Flow monitoring in clean rooms and clean zones
A direction-defined air-flow in clean rooms protects the products against contaminations and unwanted particles will safely be evacuated. To do so, a uniform air flow from the ceiling to the floor (“oriented, low-turbulence displacement flow“) is maintained in clean rooms of strong purity levels. The monitoring range is from 0.36 to 0.54 m/s flow velocity (EU GMP guide, Annex 1 in class A). In the clean room the measurements are made behind terminal filters. Since the recirculation of air is reduced during standstill periods, an extremely precise measurement of the air velocity is mandatory from 0.1 m/s on.
The solution: Air flow measurement by SCHMIDT® Flow Sensors SS 20.515 LED or SS 20.415 LED
According to the standard EN ISO 14644-3, the air velocity is measured at a distance of approx. 150 to 300 mm below the front surfcae of the filter. To facilitate an installation into ceiling or wall systems, the thermal SCHMIDT® Flow Sensors SS 20.415 LED and SS 20.515 LED are supplied with a cleanroom-compliant quick assembly kit. Both sensors are extremely compact since all electronic components are 0ces in the low-turbulence displacement flow. For a use of supply air systems, the SS 20.415 LED offers an option for simultaneous detection of the flow direction – which is interesting in case of back flows.
Measuring accuracy in black and white
One special feature of these sensors is the compensation and calibration in a novel “vertical flow channel“. This offers the advantage that effects from the measurement steps on site are eliminated and highest precision will be achieved. The renowned laser doppler measurement process (LDA) is used as reference measurement process. Upon request they will be supplied
with an additional high-precision calibration system. This system enhances the precision even more by the use of more calibration points, and the documentation of the target and actual values as ISO calibration log. This calibration can be renewed in accordance with the specifications set by the user, which in – general is after one year.
Precise measurement of small air velocities
Both SCHMIDT® Flow Sensors SS 20.415 LED and SS 20.515 LED are based on the thermal measuremement principle. The mechanical architecture of the sensor element, however, varies.
SCHMIDT® Flow Sensor SS 20.515 LED the dumb-bell head measurement principle
The Flow Sensor that is located in the stainless steel sleeve between the two ”disks of the dumb-bell“, is heated to more than 40 K above the medium temperature. Which is measured by a separate temperature sensor. The output required to maintain this excessive temperature is used as measure to determine the flow velocity that is presented as “normal velocity“. This means that it is not necessary to perform an additional pressure or temperature measurement of the medium. Both dumb-bell disks serve as flow rectifiers so that it is also possible to measure comparably irregular flows.
Always the right choice
Both sensors, the SS 20.415 LED and the SS 20.515 LED, can be used in cleanrooms and comply with the GMP and offer mounting options geared to cleanrooms as well as plug connections allowing for a quick change on site. Depending on the application, both sensors offer additional advantages:
Can be disinfected using alcohols and H2O2 (VHP-proof)
Detection even of the smallest air flows from 0.05 and/or 0.06 m/s on
Self-monitoring and transmission of error signals
Special lengths of up to 1,000 mm (straight version)
Medium :Clean air / nitrogen / additional gases on request
Measuring range : 0 ... 10 m/s
Output :0 … 10 V / 4 ... 20 mA
Operating temperature : -20 ... +70°C
Operating pressure : Atmospheric (700 … 1,300 hPa)
Type / class of protections：IP 65/III (SELV or PELV EN 50178)