooms or clean areas in which defined overpressures guarantee process safety by overflow from one area to another, an additional measurement of the directions of flow increases operational safety. There are many situations, for example when opening doors or interlocks, in which the pressure falls below the measuring range limit of the pressure sensor. In such phases, the cleanroom operator has no measured values of the actual overflow. However, the still existing overflow can be reliably detected by using an additionally installed, bidirectionally working flow sensor. If backflows take place, resulting in possible contamination, this is also reliably detected.

For applications of this type, SCHMIDT Technology has developed the flow sensors SS 20.400 and their ATEX counterpart SS 23.400. They feature integrated direction detection and detect very small air flows down to 0.05 m/s, which corresponds to a differential pressure of approx. 0.01 Pascal. To detect overflow velocities, i.e., directions of flow, a sensor of this type is mounted directly in front of a wall opening. According to Toricelli's theorem, the flow velocity is a function of the differential pressure, in which the diameter of the wall opening has virtually no influence. However, since a diameter that is too small will falsify the flow due to friction in the wall opening, a diameter of at least 25 millimeters is recommended. The functioning of the bidirectional measurement with thermal flow sensors is described in a brochure that can be downloaded from http://www.schmidttechnology.de/de/sensorik/download/datenblaetter/SS_20.400_23.400_de.pdf.

SCHMIDT® flow sensors are used in a wide range of fields in industry and cleanroom technology. A typical field of application is the detection of laminar flows, for example in RABS or insulators. The ATEX version can also be used in hazardous areas, such as modern painting booths. According to the ATEX Directive 94/9/EC, the flow sensors SS 23.400 can be used in Zone 2 (device category 3G) without any additional measures, such as a zener barrier. The sensors are DEKRA-certified.

Simple and highly accurate

The flow sensors allow simple and consistent monitoring of all clean areas, including local cleanrooms. Upon re-quest, the sensors can be supplied with high-precision adjustment. It provides a further improvement in the accuracy by the increased number of adjustment points and documentation of the nominal and actual values as ISO calibration certificate. The measuring accuracy is ┬▒1% of the measured value plus 0.04 m/s.

For laminar applications, the sensors are now adjusted in a vertical flow channel. This vertical flow channel has been developed by SCHMIDT Technology and is designed selectively for constant production of very low medium velocities, while being completely encapsulated and thermally insulated. Its setup guarantees a falling air flow, as ideally required in practical application, and excludes all external influences from actual measuring practice on site. It also takes into account the so-called thermal convection flow, which in practice amounts after all to about 10% of the downdraft flow of 0.45 m/s. This achieves the highest possible accuracy.

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