4  Troubleshooting

4.1  Electrometer problems

(a) One or more electrometers have an abnormally high noise level.

This typically happens when dust or insects deposit inside the analyzer. These foreign bodies may cause corona discharge due to high electric field inside the analyzer, create a short circuit between neighboring collecting electrodes or the central electrode.

(b) An electrometer offset current value is high (> 10 fA)

(c) An electrometer current value is missing (field empty) or the value is “Inf”.

This indicates that an electrometer is saturated and constantly resetting. During normal instrument operation electrometers reset typically after every few hours. This takes about 10 seconds. During this time the electrometer charge collecting capacitor is discharged. In extreme situations the electrometer current is so high that the capacitor is saturated again before the reset period ends.

This is typically a more severe case of high electrometer noise.


4.2  Analyzer voltage problems

4.2.1  Analyzer voltage too high

Description: A section of the mobility analyzer central electrode has an abnormally high voltage.

Solution: For older instruments the output voltage of the central electrode voltage sources may slowly drift over time. If the voltage is above 15% of the nominal value, then the voltage source needs to be adjusted. The part should be sent to Airel.

4.2.2  Analyzer voltage too low

Description: A section of the mobility analyzer central electrode has an abnormally low voltage.

Solution: The central electrode voltages reach their nominal values in about 5 – 30 minutes after the instrument has been switched on. The voltage values will slowly increase towards the nominal value. It is normal that there is a warning about the low voltages until the voltages have stabilized.

If the voltages do not reach the correct value then it needs to be established whether the problem is caused by the voltage source or there is a short circuit in the analyzer. Therefore it should be checked if the issue perists when the voltage source is disconnected from the analyzer:

  1. Switch off the instrument.
  2. Disconnect the the voltage cable connected to the bottom of the analyzer.
  3. Start the instrument, run the measurement and wait for voltage stabilization.

If the issue remains, then the problem is related to the voltage source or the data acquisition system. Please contact Airel for further help. It may be necessary to send the voltage source for repairs or replacement.

If the issue disappears, then the it is related to the analyzer. It may be necessary to clean the analyzer with special attention to the central electrode. Check the gaps between the electrodes. Measure the electrical resistance between different central electrode sections and between the sections and ground.

4.3  Charger and discharger problems

4.3.1  Current is fluctuating

Description: The standard deviation value of charger current is above 5 nA.

Solution: Corona discharge is inherently unstable. The instrument electronics constantly measures the charger current and controls the charger voltage so that the current value remains stable. Over time the charger needle becomes contaminated with dust and it is no longer possible to keep the charger current constant at the requested value.

In this case the charger current will start to fluctuate. In more severe cases the current will fluctuate between no current at all and a very high current value.

The solution is to clean the charger or the discharger.

4.3.2  Charger disabled due to overload danger

Description: The instrument has switched off a charger or discharger because there is danger of overloading the charger voltage source. This happens when the charger voltage control signal is at maximum for a couple of seconds and sufficient charger current is not produced. The instrument switches off the charger or discharger completely to prevent possible damage to the voltage source for the duration of the current measurement cycle. The charger will be reset at the beginning of the each new measurement cycle and switched off again if the problem persists.

Solution: The solution steps are identical to the “Control at maximum” charger issue.

4.3.3  Control at maximum

Description: Charger current control signal is at maximum but the measured charger current value remains too low. The charger is not operating correctly.

In case of a discharger, the measurements in all operating modes may be affected because the electrometer signal offset is not measured correctly. In case of a main charger, the particles and alternating charging modes will be affected

Warnings about charger overload danger may also be present in the software log window.


4.3.4  Control at minimum

Description: Charger current control signal is at minimum but the current value is still above the required value


4.4  Sheath air filter problems


Description: The instrument has detected a possible abnormal electrical load on a sheath air filter voltage. This may indicate that there is corona discharger happening inside the sheath air filter. Depending on the polarity of the corona discharge and its location inside the filter this may produce charged particles that would be cause invalid signal in the mobility analyzers and disturb the measurements.

Solution: Clean the sheath air filters.

4.5  Air flow problems

In case of any airflow issues you should always check the following:

  1. All the tubes inside the instrument are connected.

    E.g. the tubes leading to the preconditioning unit at the top are often forgotten to be attached after maintenance.

  2. The Venturi tubes are in the right direction.

  3. The Venturi tubes are connected to the right pressure sensor.

  4. The Venturi pressure connection tubes are not switched.

  5. If you have a flow meter available (like TSI 4000 series): Check that the flow rates match to those shown by the measurement program. Note that these are volumetric flow rates, not translated to STP conditions.

    Sample flowrate should be measured from the exhaust.

    Sheath flowrates should be measured by disconnecting the tubes going up into the preconditioning unit and measuring from the side of the blowers.

  6. If using Ashcroft pressure sensors: Check that the electric wiring of the pressure sensors is intact.

4.5.1  Flow fluctuating

Description The standard deviation of a flow sensor signal is abnormally high.

Solution In addition to the standard airflow checks, please see that strong wind is not blowing directly into the inlet.

4.5.2  Control signal is at maximum

Description The blower control signal is at maximum, however according to the flow sensors the flow speed remains below the target value. The system can not increase the blower speed any further.

Solution In addition to the standard airflow checks, check that there are no obstructions. Pay attention that all tubes and electric connections to the blowers are intact.

4.5.3  Control signal is at minimum

Description The blower control signal is at minimum, however according to the flow sensors the flow speed remains above the target value.

Solution In addition to the standard airflow checks, pay attention that all tubes and electric connections to the blowers are intact.

4.6  Distributions

4.6.1  No cluster ions visible

(A) Particles are lost before entering the instrument

Instrument inlet is not properly grounded or inlet tube is too long.

(B) Particles are lost inside the instrument before entering the mobility analyzer.

  1. Check that all plugs in the charger filter unit are connected to the correct sockets.
  2. Check that there is an electrical connection between the filter electrode and the corresponding socket.
  3. Also check the electrical connection of other sockets and the corresponding parts of the charger: charger needle, charger current electrode, postfilter electrode, discharger needle, discharger current electrode. Note however that most other broken connections would cause additional diagnostic warnings.

(C) Problem with offset measurement

If the offset operating mode is not working correctly, wrong offset estimate is subtracted from ions or particles mode signal.

(D) 9 V central electrode voltage is missing

4.6.2  Significant difference between positive and negative concentrations