System Overview; Scope; System Description - Emerson Rosemount OCX 8800 Reference Manual

1.2

System overview

1.2.1

Scope

This Instruction Manual supplies details needed to install, start up, operate, and maintain
the OCX 8800. Signal conditioning electronics output a digital signal representing oxygen
(O ) and combustibles (COe) values. This information, plus additional details, can be
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accessed with the 375 Field communicator or Emerson AMS software..
1.2.2

System description

The OCX 8800 is designed to measure oxygen and combustible concentrations in flue gas
temperatures up to 1427 °C (2600 °F). Electrical connections, power, and communications
are made through two 3/4 NPT ports in the flameproof electronics enclosure using fittings
and cables provided by you. 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 minimum 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 operating at the proper
temperature, the following Nernst equation gives the millivolt output of the cell.
EMF = KT log10 (P
where:
1.
2.
3.
4.
5.
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 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 lograrithm of
the oxygen concentration. Therefore, the output signal increases as the oxygen
concentration of the sample decreases. This characteristic enables the transmitter to
provide exceptional sensitivity at low oxygen concentrations.
Reference Manual
/P ) + C
1 2
P is the partial pressure of the oxygen in the measured gas on one side of the cell.
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P is the partial pressure of the oxygen in the reference air on the opposite side of
1
the cell.
T is the absolute temperature.
C is the cell constant.
K is an arithmetic constant.
Description and specifications
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