An NDIR gas sensor is housed within a mechanical housing made up of a can and a header housing. The header housing body contains a tunnel waveguide sample chamber or a custom dish sample chamber is formed when increased sensitivity is required. The header housing also has a top surface with a pair of windows formed in it and a signal detector, a reference detector, an infrared source and a signal processor mounted to it. The can has inner reflective surfaces and the reference detector and the signal detector are affixed to the top surface so that the inner reflective surfaces of the can and the sample chamber create a signal channel path length detected by the signal detector that is greater than a reference channel path length detected by the reference detector and an absorption bias between the signal and reference outputs can be used to determine a gas concentration in the sample chamber. Both the signal detector and the reference detector have an identical narrow band pass filter with the same Center Wavelength ("CWL"), Full Width Half Maximum (FWHM) and transmittance efficiency at the CWL.

Description
An Intrinsically Safe NDIR Gas Sensor in a Can
Field of The Invention
The present application is in the field of gas analysis, and specifically relates to apparatus using a Non-Dispersive Infrared (NDIR) gas analysis technique to determine the concentration of a gas of interest that is present in a chamber by sensing the absorption of infrared radiation passing through the gas.
Background of the Invention
We are living in a gaseous world and the type of gases surrounding our everyday life, for example in where we live, work or play, is vital to our well-being, safety, and even our very survival. Exposure to prolonged insufficient oxygen levels (-15% or less) can make us very sick or might even be fatal to us at times. Too much water vapor in the air surrounding us, especially when the temperature is very high (>90°F), can make us very uncomfortable or seriously ill. For older folks, exposure to high humidity and very high temperature for prolonged periods of time can even be fatal. Unchecked exposure to, or unintentional breathing of, toxic gases above a certain high concentration level such as Carbon Monoxide (70 -400 ppm), Hydrogen Sulfide (50-200 ppm), Formaldehyde (>50 ppb) etc., to name just a few, is extremely hazardous to one's health and often leads to unexpected deaths.
In order to prevent accidental or unintended exposure to unsafe levels of gases, humans have long devised, literally from centuries ago until today, various means of detecting all manners of gases, whether they are actually harmful to them or not. Today one can classify all the significant and still prevalent gas measurement techniques developed to date into two broad categories, namely, interactive and non-interactive types. Among the interactive types are electrochemical fuel cells, tin oxide ( Sn02) sensors, metal oxide semiconductor (MOS) sensors, catalytic (platinum bead) sensors, photo-ionization detectors (PID), flame-ionization detectors (FID), thermal conductivity sensors etc., almost all of which suffer from long-term output drifts, short life span and non-specificity problems. Non-interactive types include Non-Dispersive Infrared (NDIR), photo- acoustic and tunable diode laser absorption spectroscopy (TDLAS) gas sensors. Up and coming non-interactive techniques advanced only during the past two decades include the use of the latest micro electromechanical technologies such as MicroElectronic Mechanical Systems (MEMS) and the so-called Nano- technology. However, probably a few more years have to pass before the potential of these new non-interactive type gas sensors is fully obtainable.
With so many gas detection techniques available over the years, one could easily be misled to believe that gas sensors today must be plentiful and readily available to people to avoid harmful exposure to unhealthy or toxic gases. Unfortunately, at the present time, this is far from being the truth. The reasons are constraints arising from sensor performance and sensor cost. As a result, gas sensors today are deployed for safety reasons only in the most critical and needed circumstances. An example can be cited in the case of the kerosene heater. A kerosene heater is a very cost effective and reliable appliance used all over the world for generating needed heat during the winter months. However, it can also be a deadly appliance when used in a space where there is inadequate ventilation. In such a situation, as oxygen is being consumed without adequate replenishment, the oxygen level in the space can drop to a point (