Light source for open-path gas monitoring

Details Light source for open-path gas monitoring Light source for open-path gas monitoring

2001-09-26

2004-08-24

Adams, Russell (Department: 2851)

Optics: measuring and testing

For light transmission or absorption

Of fluent material

C362S555000, C362S558000, C362S227000, C362S257000, C250S339130, C073S001060

Reexamination Certificate

active

06781695

ABSTRACT:

FIELD OF THE INVENTION
The invention pertains to a light source used for open-path gas monitoring, particularly for the measurement of the smoke and dust content of stack gases, but also applicable to the measurement of particulates in the atmosphere.
BACKGROUND OF INVENTION
The standard method for continuous emissions measurement of particulates in stacks and ducts is optical transmissometry. The measured quantity is opacity, defined as the fraction of transmitted light which is lost in transmission through a medium.
One example of a device that measures opacity, known as a transmissometer, is the Land Combustion Model 4500mkII opacity monitor which has been used for a number of years to measure the opacity of gases in stacks and ducts. A functional diagram of the Model 4500mkII is shown in
FIG. 1
wherein the Model 4500mkII consists of two main units: a transceiver
20
mounted on one side of a stack/duct
22
and a passive retroreflector
24
mounted on the other side. A light source LS in the transceiver
20
projects a beam of light
26
along the transceiver's optical axis
27
across the duct
22
, through the dust/smoke in the open path
28
of the gas/smoke
29
(
FIG. 2
) to the retroreflector
24
which returns a reflected light beam
30
to an analyzer A in the transceiver
20
. The analyzer A then compares the intensity of the returned radiation with that measured under clear-stack conditions in order to calculate the opacity and then displays this opacity value at a remote location (e.g., a data recorder, not shown). Also see U.S. Pat. No. 5,617,212 (Stuart), whose entire disclosure is incorporated by reference herein, for a detailed description of how the analyzer A calculates the opacity.
FIG. 2
shows the Model 4500mkII mounted to the stack/duct
22
and depicts the internals of the transceiver
20
. In particular, the light source LS of the transceiver
20
comprises an LED (light emitting diode)
32
. The transceiver
20
also comprises a beamsplitter
34
, a collimating lens
36
, a folding mirror
38
, and the analyzer A which comprises a measurement detector
40
, a reference detector
42
and a processor
43
(e.g., Hitachi H8/500 microprocessor). Additional components include a flood LED
44
for drift correction, an automatic zero
46
and span
48
devices and a fail-safe shutter
50
. It should be understood that the transmissometer is autocollimated meaning that the return light
30
from the retroreflector
24
is along the same path as the projected beam
26
. External electrical power (e.g., 110VAC @ 60 Hz), not shown, is provided to the transceiver
20
for energizing the electrical components.
The divergence
52
of the projected light beam
26
means that the retroreflector
24
returns only a portion of the projected light
26
. Any change in alignment, (e.g., because of temperature changes, wind, settling, etc.) in the stack/duct
22
walls, results in a different portion of the projected beam
26
falling on the retroreflector
24
. Moreover, because the projected beam
26
is not perfectly homogeneous, i.e., the light intensity varies across the projected beam (see line
54
), a change in alignment results in a change in light intensity. This change is wrongly interpreted by the analyzer A as a change in the opacity of the stack/duct
22
gases.
Errors are also introduced where an opacity monitor (transmissometer) with an inhomogeneous light beam is calibrated in the laboratory and then installed on the stack/duct
22
. In this case, failure to perfectly reproduce the device's optical alignment between the laboratory and the duct results in a signal offset. This offset is, in many cases, the dominant source of error in the measurement. As a consequence, the detection limit of the opacity monitor may be set by this offset.
A number of factors affect the homogeneity of the projected beam
26
, including the precision and cleanliness of the optical components used. However, the principal factor is usually the inhomogeneity of the light source LS. There are a number of factors which make the pattern of light from an LED inhomogeneous. Some of these are symmetrical about the optical axis of the LED and some are not. This is especially so when a LED source is used, since the electrical contact to the center of the die results in a dark spot in the middle of the beam
26
.
One way of producing a homogeneous light source is to use an integrating sphere, such as that described in “A Guide to Integrating Sphere Theory and Applications” by Labsphere. However, an integrating sphere is both bulkier and more expensive than the present invention.
The limitations of the present state of the art are reflected in ASTM (American Society for Testing and Materials) Standard Practice for Opacity Monitor Manufacturers to Certify Conformance with Design and Performance Specifications D6216-98 (1998) which is incorporated by reference into U.S. 40 C.F.R. §60, Appendix B, EPA Performance Specification 1, and which concerns the use of opacity monitors for regulatory applications at opacity levels of 10% or higher. However, where detecting opacity levels of less than 10% is important, e.g., in the steel industry, no performance specification currently exists for the use of opacity monitors to ensure compliance with opacity limits below 10%.
Thus, there remains a need for a transmissometer that can minimize the inhomogeneity of the light source and can therefore detect opacity levels below 10% while operating within specific performance requirements.
SUMMARY OF THE INVENTION
A light source for use in an opacity monitor for measuring the opacity of gases in an open path of gases wherein the light source reduces the variation in light intensity across a beam of light projected therefrom.
A light source adapted for use in open path gas monitoring wherein the light source generates a homogeneous light beam.
An opacity monitor for measuring the opacity of gases in an open path of gases wherein opacity is defined as the fraction of transmitted light which is lost in transmission through the open path of gases. The opacity monitor comprises: an optical transmitter for projecting a light beam across the open path of gases using a light source that reduces the variation in light intensity across the projected beam; a reflector for reflecting a portion of the projected light beam back towards the optical transmitter through the open path gas of gases; an analyzer for detecting the portion of the projected light beam and calculating the opacity of the gases; and wherein the optical monitor detects opacities less than 10 percent while operating within specific performance requirements (e.g., all the requirements of ASTM D6216-98, including amendments to specific portions of ASTM D6216-98 to ensure compliance with opacity limits below 10%, such as thermal stability, insensitivity to ambient light, zero and span calibration, measurement of output resolution, calibration error, optical alignment indicator, calibration device value and repeatability, and insensitivity to supply voltage variations).
An opacity monitor for measuring the opacity of gases in an open path of gases wherein opacity is defined as the fraction of transmitted light which is lost in transmission through the open path of gases. The opacity monitor comprises: an optical transmitter having a light source that projects a homogeneous light beam across the open path of gases; a reflector for reflecting a portion of the projected homogeneous light beam back towards the optical transmitter through the open path gas of gases; an analyzer for detecting the portion of the projected homogeneous light beam and calculating the opacity of the gases; and wherein the optical monitor detects opacities less than 10 percent while operating within specific performance requirements(e.g., all the requirements of ASTM D6216-98, including amendments to specific portions of ASTM D6216-98 to ensure compliance with opacity limits below 10%, such as thermal stability, insensitivity to ambient light, zero and span calibra

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