OPERATING MANUAL OXYGEN MONITORING SYSTEM
OMS 420
9
5
O
2
sensor – operating theory
Heated zirconium oxide (ZrO
2
) is used as a ceramic solid electrolyte that is a good oxygen ion conductor at temperatures of ap-
proximately 1.550
o
F (850 °C), generated by an internal low power (20 W) heater element. The heater element is a PTC type, self-
regulating device that does not require a thermocouple for temperature regulation. Constant sensor temperature is maintained by
controlling the heater voltage and current to fixed resistance of the heater element.
The electro-motive force (emf ) that is generated across the solid electrolyte by the presence of oxygen ions can be measured as a
sensor voltage (according to Nernst law).
This voltage is measured by micro-controller based transmitter electronics and converted into a standard 4 – 20 mA signal, lin-
earized for oxygen in the range of 0 – 25 %.
The expected lifetime of this sensor is about 5 years + under normal operating conditions, and is not dependent on fuel type, but:
CAUTION:
•
If combustible gas (CO, H
2
, HC) in high concentrations are present in the sample gas, erroneous O
2
readings will re-
sult due to local combustion at the sensors’ hot surface.
•
Exposure to corrosive gases (silicone vapor, alkaline and heavy metals, P, Pb, high SO
2
, etc.) will shorten the life of
the sensor.
•
Condensation of flue gas moisture close to the sensor’s flange must be prevented.
where:
U
0
=
offset voltage (for P
O2 ref
= P
O2sample
)
R
=
universal gas constant
T
=
zirconium temperature
F
=
Faraday constant
P
O2ref
=
oxygen partial pressure reference side
P
O2sample
=
oxygen partial pressure sample side
www.
GlobalTestSupply
.com
Find Quality Products Online at:
