METLAB has further developed the dust sampling probe ESS for STL Combi Dust Sampler so that in addition to the zero-pressure difference, the dynamic pressure can also be measured continuously at the sampling point. This makes it possible to read and continuously log the value of % isokinetic deviation and the velocity at the sampling point. The isokinetic deviation is calculated via a relationship that is independent of the density of the gas, i e the gas composition in terms of O2, CO2 and moisture content, as well as the gas temperature! Only the relationship between the zero-pressure (DP0) and the dynamic pressure (Pd) affects the value of the measured isokinetic deviation. Thus, no knowledge of the sample gas flow and assumed moisture content is required! The probe may also be provided with a temperature sensor for gas temperature.
The pictures below show the two types of probe nozzle heads which are now fitted with a total pressure tube. The dynamic pressure is measured as the difference between total pressure and the probe nozzle head’s external static pressure. In this way, the influence of the gas velocity on the sensitivity of the zero-pressure control is also eliminated so accurate control can also be carried out at lower gas velocities.
The diagrams below show how the speed of a wind tunnel varied incrementally within 4-10 m/s, with zero-pressure control by help of the Automatic Motor-valve AMV-02 maintaining a low and constant level of the zero-pressure difference. The right diagram shows that measured isokinetic deviation is stable over the entire tested velocity range. The motor valve quickly resets the isokinetic deviation after the incremental velocity changes.
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