Emerson 3081FG instruction manual System Overview, Scope, System Description, System Configuration

Model 3081FG

Instruction Manual

IB-106-3081 Rev. 1.5

September 2002

1

1-2 SYSTEM OVERVIEW

a.Scope

This Instruction Bulletin is designed to sup- ply details needed to install, start up, oper- ate, and maintain the Rosemount Two-Wire In Situ Oxygen Analyzer. The analyzer con- sists of an oxygen probe and Model 3081 Transmitter. The signal conditioning elec- tronics of the Model 3081 Transmitter out- puts a 4-20 mA signal representing an O2 value. An infrared remote control (IRC) al- lows access to setup, calibration, and diag- nostics. This same information, plus additional details, can be accessed with the HART Model 275 handheld communicator or Asset Management Solutions (AMS) software.

b.System Description

The Rosemount Two-Wire In Situ Oxygen Analyzer is designed to measure the net concentration of oxygen in an industrial pro- cess; i.e., the oxygen remaining after all fu- els have been oxidized. The oxygen probe is permanently positioned within an exhaust duct or stack and performs its task without the use of a sampling system. The Model 3081 Transmitter is mounted remotely and conditions the oxygen probe outputs.

The equipment measures oxygen percent- age by reading the voltage developed across an electrochemical cell, which con- sists of a small yttria-stabilized, zirconia disc. Both sides of the disc are coated with porous metal electrodes. The millivolt output voltage of the cell is given by the following Nernst equation:

EMF = KT log10(P1/P2) + C

Where:

NOTE

For best results, use clean, dry, in- strument air (20.95% oxygen) as the reference air.

NOTE

The probe uses a Type B thermocou- ple to measure the cell temperature.

When the cell is at 550°C to 1600°C (1022°F to 2912°F) and there are unequal oxygen concentrations across the cell, oxy- gen ions will travel from the high oxygen partial pressure side to the low oxygen par- tial 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 Rosemount Two-Wire In Situ Oxygen Ana- lyzer to provide exceptional sensitivity and accuracy at low oxygen concentrations.

Oxygen analyzer equipment 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 oxy- gen. The difference will be proportional to the water content of the sampled gas stream.

c. System Configuration

1.P2 is the partial pressure of the oxygen in the measured gas on one side of the cell.

2.P1 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.

The equipment discussed in this manual consists of two major components: the oxy- gen probe and the Model 3081 Transmitter.

Oxygen probes are available in three length options, providing in situ penetration appro- priate to the size of the stack or duct. The options on length are 20 in. (508 mm), 26 in. (660 mm), or 34.625 in. (880 mm).

Rosemount Analytical Inc. A Division of Emerson Process Management Description and Specifications 1-3