output November 19, 2019 13:11 Page 5
SulfiLogger_Manual_EN_203.pdf
OPERATING PRINCIPLES
ELECTROCHEMICAL MEASUREMENTS OF HYDROGEN SULFIDE
The SulfiLogger
™
sensor contains multiple individual sensing elements which are all based on Unisense core
microsensor technology. For further information on the technology, visit: www.sulfilogger.com.
The electrochemical microsensors in the SulfiLogger
™
sensor measures dissolved H
2
S, which is one of the
three forms of dissolved sulfide in equilibrium in the water phase; H
2
S, HS
-
, and S
2-
. For all practical purposes
in sewer applications, only the first two of these are of interest. The
pKa
value for deprotonation of H
2
S is
around pH=7, which is approximately the same as typical domestic wastewater. For this reason, wastewater
will typically contain both H
2
S and
HS−
, and a separate measurement of the pH-value of the wastewater is
necessary to calculate the total amount of dissolved sulfides in the wastewater. Please contact Unisense if
you need guidance on how to find the concentration of total dissolved sulfide from the SulfiLogger
™
sensor
signal (only dissolved H
2
S). For many purposes however, the dissolved H
2
S measured by the SulfiLogger
™
sensor is indeed the parameter of interest, since the concentration of dissolved H
2
S is directly linked to the
emission of gas phase H
2
S which gives rise to the well-known odour and corrosion problems in the sewer.
Measuring the concentration of H
2
S in the liquid instead of in the gas phase gives a more reliable signal
since the measurement is independent of the daily variations in ventilation conditions in the head space
of the sewer, and it is independent of the exact mounting position of the sensor (in the liquid, the sulfide
concentration is fairly homogeneous, whereas very large differences in gas phase concentration has been
seen from top to bottom of sewer head spaces).
Electrochemical sensors are sensitive to temperature changes, and for this reason each transducer in the
sensors head is compensated for temperature effects with reference to an internal temperature measure-
ment in the SulfiLogger™ sensor’s sensor head.
MEASURING IN DIFFERENT MEDIA -UNIT CONVERSIONS
The electrochemical microsensors in the SulfiLogger
™
sensor’ sensor head measures the partial pressure of
H
2
S. In the gas phase, this can be converted to a
ppm
-reading if the total barometric pressure is known, and
in the liquid phase the measured value can be converted to a concentration measured in either
µM
or
mg/L
by using Henry’s constant and the molar weight.
Each sensor is ordered with a specification of medium type, and the signal output from the SulfiLogger
™
sensor is chosen as
ppm
for gas phase sensor heads and
S−mg/L
for liquid phase sensor heads, the "S" in
front of mg/L meaning that only the sulfur fraction of the weight is counted in the specified measurement.
The
ppm
values for gas phase sensors is calculated by using a standard barometric pressure of
1
atm =
101325
Pa, and the
mg/L
values for liquid phase sensors is calculated by using a temperature corrected
value of Henry’s constant. Please note that other dissolved substances and high salinity values may also
influence the conversion to mg/L units. For more information on this, please contact Unisense.
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