Gas sensors

Abstract

A gas sensor for measuring concentration of a predetermined gas comprises a light source arranged to emit pulses of light, a measurement volume, a detector arranged to receive light that has passed through the measurement volume, and an adaptable filter disposed between the light source and the detector. 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. The adaptable filter is arranged to change between one of said measurement state and said reference state to the other at least once during each pulse.

1 . A gas sensor for measuring a concentration of a gas comprising:
 a light source arranged to emit pulses of light;   a measurement volume;   a detector arranged to receive light that has passed through the measurement volume; and   an adaptable filter disposed between the light source and the detector and having 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 wherein the adaptable filter is arranged to change between one of said measurement state and said reference state to the other at least once during each pulse.   
     
     
         2 . The gas sensor of  claim 1 , wherein the adaptable filter comprises a micro-electromechanical system (MEMS). 
     
     
         3 . The gas sensor of  claim 2 , wherein the adaptable filter comprises a diffractive optical element having a plurality of grating bands arranged to be moved by an electrostatic potential. 
     
     
         4 . The gas sensor of  claim 2 , wherein said MEMS filter comprises an arrangement for measuring a change of capacitance therein for diagnostic purposes. 
     
     
         5 . The gas sensor of  claim 1 , further comprising a single light source and a single detector. 
     
     
         6 . The gas sensor of  claim 1 , arranged to measure a rate at which an output from the detector for no input, changes with time. 
     
     
         7 . The gas sensor of  claim 1 , 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. 
     
     
         8 . A wireless, battery-operated gas detector unit comprising the gas sensor of  claim 1 . 
     
     
         9 . A method of measuring a concentration of a gas comprising:
 passing a pulse of light through a measurement volume to a detector via an adaptable filter disposed between the light source and the detector;   switching said filter at least once in each pulse to/from a measurement state in which it passes at least one wavelength band which is absorbed by the gas and a reference state in which the wavelength band is attenuated compared to the measurement state; and   determining said concentration of the gas from a difference in light received by the detector in said measurement and reference states respectively.   
     
     
         10 . The method of  claim 9 , further comprising determining said concentration from a single pulse. 
     
     
         11 . The method of  claim 9 , further comprising measuring said concentration using a single light source and the detector. 
     
     
         12 . The method of  claim 9 , further comprising repeatedly switching said filter between said measurement and reference states a plurality of times during each pulse. 
     
     
         13 . The method of  claim 12 , further comprising measuring said concentration using a modulation amplitude of a signal detected by the detector. 
     
     
         14 . The method of  claim 9 , further comprising measuring a rate at which an output from the detector for no input, changes with time. 
     
     
         15 . The method of  claim 9 , 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 each pulse.