Monitoring gas and vapor concentrations

• US 4,563,585 A
• Filed: 09/23/1982
• Issued: 01/07/1986
• Est. Priority Date: 09/23/1982
• Status: Expired due to Fees

• Current Assignee: Moniteq Ltd.
• Original Assignee: Moniteg Limited
• Inventors: Ward, T. Victor
• Primary Examiner(s): EVANS, FANNIE L
• Application Number:
  US06/422,478
• Time in Patent Office: 1,202 Days
• Field of Search: 356/320, 356/331, 356/334, 356/51, 356/310, 356/325, 250/343, 250/373
• US Class Current: 250/373
• CPC Class Codes: G01J 2003/2866 Markers; Calibrating of scan
• CPC Class Codes: G01J 3/457 Correlation spectrometry, e.g. of the intensity G01J3/453 takes precedence

Abstract

In the monitoring of gas or vapor concentrations in a gas sample by comparing the transmission of light through the sample at different wavelengths corresponding to behaviorial maxima and minima of the gas, the different wavelengths are selected by permitting light to fall alternately on different sets of slits in an entrance mask of a spectroscope so that light diffracted by a grating in the spectroscope at the wavelengths represented by the slits in the two sets will fall on an exit slit in an exit mask of the spectroscope and thence pass to a photodetector. This arrangement enables the shift between the different sets of wavelengths to be achieved without introducing mechanical jitter, since the photodetector output may be electronically gated so as to be measured only for predetermined periods when light is passing first through one and then through the other set of slits.

Claims

1. In a method of monitoring low gas or vapour concentrations in a sample by correlating the behaviour thereof in respect of light at at least one first wavelength corresponding to at least one behavioural maximum of the gas being monitored with that in respect of at least one second adjacent wavelength at which the gas shows a behavioural minimum, the improvement wherein the first and second wavelengths are selected by passing light from a common entry beam through a spectrometer comprising slitted entrance and exit masks and an optically diffractive component, one of said masks having a plurality of slits determining different angles of diffraction of said first and second wavelengths, and selecting said first and second wavelengths alternately by permitting alternate passage of said light through slits in said mask with plural slits corresponding to angles of diffraction characteristic of said first and second wavelengths respectively, wherein light is selectively masked from slits in said mask with plural slits by movement of a slotted shutter, and movement of said slotted shutter is monitored and utilized to trigger a precision electronic gating signal operating to gate signals from a photodetector receiving light from the spectrometer for precisely controlled durations fully within periods when said selected slits are fully masked or unmasked, and wherein said light is passed through said sample between said exit slit and said photodetector.

2. A method according to claim 1, wherein the entrance mask has the plural slits, and the slits are in sets corresponding to a plurality of behavioural maxima and a plurality of behavioural minima respectively.

3. A method according to claim 1 or 2, wherein the responses compared are absorption maxima and minima of the gas being monitored.

4. Apparatus for monitoring a low concentration of a gas or vapour in a gas sample comprising:

a source of light containing certain predetermined wavelengths in a desired spectral region,

a collimator receiving said light and concentrating it upon a slitted area in an entrance mask,

a slitted shutter movable adjacent said entrance mask into a first position such that slits in the shutter permit light to pass through a first set of parallel slits in the entrance mask and a second position such that first slits in the shutter permit light to pass through second slightly displaced set of parallel slits in the entrance mask,

means to move the shutter cyclically into said positions,

a dispersion grating disposed to receive and diffract light from said parallel slits in said entrance mask,

an exit mask defining at least one exit slit disposed to receive light diffracted by said grating at specific wavelengths determined by the relative dispositions of said entrance mask slits, said grating and said exit slit, said wavelengths comprising a first set of wavelengths characteristic of the absorption maxima of the gas being monitored and determined by said first set of slits and a second set of wavelengths determined by said second set of slits which set is displaced from said first set of wavelengths and characterized by minimum absorption by the gas being monitored,

means establishing an optical path through a gas sample being monitored,

an optical relay system extending from said exit slit to the beginning of said optical path,

a photo-electric sensor device,

an optical relay system extending from the end of said optical path to said sensor device,

means to generate signals in response to the entry of said shutter into said positions,

a frequency stable electronic clock pulse generator generating pulses at a frequency which is very high compared to the cycling rate of the means moving said modulator,

