2
3
4
Oxygen Meter
This sensor is a galvanic cell-type oxygen sensor that
measures oxygen gas (O
2
) in air. It has a lead anode, a gold
cathode, an acid electrolyte, and a fluorine resin membrane.
The current flow between the electrodes is proportional to
the oxygen concentration being measure. An internal bridge
resistor is used to provide an mV output linearly proportional
to O
2
. Unlike polargraphic oxygen sensors, galvanic cells do not
require a power supply. The hand-held meter used to measure
the sensor uses a 3 V coin cell battery.
The mV output responds to the partial pressure of oxygen in
air. The standard units for partial pressure are kPa. However,
gas sensors that respond to partial pressures are typically
calibrated to read out in mole fraction of the gas in air, or units
of moles of oxygen per mole of air. These units can be directly
converted to % O
2
in air, or ppm O
2
in air. The concentration
of oxygen in our atmosphere is 20.95%, and this precise
percentage has not changed for decades. It is also constant
across changing temperatures or pressures. This allows for
precise calibration of the instrument.
Being a galvanic cell type sensor, a small amount of oxygen
is consumed in the reaction in order to produce the current
flow and subsequent mV output. The oxygen consumption
was measured to be 2.2 µmol O
2
per day when the O
2
concentration was 20.95% (3240 mmol) at 23 C.
Humidity Changes
The graph below shows an example of humidity dependency.
The sensor chemistry is not influenced by humidity, but its
output decreases because O
2
is displaced by water vapor
molecules in the air. The effect of humidity is larger at warmer
temperatures because there is more water vapor in the air. To
eliminate humidity effects, simply recalibrate.
For use in high humidity, such as soil, remove
the head and take the calibration measurement
over water in a sealed container as shown at
right.
Oxygen Meter
MO-200 Series
Effects on Output
For the most reliable measurements, the O
2
sensor is designed
to be used in a vertical position, as shown. A flow-through or
diffusion head is not required for making measurements.
Influences from Various Gases
The sensor is unaffected by CO, CO
2
, NO, NO
2
, H
2
S, H
2
, and
CH
4
. There is a small effect (approximately 1%) from NH
3
, HCI,
and C
6
H
6
(benzene). The sensor is sensitive to SO
2
and can be
damaged by O
3
.
