System Description - Emerson Rosemount OCX8800 Instruction Manual

Instruction Manual
IM-106-880C, OI
1.3.2

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
Where:
1. P is the partial pressure of the oxygen in the measured gas on one side of the cell.
2
2. P is the partial pressure of the oxygen in the reference air on the opposite side of the cell.
1
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.
The Hazardous Area OCX 8800 measures net oxygen concentration in the presence of all the
products of combustion, including water vapor. Therefore, it may be considered an analysis
on a "wet" basis. In comparison with older methods, such as the portable apparatus, which
provides an analysis on a "dry" gas basis, the "wet" analysis will, in general, indicate a lower
percentage of oxygen. The difference will be proportional to the water content of the
sampled gas stream.
System
Description
/P ) + C
1 2
Section 1: Description and
Specifications
April 2017
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