The isokinetic sampling of the exhaust gas in biomass heating plants is challenging due to high fluctuations in mass flow during a combustion process. These fluctuations occur because of changes in the pressure setup in the combustion chamber as a result of the inhomogeneous combustible material. To measure the particulate matter in the exhaust gas the isokinetic criteria must be strictly adhered. As a consequence most conventional systems with a manual adaption of the isokinetic state are unable to stay within the allowed limits of the suction condition. This can lead to significant uncertainty with respect to the standard .
To compensate this issue a controlled isokinetic sampling unit is engineered. The unit consists of a Prandtl probe for main flow velocity, an air mass flow meter (HFM) for the measurement of the velocity in the probe nozzle, a proportional valve for air flow adjustment and sensors for temperature and humidity compensation. For the processing of the measured data a process control unit provides the needed calculations and adapts the enhanced control loop to fulfill the isokinetic condition automatically.
To ensure proper results the functional capability of the isokinetic sampling unit is benchmarked with a single phase test to evaluate the control loop and a multiphase test to confirm the overall system.
The results show that the mean deviation of the velocity in the exhaust pipe with respect to the reference value is smaller than ±5%. The deviation of the mass concentration values is in the range of -6.6% to +13.8%. This is mainly because of the fact that these maximum deviations are a result of the dosing systems lower mass-flow limit.
 VDI 2066 Blatt 1: Staubmessung in strömenden Gasen: Gravimetrische Besteimmung der Staubbeladung, 2006th ed. Düsseldorf: Beuth, 2006