Electrolysis Device SGM5EL Functional description
8
4 Functional description
4.1 Measuring principle
Basic element of the SGM5-EL is the “combi cell”. An electrolysis cell and
additionally an analysis cell are located on a ZrO
2
solid electrolyte pipe
(general composition see chapter 6). Electrolysis and analysis cell are
electrochemical cells with two platinum electrodes each. The combi cell
separates the working substance from the ambient air.
If a current flows through the electrolysis cell, oxygen in chemically pure form
will be transferred from or into the gas room based on
FARADAY
`s Law:
F
n
t
I
⋅
⋅
=
⋅
4
(I)
I - current in A
n - amount of substance
t - time in s
4 - charge number
F - Faraday constant
The driving force for the current flow through the cell is the resultant from the
cell’s outside voltage u and the cell’s voltage U
eq.
If the sign of the resultant
voltage corresponds to the sign of the cell voltage, oxygen flows through the
solid electrolyte wall in the direction of the concentration gradient. If the sign
of the resulting voltage reverses, oxygen is pumped against the
concentration gradient.
For the pumping performance (in cm
3
s
-1
) the following applies:
v (O
2
) = 0,06336·I
/A
The real oxygen concentration of the measuring gas is determined by the
analysis cell. Its cell voltage U
eq
in currentless condition depends on the
oxygen partial pressure in the working substance and in the air as well as on
the cell temperature. This correlation is based on the
NERNST
Law:
air
O
gas
meas
O
p
p
F
RT
U
,
.
2
,
2
ln
4
=
(II)
U - cell voltage in V
R - molar gas constant
T - measuring temperature in K
F - Faraday constant
p
O2,air
– oxygen partial
pressure at the reference
electrode in dry air in Pa
p
O2,meas.gas
– oxygen partial
pressure at the measuring
electrode in Pa.