Optical instrument for measuring concentrations of polluting gases on long and short geometrical paths

• US 4,060,326 A
• Filed: 07/03/1975
• Issued: 11/29/1977
• Est. Priority Date: 07/04/1974
• Status: Expired due to Term

• Current Assignee: Tecneco SPA
• Original Assignee: Tecneco SPA
• Inventors: Cesari, Giulio, Bonafe, Ubaldo, Giovanelli, Giorgio, Tirabassi, Tiziano, Antisari, Ottavio Vittori
• Primary Examiner(s): Evans, F. L.
• Application Number:
  US05/592,727
• Time in Patent Office: 880 Days
• Field of Search: 250/343, 250/237 R, 250/237 G, 356/74, 356/96, 356/97, 356/99, 356/100
• US Class Current: 356/328
• CPC Class Codes: G01J 2003/2866 Markers; Calibrating of scan
• CPC Class Codes: G01J 3/0229 using masks, aperture plates, spatial light modulators or spatial filters, e.g. reflective filters
• CPC Class Codes: G01J 3/04 Slit arrangements slit adjustment
• CPC Class Codes: G01J 3/427 Dual wavelengths spectrometry
• CPC Class Codes: G01J 3/457 Correlation spectrometry, e.g. of the intensity G01J3/453 takes precedence

Abstract

The present invention provides an optical instrument for measuring concentrations of polluting gases on long and short geometric paths. The apparatus comprises a telescope for focussing a ray of light into a spectrometer, a comparison cell containing a known concentration of a polluting gas, control means and an electronic apparatus adapted to determine the concentration of the polluting gas in the analytical sample.

Claims

1. An optical instrument for measuring concentrations of polluting gases on long and short geometrical paths, characterized in that it comprises a spectrometer;

a telescope focussing a light beam coming from a source located at a predetermined distance, into a slit of said spectrometer;

at least a cell, containing a known concentration of the polluting gas under examination, brought by first control means along the path of the light beam between said telescope and said slit;

said spectrometer dispersing and focussing said light beam on its own outlet focal plane;

a mask provided with movable slits, moved by second control means and located on the outlet focal plane of said spectrometer, said mask provided with movable slits that are adapted to effect the sampling of said dispersed light beam in at least four series of wave length ranges;

a photodetector giving at the outlet electrical signals when receiving at the inlet light signals caused by said sampling;

an electronic apparatus, located at the outlet of said photodetector, including means to elaborate at least four numerical values, of which;

a first numerical value relating to the ratio between two electrical signals referring respectively to the sampling of a first and second of said series of wave length ranges of the dispersed light beam, a second numerical value referring to the ratio between two electrical signals referring respectively to the sampling of a third and fourth of said series of wave length ranges of the dispersed light beam, a third numerical value referring to the ratio between two electrical signals referring respectively to the sampling of said first and second series of wave length ranges of the dispersed light beam when said cell is interlocated along the light beam, and a fourth numerical value relating to the ratio between two electrical signals referring respectively to the sampling of said third and fourth series of wave length ranges of the dispersed light beam when said cell is interlocated along the path of the light beam;

said elaboration of the numerical data being such to give at the outlet a numerical value given by the value of the known concentration of the polluting gas contained in said cell multiplied by the length of the cell and by a ratio having as numerator the difference between said first and second numerical value and as denominator the product of the distance between said source and the measuring device by the value obtained substracting from the difference between said third and fourth numerical value the said difference between the first and second numerical value.

2. An optical measuring instrument according to claim 1, characterized in that said first series of ranges where the sampling occurs, corresponds to wave length values relating to the windows of the gas absorption spectrum in question;

said second series corresponds to wave length values relating to the bands;

said third and fourth series correspond to the wavelength slightly shifted as to the windows and bands respectively.

3. An optical measuring instrument according to claim 1, characterized in that said mask is a disk, rotated at constant speed by said second control means, which presents four series of slits arranged on a circular arc, having like width and depth, where each series presents an equal number of slits and the slits are equally spaced from each other;

said series being differently spaced as to the disk rotation axis and each slit series being suitable to effect the sampling in one of said series of wave length ranges.

