Regular maintenance of the NAIS and EAS instruments is critical to ensure the reliability of measurement results. The maintenance procedures include instrument cleaning, leak tests, and checks on the condition of corona-needles, proper insulation between the inner and outer electrodes, and proper instrument grounding and inlet operation.
The inlet net and inlet tubing should be cleaned thoroughly in 1 – 3 week intervals to maintain the optimal aerosol sample flow and reduce the amount of dirt settling on the analyzer. The required interval depends on the local aerosol concentrations.
Routine instrument cleaning
During long-term operation the NAIS should be cleaned every 1 – 3 months due to the deposition of particulate matter inside the instruments. Within the cleaning procedures all parts, which are in contact with the sample- and sheath-flow, should be thoroughly wiped using delicate task disposable wipes (e.g. Kimberly-Clark Kimtech Science Kimwipes) and alcohol (e.g. 2-propanol). The wipes which get easily worn out and leave fibers should be avoided. The metallic nets inside the Venturi flow tubes (i.e. tubes with narrow slits for adjusting the volumetric flow) should be cleaned carefully. When dirt settles onto the nets, the flow resistance increases, and consequently the volume flow through the Venturi tubes decreases. This alters the mobility classification. An ultrasonic bath is recommended for cleaning these nets. Instead of nets, the instruments with 3 or 4-blowers have typically honeycomb-shaped pieces to make the flow laminar. These are less likely to become dirty and a careful cleaning with a brush or pressurized air is sufficient. Overall, the 3- and 4-blower instruments are significantly less susceptible to flow deviation issues. The corona-needle chargers should be cleaned (scraped with a sharp knife) regularly (1 – 3 month intervals) to make sure that the corona-generated ion concentration is maintained at a constant level.