Re-calibration methodology for NDIR gas sensors

A re-calibration method for a dual-beam NDIR gas sensor uses a calibration curve based upon a combination of physics and sensor measurement components of the sensor to calculate sample gas concentration, then determines a second gas concentration measurement by a secondary gas standard which is used with a reversed calibration curve algorithm to adjust the sensor measurement component. The calibration curve is based upon a gamma ratio (“G”) that has been normalized by G when no sample gas is present in the sample chamber (“G₀”), G being a ratio of a signal channel output (“V_S”) of the sensor divided by a reference channel output (“V_R”) of the sensor. The concentration (“P”) of sample gas in the sensor is calculated through use of the calibration curve by a gas detection equation of P=F(x)=F(y/G₀), where x is a normalized ratio of V_S/V_R and y is G. The reversed calibration curve algorithm is P=F(x)=F(y/G_0N), where G_0N=y₁/x₂, y₁=G for the sensor, x₂=F⁻¹ (P₂) and P₂ is the second gas concentration of the sample gas.

Description

FIELD OF THE INVENTION

The present invention is in the field of measuring instruments, and specifically relates to a method for re-calibrating non-dispersive infrared (NDIR) gas sensors whose outputs have drifted over time and no longer correctly reflect their measurement accuracy.

BACKGROUND OF THE INVENTION

Output stability or drift over time leading to measurement inaccuracies has long been a major deficiency for gas sensors irrespective of what technology or methodology is used for their conception or realization. Output software correction may alleviate the problem somewhat but it is in many instances inaccurate and not even always applicable. It has long been the objective of many researchers in this field to overcome this problem fundamentally and for good. Recently the present inventor in U.S. application Ser. No. 12/859,749 advanced the teaching of an Absorption Biased NDIR Gas Sensing Methodology which is capable of eliminating substantially all NDIR gas sensor output drifts over time without the need for re-calibration (Wong, filed 19-AUG-2010). As it turns out, the solution to solving this output drift problem for gas sensors actually lies deeper than the availability of superior NDIR gas sensor types even though they can indeed be designed to be capable of maintaining measurement accuracy over time. The fact of the matter is that people have experienced gas sensor output instability for such a long time in the past that when output stable sensors really come along nobody would believe it. Until such time that stable gas sensors become widely available and users begin to consider their performance as trustworthy and truly believable, the real need today must be viewed at a completely different perspective and that is to be able to come up with a fast, inexpensive and simple methodology that can easily check the accuracy of gas sensors and more importantly, just as easy and simple, hence inexpensive, to re-calibrate them when they are found to be inaccurate.

It is therefore the primary objective of the present invention for the present author to advance a novel methodology to simply and easily re-calibrate an NDIR gas sensor.

SUMMARY OF THE INVENTION

The present invention is generally directed to a method for recalibrating a dual-beam NDIR gas sensor by using a calibration curve that is based upon a combination of a physics measurement component of the NDIR gas sensor and a sensor measurement component of the NDIR gas sensor to calculate a first concentration of sample gas, then using a secondary gas standard to determine a second gas concentration of the sample gas, and then recalibrating the NDIR gas sensor by using the second gas concentration and a reversed calibration curve algorithm which adjusts the sensor measurement component to correct for any difference between the first concentration and the second gas concentration when the difference between the two exceeds a preselected threshold.

In a separate group of aspects of the present invention, the calibration curve (which expresses the concentration of the sample gas as an nth order, e.g., a third order polynomial of G) is based upon a gamma ratio (“G”) that has been normalized by the gamma ratio when no sample gas is present in the sample chamber (“G₀”), G being a ratio of a signal channel output (“V_S”) of the NDIR gas sensor divided by a reference channel output (“V_R”) of the NDIR gas sensor. The concentration (“P”) of sample gas in the NDIR gas sensor is calculated through use of the calibration curve by a gas detection equation of P=F(x)=F(y/G₀), where x is a normalized ratio of V_S/V_R and y is G. The reversed calibration curve algorithm, which is a non-linear equation, is P=F(x)=F(y/G_0N), where G_0N=y₁/x₂, y₁=G for the NDIR gas sensor, x₂=F⁻¹ (P₂) and P₂ is the second gas concentration of the sample gas.

In another separate group of aspects of the present invention, the secondary gas standard (which can be a second NDIR gas sensor) is calibrated within a preselected time period prior to determining the second gas concentration and both the first concentration and the second concentration detect substantially the same concentration within a pre-selected space (e.g., a still space of less than 1,000 cubic feet). The second concentration can be transmitted to the NDIR gas sensor being recalibrated which receives the transmission and calculates gas concentration through electronics which use its calibration curve and recalibrate the NDIR gas sensor and then provide indication of recalibration.

Accordingly, it is a primary object of the present invention to provide an improved methodology for re-calibrating NDIR gassensors whose outputs have drifted over time and no longer correctly reflect their measurement accuracy.

This and further objects and advantages will be apparent to those skilled in the art in connection with the drawings and the detailed description of the invention set forth below.

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