Gas sensors

Abstract

A gas sensor for measuring concentration of a predetermined gas includes a light source (2) arranged to emit pulses of light, a measurement volume (10), a detector (4) arranged to receive light that has passed through the measurement volume (10), and an adaptable filter (6) disposed between the light source (2) and the detector (4). The gas sensor has a measurement state in which it passes at least one wavelength band which is absorbed by the gas and a reference state in which said wavelength band is attenuated relative to the measurement state. A controller is connected to each of the light source, the detector and the adaptable filter to change the adaptable filter between one of said measurement state and said reference state to the other at least once during a gas sensor operation period.

What is claimed is: 
     
         1 . A gas sensor for measuring a concentration of a gas, the gas sensor comprising:
 a light source configured to be switched on to emit light and to be switched off;   a measurement volume;   a detector configured to receive light that has passed through the measurement volume to output a photo detector signal;   an adaptable filter disposed between the light source and the detector and having a measurement state in which the adaptable filter passes at least one wavelength band which is absorbed by the gas and a reference state in which said wavelength band is attenuated relative to the measurement state; and   a controller connected to each of the light source, the detector and the adaptable filter, the controller being configured:   to switch the adaptable filter between the measurement state and the reference state;   to switch on the light source at a start of a gas sensor operation period and to control power to the light source during said gas sensor operation period;   to switch off the light source at an end of the gas sensor operation period so as to enter a low power or shutdown mode, wherein the light source is switched off during the low power or shutdown mode, and wherein:
 the control of power to the light source during said gas sensor operation period includes initiating an initial powering of the light source during a pre-heat stage of the gas sensor operation period to preheat the light source to a temperature below a desired source temperature; 
 the control of power to the light source during said gas sensor operation period includes controlling power to the light source during a measurement stage of the gas sensor operation period to reach a desired source temperature; 
 the measurement stage of the gas sensor operation period is subsequent to the pre-heat stage of the gas sensor operation period; and 
 the switching of the adaptable filter comprises changing the adaptable filter between one said measurement state and one said reference state during the measurement stage of said gas sensor operation period and subsequent to the light source reaching the desired source temperature; 
   to sample the photo detector signal subsequent to initiating the initial powering of the light source and prior to reaching the desired source temperature;   to sample the photo detector signal with the adaptable filter in the measurement state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature; and   to sample the photo detector signal with the adaptable filter in the reference state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature.   
     
     
         2 . The gas sensor of  claim 1 , wherein:
 the controller is configured to sample the photo detector signal subsequent to initiating the initial powering of the light source and prior to reaching the desired source temperature a plurality of times to provide a plurality of samples; and   the controller is configured to measure a rate at which an output from the detector, represented by the plurality of samples, changes with time.   
     
     
         3 . The gas sensor of  claim 1 , wherein the adaptable filter comprises a micro-electromechanical system (MEMS). 
     
     
         4 . The gas sensor of  claim 3 , wherein said MEMS filter comprises an arrangement for measuring a change of capacitance therein for diagnostic purposes. 
     
     
         5 . The gas sensor of  claim 1 , wherein the adaptable filter comprises a diffractive optical element having a plurality of grating bands arranged to be moved by an electrostatic potential. 
     
     
         6 . The gas sensor of  claim 1 , wherein said light source is the only light source and wherein said detector is the only detector. 
     
     
         7 . The gas sensor of  claim 1 , wherein the adaptable filter comprises a plurality of measurement states in each of which the adaptable filter passes at least one wavelength band which is absorbed by the gas and for each measurement at least one reference state in which the wavelength band corresponding to the measurement state is attenuated relative to said measurement state. 
     
     
         8 . The gas sensor of  claim 1 , wherein the controller is adapted to change the adaptable filter between said measurement state and one said reference state a plurality of times during said gas sensor operation period and subsequent to the light source reaching the desired source temperature and the sampling of the photo detector signal is synchronized by the controller to occur each time the adaptable filter is in the measurement state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature and to occur each time the adaptable filter is in the reference state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature. 
     
     
         9 . The gas sensor of  claim 1 , wherein:
 the control of power to the light source during said gas sensor operation period heats the source to a temperature that is low enough not to be measured by the detector from a time of initiating the initial powering of the light source during the pre-heat stage up to a time of the controlling of the power to the light source to reach a desired source temperature; and   the sampling of the photo detector signal subsequent to initiating the initial powering of the light source and prior to reaching the desired source temperature occurs with the light source heated to a temperature that is low enough not to be measured by the detector.   
     
     
         10 . The gas sensor of  claim 9 , wherein:
 the controller is configured to power the light source for a period to provide the pre-heat stage and to subsequently not power the light source for a period subsequent to the pre-heat stage and up to a time of the controlling of the power to the light source to reach a desired source temperature; and   the sampling of the photo detector signal subsequent to initiating the initial powering of the light source and prior to reaching the desired source temperature occurs with the controller not powering the light source.   
     
     
         11 . A wireless, battery-operated gas detector unit comprising the gas sensor of  claim 1 . 
     
