3.6  Diagnostic Checklist

The following list of conditions can be used to check that the instrument is operating properly.

Many of these checks are also done by the measurement software and a warning will be shown if some condition is not met. But here are also several checks that

3.6.1  Electrometers

  1. Electrometer noise levels are around 0.1 fA for 10 s average records.
    May be 0.2 fA for electrometers number 17 and larger.

  2. Electrometer currents in offset mode do not exceed ±10 fA.

  3. Electrometer integrator output voltages are between –4 and +4 V.

  4. Electrometer integrator output voltages are different and quite random.

  5. No electrometer has resets more frequently than once every 5 minutes.

  6. The is no visible systematic drift in electrometer currents when looking at 1 s data for several consecutive measurement cycles.

  1. Check the electrometer time series plots of raw records. The fluctuations in all electrometers should be random. There should be no obvious waveform or beat pattern.

  2. Final block average electrometer currents are positive or at least not below –2σ in any operating mode.

3.6.2  Air Flows

  1. Barometric pressure measurement is correct within 1 hPa.

  2. All flow rate feedback controls are between 0.1 and 4.9 V.

  3. Total sample flow rate is 54 l/min
    pos and neg sample flow rates are both 27 l/min (if measured separately).

  4. Pos. and neg sheath flow rates are around 60 l/min.
    (higher than 60 l/min if the barometric pressure is below 1000 hPa, lower flow rate at higher pressure).

  5. There is no audible periodic fluctuation of the flow rates, pump noise.

  6. Pumps react when obstructing the inlet or outlet with hand.

  7. Pumps return to normal after previous obstruction is removed.

  8. Airtightness: Close the inlet completely with hand and wait until the pumps have stabilized. The total sample flow rate should not exceed 10 l/min.

3.6.3  Sample Conditioning

  1. Pos and neg charger currents are about 20 nA in particle and alternating charging modes and 0 nA in offset and ions modes.

  2. Standard deviation of pos and neg charger currents is less than 2 nA.

  3. Positive and negative discharger currents are around 25 nA in offset mode,
    around 10 nA in alternating charging mode,
    and 0 nA in particles and ions modes.

  4. Standard deviation of pos and neg discharger currents is less than 2 nA.

  5. Prefilter is active in offset mode.

  6. In case the charger current standard deviation parameters are not shown: check charger currents in raw records to see that they don’t fluctuate more than a few nA.

3.6.4  Postfilters

  1. Pos and neg postfilter voltages are between 20 and 80 V in particles and alternating charging modes.

  2. Pos and neg postfilter voltages are around 0 V in ions and offset modes.

  3. Pos and neg postfilter voltages are automatically adjusted: there are small variations in the voltage for each next measurement cycle.

3.6.5  Sheath Air Filters

  1. There is should be no wrong polarity corona in the filter.

3.6.6  Spectra

  1. Pos and neg ion distributions are similar to within 10% for mobilities below 1 cm2/V/s.

  2. Pos ions cluster band peak is at ~1.3 cm2/V/s

  3. Neg ions cluster band peak is at ~1.7 cm2/V/s

  4. Pos and neg particle distributions are similar to within 10% for sizes above 2.5 nm.