5.5  Flow System

All the flow rates – sheath flow for both polarities and sample flow (either single or separate) – are measured using Venturi tubes and differential pressure sensors. The sensors are calibrated at Airel.

The blowers are automatically adjusted using a software PID controller based on the flow sensors and barometric sensor so that the mobility analysis and sampling volume flow rate remain constant.

Figure 1: The principle of the airflow speed control

5.5.1  Sheath Air Filters

The sheath air of the mobility analyzer is recirculated from the exhaust air after it has been cleaned by the sheath air filter. It is only necessary to remove charged particles from the sheath air. Therefore a simple electric filter is used which can be easily cleaned and does not need replacement when the filter becomes clogged. Both analyzer columns have a separate sheath air filter.

The filter voltage is typically about 4.5 kV. When the filter has not been cleaned for some time then there is increasing probability that some fibers or dirt inside the filter will start coronating and producing charged particles. This may disturb the measurements of the NAIS. How severely, depends on the polarity and the location of the corona discharger. See Sheath Air Filter Cleaning.