     
         12 . A method of measuring a concentration of a gas, the method comprising the steps of:
 providing a gas sensor for measuring a concentration of a gas, the gas sensor comprising a light source configured to be switched on to emit light and to be switched off, a measurement volume, a detector configured to receive light that has passed through the measurement volume to output a photo detector signal, an adaptable filter disposed between the light source and the detector and having a measurement state in which the adaptable filter passes at least one wavelength band which is absorbed by the gas and a reference state in which said wavelength band is attenuated relative to the measurement state; and a controller connected to each of the light source, the detector and the adaptable filter;   switching, with the controller, on the light source at a start of a gas sensor operation period;   switching, with the controller, off the light source at an end of the gas sensor operation period so as to enter a low power or shutdown mode, wherein the light source is switched off during the low power or shutdown mode;   controlling, with the controller, power to the light source during said gas sensor operation period, including initiating an initial powering of the light source during a pre-heat stage of the gas sensor operation period to preheat the light source to a temperature below a desired source temperature, and subsequent to said initiating an initial powering, controlling power to the light source during a measurement stage of the gas sensor operation period to reach a desired source temperature, wherein the measurement stage of the gas sensor operation period is subsequent to the pre-heat stage of the gas sensor operation period;   switching, with the controller, the adaptable filter between the measurement state and the reference state comprising changing the adaptable filter between one said measurement state and one said reference state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature;   sampling, with the controller, the photo detector signal subsequent to initiating the initial powering of the light source and prior to reaching the desired source temperature;   sampling, with the controller, the photo detector signal with the adaptable filter in the measurement state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature; and   sampling, with the controller, the photo detector signal with the adaptable filter in the reference state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature.   
     
     
         13 . The method of  claim 12 , wherein the sampling during said gas sensor operation period and subsequent to the light source reaching the desired source temperature provides a gas concentration measurement using only the light source as a single light source and using only the detector as a single light detector. 
     
     
         14 . The method of  claim 12 , wherein:
 the switching of the adaptable filter between the measurement state and the reference state comprises changing the adaptable filter between said measurement state and said reference state a plurality of times during said gas sensor operation period and subsequent to the light source reaching the desired source temperature;   the sampling of the photo detector signal occurs each time the adaptable filter is in the measurement state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature; and   the sampling of the photo detector signal occurs each time the adaptable filter is in the reference state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature.   
     
     
         15 . The method of  claim 12 , wherein the sampling during said gas sensor operation period and subsequent to the light source reaching the desired source temperature provides a gas concentration measurement using a modulation amplitude based on the photo detector signal. 
     
     
         16 . The method of  claim 12 , wherein the sampling of the photo detector signal subsequent to initiating the initial powering of the light source and prior to reaching the desired source temperature comprises a plurality of samples and further comprising measuring a rate at which an output from the detector, represented by the plurality of samples, changes with time. 
     
     
         17 . The method of  claim 12 , wherein the adaptable filter comprises a plurality of measurement states in each of which it passes at least one wavelength band which is absorbed by the gas and for each measurement at least one reference state in which the wavelength band corresponding to the measurement state is attenuated relative to said measurement state, and the method further comprises switching to each of said measurement states at least once during said gas sensor operation period and subsequent to the light source reaching the desired source temperature. 
     
     
         18 . A gas sensor for measuring a concentration of a gas, the gas sensor comprising:
 a light source configured to be switched on and to be switched off;   a measurement volume;   a detector configured to receive light that has passed through the measurement volume to output a photo detector signal;   an adaptable filter disposed between the light source and the detector and having a measurement state in which the adaptable filter passes at least one wavelength band which is absorbed by the gas and a reference state in which said wavelength band is attenuated relative to the measurement state; and   a controller connected to each of the light source, the detector and the adaptable filter, the controller being configured:   to switch on the light source at a start of a gas sensor operation period to control power to the light source during said gas sensor operation period and to switch off the light source at an end of the gas sensor operation period, wherein the control of power to the light source during said gas sensor operation period includes initiating an initial powering of the light source to preheat the light source to a temperature below a desired source temperature during a pre-heat stage of the gas sensor operation period and subsequent to the pre-heat stage controlling power to the light source to reach a desired source temperature;   to sample the photo detector signal subsequent to the pre-heat stage and prior to reaching the desired source temperature;   to switch the adaptable filter between the measurement state and the reference state, including changing the adaptable filter between one said measurement state and one said reference state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature;   to sample the photo detector signal with the adaptable filter in the measurement state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature; and   to sample the photo detector signal with the adaptable filter in the reference state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature.   
     
     
         19 . The gas sensor of  claim 18 , wherein:
 the controller is configured to sample the photo detector signal subsequent to the pre-heat stage and prior to reaching the desired source temperature a plurality of times to provide a plurality of samples; and   the controller is configured to measure a rate at which an output from the detector, represented by the plurality of samples, changes with time.   
     
     
         20 . The gas sensor of  claim 18 , wherein the controller is adapted to change the adaptable filter between said measurement state and one said reference state a plurality of times during said gas sensor operation period and subsequent to the light source reaching the desired source temperature and the sampling of the photo detector signal is synchronized by the controller to occur each time the adaptable filter is in the measurement state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature and to occur each time the adaptable filter is in the reference state during said gas sensor operation period and subsequent to the light source reaching the desired source temperature.