Thermal anemometers for process control

SCHMIDT Technology: In innumerable processes, the prevailing process gases must be detected, monitored and controlled. There are some methods of measuring their flow velocities or volumetric flows, but thermal anemometers are the only ones that measure independently of pressure and temperature. This gives them a very broad range of applications in process technology and engineering.

Monitoring and control of industrial processes require all relevant parameters to be measured, including volumetric flows or flow velocities of the process gases used. Different measurement principles are available for their determination. Thermal anemometry, which has two big advantages, is widely used. On the one hand, the measurements are independent of the prevailing pressure and temperature conditions, as the sensor only detects the number of molecules flowing past it. On the other hand, an anemometer does not require any mechanical components. This results in a long service life of the sensors and insensitivity to impacts and vibrations. Moreover, the sensor is not subject to any drift during operation as a result of this measurement principle.

The challenge presented by the environmental and process conditions

The large variety of different processes in industrial production present various challenges to the measuring device with respect to the environmental and process conditions. Suitable flow sensors are available for all measurement requirements. The so-called dumbbell head sensors allow measurements from all directions. The design free of undercuts makes the sensor insensitive to dusts and impurities, preventing them from affecting the quality of the measurement. In addition to that, thanks to its design free of undercuts, the sensor can be very thoroughly cleaned or disinfected. An optional protective coating makes the sensor resistant to aggressive media such as acids, acetone or sulfuric acid, allowing its use in this ATEX version in potentially explosive atmospheres. For measurements at high temperatures of up to 350°C, versions are available in which the sensor element is positioned protected in a chamber head made of heat-resistant ceramic material that favors flow.

Practical examples
The exact supply of these air volumes in the freeze-drying process is monitored by high-precision thermal anemometers. This is due to the fact that they meet two specific requirements of this application: They measure precisely even at temperatures around –35° C and precisely detect the flow velocity in the range below 0.1 m/s. Another field of application for thermal anemometers is the elimination of volatile substances such as solvents from the soil. Its relative insensitivity to dust loads in the medium to be measured and its broad measuring range from 0.06 to 35 m/s qualify the flow sensor for soil air cleaning. Owing to their measuring range of up to 350°C at pressures of up to 16 bar, thermal anemometers also monitor atmosphere control in ceramic kilns. In addition, they also support heat recovery, due to their high resistance to heat, for the purpose of effective use of energy.