Oxygen Monitor SGM7.2.4/7.2.6
4 Functional description
11
Based on the assumption that the total pressures of the gases are almost
the same at both electrodes (in this case the volume concentrations may
be used in the calculation instead of the partial pressures) and replacing
the parameters by numbers in equation (
I
) the following equation applies:
Equation
for oxygen
concentr.
ϕ
O
2
=
20.64 · e
(-46,42 ·
)
(II)
ϕ
O2
– oxygen concentration in the measuring gas in vol%
U
– potential difference in mV
T
– measuring temperature in K
20.64
– oxygen concentration in air with a relative
humidity of 50% in vol%
4.2
General recommendation
The oxygen may be in free or bound form inside the measuring gas
1
. (see
chapter 10.1)
Thereby, the following dependencies are valid:
– for free oxygen
– for bound oxygen
The equation
(II)
for calculating the oxygen concentration is valid for
measuring gases with free oxygen as well as for reducing gas compounds in
which oxygen only exists in bound form (e.g. in H
2
/H
2
O- or CO/CO
2
-
compounds).
4.2.1
Gas flow rate
The constant flow rate of the measuring gas is ensured by an internal pump.
It can be chosen between
HIGH
(35 l/h, maintenance interval runs 3 times
faster than in the LOW mode) and
LOW
(8 l/h).
1
D
ifferent conditions of oxygen in the measuring gas must be distinguished:
Free oxygen:
Oxygen molecules in the gas are independent without a bond to other gas components
(inert gases such as N
2
or Ar).
Bound oxygen:
Free oxygen molecules do not exist in the gas, only in bound form e.g. as water
vapor. Higher temperatures cause a dissociation and oxygen molecules are available. Since the
dissociation degree increases with the temperature, the measurement result depends on
the
temperature.
Possibly, free oxygen can react with potential burnable gases at the hot platinum electrode. The result
can be a reducing gas.
U
T
U
T
~
U
T
~
1
