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Instruction Manual
IM-106-880C, OI
September 2009
1-3
Hazardous Area OCX 8800
SYSTEM OVERVIEW Scope
This Instruction Manual supplies details needed to install, startup, operate,
and maintain the Hazardous Area OCX 8800. Signal conditioning electronics
outputs a digital signal representing oxygen (O
2
) and combustibles (COe)
values. This information, plus additional details, can be accessed with the 375
Field communicator or Emerson Process Management AMS software. The
optional local operator interface (LOI) also provides a communications
interface with the electronics.
System Description
The Hazardous Area OCX 8800 is designed to measure oxygen and
combustible concentrations in flue gas temperatures up to 2600°F (1427°C).
Electrical connections, power and communications are made through two 3/4
NPT ports in the flameproof electronics enclosure using fittings and cables
provided by the customer. Cable installation must meet NEC, IEC and/or
other applicable national or local codes for Class I, Zone 1, Group IIB +H2
T3/T6 permanently mounted equipment. The transmitter is close coupled to
the process and requires minimal sample conditioning requirements.
The equipment measures oxygen percentage by reading the voltage
developed across a heated electrochemical cell, which consists of a small
yttria-stabilized, zirconia disc. Both sides of the disc are coated with porous
metal electrodes. When operated at the proper temperature, the millivolt
output of the cell is given by the following Nernst equation:
EMF = KT log10 (P
1
/P
2
) + C
Where:
1. P
2
is the partial pressure of the oxygen in the measured gas on one side
of the cell.
2. P
1
is the partial pressure of the oxygen in the reference air on the
opposite side of the cell.
3. T is the absolute temperature.
4. C is the cell constant.
5. K is an arithmetic constant.
NOTE
For best results, use clean, dry instrument air (20.95% oxygen) as the
reference air.
When the cell is at operating temperature and there are unequal oxygen
concentrations across the cell, oxygen ions will travel from the high oxygen
partial pressure side to the low oxygen partial pressure side of the cell. The
resulting logarithmic output voltage is approximately 50 mV per decade. The
output is proportional to the inverse logarithm of the oxygen concentration.
Therefore, the output signal increases as the oxygen concentration of the
sample gas decreases. This characteristic enables the Hazardous Area OCX
8800 to provide exceptional sensitivity at low oxygen concentrations.