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November 19, 2019
13:11
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SulfiLogger_Manual_EN_203.pdf
TROUBLESHOOTING
Problem:
The SulfiLogger™ sensor is showing values close to zero all the time even though grab sample
analysis of the medium shows significant sulfide concentrations.
Solution:
The inlet of the SulfiLogger™ sensor my be blocked by fouling. Try rinsing the sensor and gently
wiping the front with a wet cloth, taking care not to apply force around the inlets of the sensing elements.
If the measurements are performed in liquid phase (e.g. in wastewater), check the pH-value of the liquid.
If the medium is alkaline (above pH-8) then the dissolved sulfide is primarily on the
HS
−
-form, which is
not detected by the SulfiLogger™ sensor. For slightly alkaline pH-values (pH-7-8), the pH-effect can be
corrected for, but for higher pH-values, we recommend using the SulfiLogger™ sensor in a low-volume
sidestream reactor, where the wastewater is acidified to ensure that all dissolved sulfide is on the H
2
S-form.
Problem:
The SulfiLogger™ sensor does not measure correct values according to our standard tests.
Solution:
When judging the accuracy of the SulfiLogger™ sensor we always recommend to apply a gas phase
measurement with gas from a pre-mixed gas bottle containing a known H
2
S concentration. Please note
that the accuracy (grade) of gas bottle concentrations vary. The accuracy should be stated on the bottle or
can be provided by the vendor. If judging the accuracy of the SulfiLogger™ sensor based on liquid phase
measurements there are various ways to do this, all of which are prone to higher degrees of uncertainty than
the gas phase testing: One option is to prepare a standard (stock solution) mixture with a known concentra-
tion of H
2
S. Experience shows that this is not an easy task and that the concentration should continuously
be checked with laboratory equipment such as spectrophotometric methods. For more information on
this, please contact Unisense. Another option is to do grab sample analysis of wastewater and compare
the results of a sulfide analysis (e.g. Clines’ method) with the measurement values of the SulfiLogger™
sensor. Please be aware, though, that standard sulfide analyses of wastewater grab samples measures the
Total Sulfide
and not only the
Total Dissolved Sulfide
. For a comparison between the SulfiLogger™ sensor
measurements and grab sample values it is thus needed to filter out any particulate sulfide of the samples,
and it is necessary to do a pH-based calculation of the total dissolved sulfide from the SulfiLogger™ sensor
signal. Furthermore, it should be noted that correct sampling techniques and sample preservation should
be conducted with utmost care to avoid significant influences on the measured sulfide concentrations in the
grab samples. For further information and guidance on procedures for liquid phase comparisons, please
contact Unisense.
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