Optics: measuring and testing – For light transmission or absorption – Of fluent material
Reexamination Certificate
1999-03-02
2001-01-09
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
Of fluent material
C422S051000, C436S164000
Reexamination Certificate
active
06172759
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to detection systems used to detect harmful levels of a target gas in the air and, more particularly, relates to a target gas detection system comprising a set of optically-responding sensors that are designed to determine the concentration of a target gas by measuring sensor photon transmission levels, and be rapidly regenerating.
BACKGROUND OF THE INVENTION
The present invention relates to a sensing system comprising a set of optical responding sensors (a set consisting of at least one sensor) for detecting the presence of at least one target gas and, more particularly, to an apparatus incorporating two sensors in the sensing system, and a method for determining the concentration of at least one target gas by monitoring the optical response of the sensor within a given period of time, for use as monitoring devices in a wide variety of applications, such as home safety, occupational health, industrial, commercial, recreational and medical, and specifically to meet the British Standard Institute carbon monoxide standard.
SUMMARY OF THE INVENTION
There is provided several preferred embodiments of the present invention, i.e., an apparatus and method for determining the concentration or hazard of any particular target gas by a sensor or sensor system that changes its optical properties in response to said gas and return to the normal state after the target gas is removed very rapidly. The basic method is to measure the rates of the reactions, i.e., the forward and regeneration. A simplification of the exponential Coburn type equation into a simple differential equation so they may be easily approximated by the simpler equation(s) such as equation 1 or 2 or by means of a look up table system. These simple equation can be used in small inexpensive microprocessors or other logic circuits. A series of sensors are placed close together within a sensing system, e.g., in a row so that they may be monitored conveniently. Each of these sensors may be best used over the linear portion of its response curve (if the carbon monoxide concentration is above a certain level D) thus simplifying most of the complex equations to very simple programming to measure slope, as previously described in U.S. patent application Ser. No. 80/026,534. Then under special low carbon monoxide (CO) concentration (less than L) the most sensitive sensor response may be approximated by a simple equation or a look-up table.
EXAMPLE of a Carbon Monoxide Sensing System to meet British Standard Institute (BSI) carbon monoxide standard.
For example, a low power application of the invention includes the steps of intermittently measuring optical characteristics of an optical responding sensor or sensor system using pulsed technology, differentiating a plurality of measurements over time to obtain both the I(n) value(s) at time t and the rate of change of the measured optical characteristics of the sensors, and determining the concentration of target gas as a function of both I(n) at time t and the rate of change of the measured optical characteristics at time interval t(0) and time t. The target gas concentration information may be calculated by means of a look up table or may be calculated by an algorithm and then set to signal and or display the alarm. The simplest way to meet the BSI CO standard is to set the software to the specific requirements of that standard which included 45 ppm no response in 60 minutes, 150 ppm in 35 minutes, 350 in 6 minutes both alarm levels must reset within 15 minutes in clean air. There are temperature and humidity tests that don't present any problems for standard products that meet UL 2034. The over range test is unique, it requires 3 separate exposure over 30 day periods to 5000 ppm with reset with 30 minutes for each test.
Exemplary apparatus according to the present invention for implementing the method includes at least one and in some cases two light emitting diodes (LEDs) and two photodiodes with the target gas sensors located so that the photons pass through the sensors and the amount of light transmitted is measure by the photodiode response to that light. Each LED illuminates a corresponding sensor, and the light transmitted through the sensor is received by the photodiode.
For a low power low cost embodiment of the invention designed to determine the concentration of CO as the target gas, it is desirable to change the analog signal to digital (at very low power and low cost), the resulting photocurrent from the photodiode, proportional to the transmitted light received by the photodiode, charges a series of capacitors set to a threshold value programmed into the microprocessor as an action level such as an alarm point. The charging time of the series capacitors along with the voltage or current measured by the series of photodiodes I(n)s is recorded by the microprocessor to produce a plurality of sensor readings proportional to the darkness of the CO sensor and its rate of change. Differences of adjacent readings in the second sensor are determined only when the first sensor is saturated (no longer changing or is below a certain level such as 20% transmission. If the detector register exceeds a predetermined value or the concentration goes above a certain value, or the TWA exceed a specific value the a condition of alarm may be so indicated. And if the second sensor regenerates do to clean air it can be seen long before the first sensor regeneration can be seen. This is the key to meeting the BSI standard at 5000 ppm CO over range tests.
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Christie Parker & Hale LLP
Nguyen Sang H.
Pham Hoa Q.
Quantum Group Inc.
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