4. An optical measuring instrument according to claim 1, characterized in that said mask consists of a small slab presenting a series of parallel slits suitable to effect the sampling in one whatever of said series of ranges, said small slab being operated, so as to effect a rectilinear reciprocal periodical motion, by said second means, which allow, further, to shift along the focal plane the said small slab motion.

5. An optical measuring instrument according to claim 1 characterized in that said electronic apparatus comprises:

at least three optical choppers detecting corresponding slits provided in a mask, a first chopper being suitable to give a first electrical signal, a second chopper being suitable to give a second and a third electrical signals and finally a third chopper suitable to give a plurality of electrical signals controlling through a suitable programmer, the second control means;

said programmer, at whose inlet there are said first, second and third electrical signals, being further suitable to act on said first means, a preamplifier, arranged at the outlet of said photodetector, which amplifies said electrical signal relating to said sampling of the dispersed light beam;

gate arranged at the outlet of said preamplifier, said analogic gate being switched on by said second signal and switched off by said third signal, said second and third signals being fed by said programmer;

a digital analogic converter which converts the electrical signal coming from said analogic gate, relating to said sampling, into corresponding pulse trains expressed in binary code;

a sequential network for addressing data, arranged at the outlet of said digital converter, presenting four outlets connected with as many digital analogic accumulators, said sequential network being suitable, through said first signal sent by said programmer, to address to a first digital accumulator the pulse train corresponding to the amplified electrical signal relating to the sampling of said first series of wave length ranges of the dispersed light beam, to address to a second digital, accumulator the pulse train corresponding to the amplified electrical signal relating to the sampling of said second series of wave length ranges of the dispersed light beam, to address to a third digital accumulator the pulse train corresponding to the amplified electrical signal relating the sampling of said third series of wave length ranges of the dispersed light beam and to address to a fourth digital accumulator the pulse train corresponding to the amplified electrical signal relating to the sampling of said fourth series of wave length ranges of the dispersed light beam, said addressing recurring cyclically for a predetermined number of sequences established by said programmer;

a first ratio circuit at whose inlet there are said first and second digital analogic accumulators, suitable to give, through said programmer, a first numerical value and a third numerical value obtained with said cell arranged between said telescope and said spectrometer slit;

a second ratio circuit at whose inlet there are said third and fourth digital analogic accumulators, suitable to give, through said programmer, said second numerical value and said fourth numerical value obtained with said cell arranged between said telescope and said spectrometer slit;

a difference circuit at whose inlet are connected said first and second ratio circuits, said difference circuit being suitable to give a first difference between said first and second numerical values and to give a second difference between the third and fourth numerical values;

a switch arranged at the outlet of said difference circuit, said switch being suitable, through an order of said programmer, to feed to a first store, and a second store said first and second differences, respectively a store discharger at whose inlet there are said first and second stores, said store discharger being suitable to send, through an order of the programmer, said first difference and said second difference stored into the corresponding stores to a ratio-difference circuit said ratio-difference circuit being suitable to give at the outlet a value equal to a fraction whose numerator is said first difference and denominator is the difference between said second difference and first difference, said ratio-difference circuit being further suitable to multiply the preceding ratio by a ratio presenting at the numerator the product of the known concentration of the cell by its length and at the denominator the distance between the source and the measuring instrument in question, obtaining thereby accordingly a value equal to the concentration of the polluting gas expressed in a suitable binary code;

a numerical display connected at the outlet with said ratio-difference circuit, said numerical display being suitable to visualize the above said value of concentration, expressed in binary code, into the corresponding numerical value.

6. An optical measuring instrument according to claim 1, characterized in that said photodetector is a photomultiplier.

7. An optical measuring instrument according to claim 1, characterized in that said telescope is a Cassegrain telescope.

8. An optical measuring instrument according to claim 1, characterized in that the spectrometer is a grating spectrometer.

