Teledyne Max-5 - Combustion efficiency analyzer - Manual (Page 8)

* continuous flow of all six readings
F. Air
Flowmeter with flow set valve to regulate air flow.
G. Sample
Flowmeter with flow set valve to regulate sample flow.
H. Chemical
Scrubber for removal of corrosive compounds, such as SO
2
,, H
2
S, and NOx.
I. Sample-in
Port.
J. Coalescing
Filter (to remove particulates and water) and filter-drop out pot.
PRINCIPLE OF OPERATION
SENSOR DESCRIPTION
Oxygen. The oxygen section uses Teledyne’s Micro-Fuel Cell, Class B-3 oxygen sensor (U.S.
Patent No. 3,429,796). The B-3 Micro-Fuel Cell measures the concentration of oxygen in a gas
stream. The analysis is specific for oxygen; i.e., the measuring cell will not generate an output
current unless oxygen is present in the sample gas. Therefore, the oxygen channel has absolute
zero, and no zero gas is required to standardize the analyzer. The standard range is 0-25%
oxygen, and you perform span calibration with atmospheric air as the span gas, where feasible.
Carbon Monoxide. The class F-1R* solid-state electrochemical sensor measures CO. The
solid- state design eliminates leakage. You zero the CO section against air and span with a span
gas that contains the measured component in a concentration equivalent to 80-90% of the
instrument’s maximum fullscale value. TBE/AI sets the standard measurement range at 0-1000
PPM.
Combustibles. The combustibles sensor is a low-temperature, catalytic bead type transducer in
a constant current-excited Wheatstone Bridge circuit. Two legs of the bridge are exposed to the
sample gas. The other two legs are passive elements.
Gas diffuses into the sensing element and oxidizes at the catalytic surface of the active or
measuring bead, causing its temperature to rise. The reference head is not catalytically coated,
so it is not heated by the combustibles. The difference in temperature and therefore resistance of
the otherwise matched pair of catalytic beads creates a signal in the bridge circuit.
Use of the uncoated reference bead compensates for the effects of temperature variations,
humidity changes, ambient pressure changes, and variations in line resistance. The beads are
installed in a housing that has a flashback arrestor screen at the sensing aperture to prevent flame
propagation into the process.
The combustibles readout is expressed in percentage LEL (methane equivalents). Air is the zero
gas, and the span gas follows the 80-90% fullscale rule. By far the most common range is 0-5%
methane equivalence (0-100% LEL), and the span gas for that range would be 3% to 4%