Ion Science MiniPID - Manual (Page 15)

MINIPID 3PIN MANUAL Ion Science Ltd
Page 15 of 27
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General
– Best Practice
Balance Gas
When calibrating, ensure that the gas carrier (e.g. nitrogen cylinder or compressed air) is clean - defined as
“zero air”. Lubricating oils in compressed air lines should be avoided as they will foul PIDs if exposed to the
gas stream for extended times.
Some gases absorb UV light without causing any PID response (e.g. methane, ethane). In ambient
atmospheres where these gases are present the measured concentration of target gas will be less than is
actually present. Methane absorbs UV strongly, so for accurate measurements in methane containing
atmospheres, calibrate with a calibration gas containing the expected methane concentration. 50% LEL
methane reduces the reading by up to 50%. Gases such as nitrogen and helium do not absorb UV and do
not affect the relative response.
High Backgrounds
After periods of storage or non-usage, the MiniPID may be susceptible to baseline settlement issues. In such
an event, it is recommended that the MiniPID Sensor is left powered in a clean air environment for an hour or
two. For detection at very low gas concentrations (ppb) after an extended epoch of storage the MiniPID may
require operation in clean air for several hours to get to a stable baseline reading.
The 2 mV and 20 mV increase from 50 mV is purely dependent upon cell contamination which can be one or
more of the factors given below (the first two points are highly dependent upon the type of usage).

Temporary contamination within the layers of the pellet which may require some minutes of lamp
illumination to burn-off the debris.

Excessive permanent contamination through salts (or the suchlike) deposited along the walls which
bridges the fence electrode and reduces its effectiveness. The cell needs to be replaced if this is
suspected to be the cause.

A much lower signal caused by photo-ejection from the back-electrode that is used to monitor the
status of the lamp condition to create our error status messages.
This combined signal is part of the ‘lamp-out-detection’ circuit presently unique to this type of sensor and
thus allows for continuous real time ‘in-cell’ monitoring.
‘lamp-out detection’ failing to occur is likely due to a heavily contaminated pellet – where surface leakage
and/or salt build-up within the cell creates unwanted currents similar to that created by lamp illumination.
Always ensure a clean pellet is used. Excessive cell contamination can always be checked with the lamp
removed but with the pellet in place to give a lamp error status in normal operation.
! Caution ! Note on Silicones:
PIDs are not permanently damaged by Silicones but they do potentially foul the windows of the lamps and
reduce response to some gases. This can usually be remedied by polishing the lamp window with alumina
powder. However, instrument manufactures incorporating the MiniPID sensors should be careful to avoid
any silicones such as those which may occur in labels and moulding release agaents for plastics. Over
months of storage the silicones may leach into the sensor and lead to window fouling and sensitivity lost.
Temperature correction
Increasing temperature increases slightly the PID sensitivity. At 50

C the sensitivity is typically a few percent
higher than at 20

C. At -20

C sensitivity will be approximately 20% less than at 20

C. It is always possible to
switch the sensor off and on during large temperature excursions to ensure the most accurate readings are
obtained.