9. An optical instrument for measuring concentrations of polluting gases on long and short geometrical paths, characterized in that it comprises:

a grating spectrometer, a telescope focussing a light beam coming from a source located at a predetermined distance, into a slit of said grating spectrometer;

at least a cell containing a known concentration of the polluting gas under examination, brought by first control means along the path of the light beam between said telescope and said slit;

said grating spectrometer dispersing and focussing said light beam on its own outlet focal plane;

a mask provided with movable slits that are adapted to effect the sampling of said dispersed light beam in at least four series of wave length ranges;

said movable slits being moved by second control means and located on the outlet focal plane of said spectrometer, a photodetector giving at the outlet electrical signals when receiving at the inlet light signals caused by said sampling;

an electronic apparatus comprising of at least three optical choppers detecting corresponding slits provided in said mask a first chopper suitable to give a first electrical signal, a second chopper being suitable to give a second and a third electrical signal and finally a third chopper suitable to give a plurality of electrical signals controlling, through a suitable programmer, the second control means;

said programmer, at whose inlet there are said first, second and third electrical signals, being further suitable to act on said first means;

a preamplifier, arranged at the outlet of said photodetector which amplifies said electrical signals relating to said sampling of the dispersed light beam;

an analogic gate arranged at the outlet of said preamplifier, said analogic gate being switched on by said second signal and switched off by said third signal, said second and third signals being fed by said programmer;

a digital analogic converter which converts the electrical signal coming from said analogic gate, relating to said sampling, into corresponding pulse trains expressed in binary code;

a sequential network for addressing data, arranged at the outlet of said digital converter, presenting four outlets connected with as many digital analogic accumulators, said sequential network being suitable, through said first signal sent by said programmer, to address to a first digital acculumator the pulse train corresponding to the amplified electrical signal relating to the sampling of said first series of wave length ranges of the dispersed light beam, to address to a second digital accumulator the pulse train corresponding to the amplified electrical signal relating to the sampling of said second series of wave length ranges of the dispersed light beam, to address to a third digital accumulator the pulse train corresponding to the amplified electrical signal relating to the sampling of said third series of wave length ranges of the dispersed light beam and to address to a fourth digital accumulator the pulse train corresponding to the amplified electrical signal relating to the sampling of said fourth series of wave length ranges of the dispersed light beam, said addressing recurring cyclically for a predetermined number of sequences established by said programmer;

a first ratio circuit at whose inlet there are said first and second digital analogic accumulators, suitable to give, through said programmer, a first numerical value and a third numerical value obtained with said cell arranged between said telescope and said spectrometer slit;

a second ratio circuit at whose inlet there are said third and fourth digital analogic accumulators, suitable to give, through said programmer, a second numerical value and a fourth numerical value obtained with said cell arranged between said telescope and said spectrometer slit;

a difference circuit at whose inlet are connected said first and second ratio circuits, said difference circuit being suitable to give a first difference between said first and second numerical values and to give a second difference between the third and fourth numerical values;

a switch arranged at the outlet of said difference circuit, said switch being suitable, through an order of said programmer, to feed to a first store and a second store said first difference;

a store discharger at whose inlet there are said first and second stores, said store discharger being suitable to send, through an order of the programmer, said first and second differences stores into the corresponding stores to a ratio-difference circuit, said ratio-difference circuit being suitable to give at the outlet a value equal to a fraction whose numerator is said first difference and denominator is the difference between said second difference and first difference, said ratio-difference circuit being further suitable to multiply the preceding ratio by a ratio presenting at the numerator the product of the known concentration of the cell by its length and at the denominator the distance between the source and the measuring instrument in question, obtaining thereby accordingly a value equal to the concentration of the polluting gas expressed in a suitable binary code;

a numerical display connected at the outlet with said ratio-difference circuit, said numerical display being suitable to visualize the above said value of the concentration, expressed in binary code, into the corresponding numerical value.