Teledyne 9070 - O2 and CO2 Food package analyzer (preliminary) - Manual (Page 11)

DESCRIPTION
2.1 THE ZIRCONIA SENSOR
The oxygen Analyzer input is provided from a solid electrolyte oxygen sensor which contains a zirconia element and
thermocouple. The sensor is designed to have a small sample of the unknown gas passed into the inside of the sensor
tube, and air (20.95% oxygen) around the outside. A heater is mounted around the sensor to keep the sensor hot. The
sensor construction is shown in Figure 2.1.
Internal electrode wire
Ziconia Disc
Thermocouple
Wires
Electrode Material
(required only
Heater
with heated
probes)
External Wire
Contact
Thermocouple
Junction
Internal Electrode
Four-Bore Thermocouple
Insulating Tubing
Alumina Tube
Fig 2.1 Schematic View of a Sensor Assembly
The heater control is a time proportioning temperature controller and triac so that the thermocouple junction is
controlled to between 720°C (1320°F) and 720°C.
When exposed to different oxygen partial pressures at the outside and inside of the sensor, an EMF (E) is developed
which obeys the Nernst equation:
E (millivolts) =
RT
4F log e 


 (PO
2
) INSIDE
(PO
2
) OUTSIDE
Where T is the temperature (K) at the zirconia disc (>650°C (>1200°F) ), R is the gas constant, F is the Faraday
constant and (PO
2
) INSIDE and (PO
2
) OUTSIDE are the oxygen partial pressures at the inner and outer electrodes,
respectively, with the higher oxygen partial pressure electrode being positive.
If dry air at atmospheric pressure, (21 % oxygen) is used as a reference gas at the inner electrode, the following
equations are obtained:
E (millivolts) = 2.154 x 10 -2
T log
e
0.21
(PO
2
) OUTSIDE
Transposing this equation
(%O
2
) OUTSIDE (ATM) = 0.21 EXP
-46.421E
T
The 9070 transmitter solves this equation which is valid above 650°C (1200°F). The oxygen sensor heater maintains
the sensor temperature at this level.
2.2 THE OXYGEN SENSOR ASSEMBLY
The oxygen sensor assembly provides a means of exposing the zirconia sensor to the atmosphere to be measured on the
inside of the sensor, and maintain air as a reference gas on the outside of the sensor. A small volume, (from as little as
150 cc (20 scfm) /min) of the gas to be sampled is either pumped through the assembly at a known rate by the internal
pump. The gas flow rate may also be monitored by a flow rate switch, which will cause an alarm if the rate falls below
120 cc (15 scfm) /min.
Teledyne Analytical Instruments 9070 Oxygen Analyzer Page 11