The concentration of a gas species is detected by using a single beam
NDIR gas sensor in which an infrared source element is driven at two
different temperatures, a feed back loop senses an operation voltage of
the source, a differential gain amplifier creates a high cycle amplified
output during a high cycle and a low cycle amplified output during a
low cycle while a controller synchronizes the source driver so that a
signal processor can determine the gas concentration through use of the
high cycle amplified output and the low cycle amplified output. The
infrared source can be a non-genuine blackbody source such as an
incandescent miniature light bulb when the sample chamber is a thermally
insulated aluminum tube that is maintained at a preselected temperature
greater than ambient so that the glass envelope of the bulb is
maintained at an equilibrium temperature (such as approximately 30
degrees Celsius plus or minus two degrees Celsius) during its low cycle
operation state.
Description
FIELD OF THE INVENTION
The present invention generally relates to the field of gas sensing devices and, more particularly, to NDIR gas analyzers.
BACKGROUND OF THE INVENTION
Non-Dispersive infrared (NDIR) gas analyzers have been used for
detecting the presence and concentration of various gases for over four
decades. The NDIR technique has long been considered as one of the best
methods for gas measurement. In addition to being highly specific, NDIR
gas analyzers are also very sensitive, stable and easy to operate and
maintain.
In contrast to NDIR gas sensors, the majority of other types of gas
sensors today are in principle interactive. Interactive gas sensors are
less reliable, generally nonspecific, and in some cases can be poisoned
or saturated into a nonfunctional or irrecoverable state.
Despite the fact that interactive gas sensors are mostly unreliable and
that the NDIR gas measurement technique is one the of best there is,
NDIR gas analyzers have still not enjoyed widespread usage to date
mainly because of the fact that their cost is still not low enough as
compared to other inferior gas sensors for many applications.
In the past, NDIR gas analyzers typically included an infrared source, a
motor-driven mechanical chopper to modulate the source, a pump to push
or pull gas through a sample chamber, a narrow bandpass interference
filter, a sensitive infrared detector plus expensive infrared optics and
windows to focus the infrared energy from the source to the detector.
In an attempt to reduce the cost and simplify the implementation of the
NDIR methodology, a low-cost NDIR gas sensor technique was earlier
developed. This low-cost NDIR technique employs a diffusion-type gas
sample chamber of the type disclosed in U.S. Pat. No. 5,163,332, issued
on Nov. 17, 1992 to Wong, one of the present applicants. This
diffusion-type gas sample chamber eliminates the need for expensive
optics, mechanical choppers and a pump for pushing or pulling the gas
into the sample chamber. As a result, a number of applications using
NDIR gas sampling technique, which were previously considered
impractical because of cost and complexity, have been rendered viable
ever since.
In the ensuing years since the U.S. Pat. No. 5,163,332 (1992) was
issued, Wong, one of the present applicants, continued to refine and
improve low-cost NDIR gas sampling techniques as evidenced by the
issuance of U.S. Pat. No. 5,222,389 (June 1993), U.S. Pat. No. 5,341,214
(August 1994), U.S. Pat. No. 5,347,474 (September 1994), U.S Pat. No.
5,453,621 (September 1995), U.S. Pat. No. 5,502,308 (March 1996), U.S.
Pat. No. 5,747,808 (May 1998), U.S. Pat. No. 5,834,777 (November 1998)
and U.S. Pat. No. 6,237,575 (May 2001) to same. However, it has been
quite apparent that despite the intense efforts over the years by Wong
and others in the field, the unit sale price of NDIR gas sensors is
still too high for many applications. It is of interest to note that
back in 1991 and prior to the issuance of U.S. Pat. No. 5,163,332 (1992)
to Wong, the same inventor has earlier advanced the concept of a
simpler
NDIR sensor methodology using spectral ratioing technique with a
differential temperature infrared source in U.S. Pat. No. 5,026,992
(1991). However, even after almost 15 years, this concept has to date
neither been proven to be viable in theory nor has it been
experimentally demonstrated to illustrate its practicality. It was found
out only very recently and experimentally by Wong, the original
inventor of U.S. Pat. No. 5,026,992 (1991) and one of the present
applicants, that although the concept as suggested by the author was
sound, the method does not work if the prescribed steps were followed
exactly according to the teaching of the patent. Furthermore, it was
found out by the present applicants that the methodology itself has to
be completely reformulated taking into consideration the shortcomings of
both the method and the system components as suggested by the original
inventor.
There is still a long felt need in a variety of industries and
applications to use lower cost
NDIR gas sensors, and so far this desire
has gone unanswered. It is this need that the current application seeks
to address and bring about a new and novel technique for the design and
implementation for ultra low cost NDIR gas sensors.