gate control means receiving said clock pulse signals from said clock pulse generator and operative to count said pulses and generate gating signals commencing a predetermined time after receipt of a response signal from said signal generating means and lasting a period corresponding to a predetermined count of said gate pulses related to the rotation rate of the shutter such that the gating signals commence after the shutter has entered its said positions and end before it leaves them,

a transmission gate controlled by said gating signal and passing the signals from said sensor device in response thereto,

comparator means receiving the signals from said gate and comparing the magnitude of the signals passed when the shutter is in said first and second positions, and

wherein said shutter has a third position such that light can pass neither set of slits in the entrance mask, and further comparator means receives signals from said gate when the shutter is in said third position for comparison with the signals passed in one of said first and second positions.

5. Apparatus according to claim 4, wherein the light source provides light in the ultraviolet region.

6. Apparatus according to claim 5, wherein the first set of wavelengths correspond to absorption maxima of sulfur dioxide.

7. Apparatus according to claim 4, wherein means to introduce calibration cells into the light path are associated with the collimator.

8. Apparatus according to claim 4, wherein an automatic gain control circuit receives the output from the further comparator means and applies it as a gain control signal to a variable gain amplifier between said sensor and said comparators.

9. Apparatus for monitoring low concentrations of a particular gas or vapour in a gas sample, comprising a source of a beam of radiation, a spectrometer receiving said beam and an optical sensor, all forming part of an optical path between said source and said sensor, said spectrometer comprising an optically diffractive component and slitted entrance and exit masks, at least one of which has multiple slits, the diffractive component and the slits in said mask being disposed relative to one another so that only light of selected wavelengths, corresponding respectively to at least one absorption maximum and at least one adjacent absorption minimum of said particular gas or vapour, can traverse said optical path, each of the multiple slits passing a different one of said wavelengths, means adjacent said multiple slitted mask and operable so as selectively to block and unblock said slits whereby to permit the passage at different times of first wavelengths corresponding to said at least one absorption maximum, and second wavelengths associated with said at least one absorption minimum, means to introduce said sample into said optical path between said spectrometer and said sensor in a manner such that it can influence the spectral composition of the light passing along said path, and means to correlate the outputs of said sensor when receiving light of said first and second wavelengths respectively, said means including comparator means, and gating means phase locked to said blocking means and operative to prevent passage of said sensor output to said comparator means when any of said slits in said multiple slitted mask are partially blocked, whilst passing said output for precisely predetermined periods when those of said slits passing wavelengths corresponding to said absorption maxima are unblocked and for precisely defined periods when those of said slits passing wavelengths corresponding to said absorption minima are unblocked.

10. Apparatus according to claim 9, wherein the blocking means comprises a shutter rotatable in a plane adjacent said multiple slitted mask and having radially and circumferentially spaced sets of arcuate slits therein, the sets of arcuate slits being selectively alignable with different sets of slits in the mask on rotation of the shutter, and wherein the gating means comprises means to generate a signal responsive to entry of a set of slits in the shutter into alignment with a set of slits in the mask, a clock pulse generator, means responsive to said signal to count pulses from said clock pulse generator and to generate an enabling signal continuity between predetermined counts, and a transmission gate receiving said enabling signal and operative in response thereto to connect said sensor output to said comparator means.

11. Apparatus according to claim 10, wherein the signal generating means comprises additional radially and circumferentially spaced slits in the shutter, and photosensors in radial alignment with said additional slits.

12. Apparatus according to claim 10, wherein the blocking means is adjacent the entrance mask.

13. Apparatus according to claim 10, wherein the sample introduction means is optically connected to the exit mask and the light sensor by optical relay systems.

14. Apparatus according to claim 9, wherein the gating means are further operative to pass said output for further precisely defined periods when all of said slits in said multiple slitted mask are blocked, and further comparator means are provided to compare said output during said further periods with said output during said periods when those of said slits passing wavelengths corresponding to said absorption minima are unblocked, whereby to provide at the output of said further comparator a reference for the correlation output from the first comparator.

15. Apparatus according to claim 14, wherein an automatic gain control circuit receives the output from the further comparator means and applies it as a gain control signal to a variable gain amplifier between said sensor and said comparators.