Optics: measuring and testing – By shade or color – With color transmitting filter
Patent
1995-08-03
1997-02-25
McGraw, Vincent P.
Optics: measuring and testing
By shade or color
With color transmitting filter
356346, G01J 351, G01N 2161
Patent
active
056064195
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns equipment for the spectroscopic measurement of a gas in a gas mixture, in which a filter device is used to filter out one spectral line from other spectral lines belonging to other gases and in which the transmitted light is detected by a detector which sends a signal to devices for calculating and displaying the measurement data.
Gas measurement instruments based on spectroscopic methods are available for a number of different gases and are more or less advanced/complicated. The simplest instruments use optical interference filters which select a certain spectral range in which the gas in question absorbs. For reasons of production technology the filters are relatively wide and there is a high probability that several gases will have adsorption lines within the filter's transmission band. Such instruments can therefore not be made particularly sensitive unless there is certainty that all interfering gases have been removed in advance. The greatest problem in this connection is often water vapour which has numerous adsorption lines over a broad spectrum. When used in industrial circumstances it is necessary for the gas samples to be dried before measurement can take place and this makes continuous monitoring of discharges impossible in practice.
At the other end of the scale there are so-called Fourier Transform Instruments (FTS) which are designed to measure a number of gases and which, for this reason, cover a broad spectrum. The instruments usually have a built-in "library" with spectra which are used for identifying individual elements. The instruments are very expensive and are in practice not very well suited for practical industrial applications.
The equipment of the present invention has been designed for spectroscopic measurement of a gas and represents a practical solution which is exceptionally well suited for industrial applications and which is considerably simpler in its design and less expensive than the existing solutions.
The present invention is characterised in that the filter device consists of an interference filter, at least one fixed etalon and a Fabry-Perot interferometer.
With the solution defined here it is possible to realise a spectral resolution of approximately 0.1 cm.sup.-1 which is necessary to achieve a measurement over a single absorption line.
The present invention will now be described in more detail by means of examples and with reference to the attached drawings, of which
FIG. 1 shows a spectrum of the atmosphere,
FIG. 2 shows the basic optical elements of the equipment in accordance with the present invention, and
FIGS. 3-7 show how the transmission takes place between the components shown in FIG. 2.
Development of the equipment in accordance with the present invention was based on in situ measurement in real time of the gas concentration of hydrogen fluoride (HF). In situ here means that the measurement and the measuring equipment are placed where it is relevant to determine the gas concentration of HF, for example in the atmosphere, and real time means that the measuring result exists after an interval of time which is shorter than the time resolution with which the measurement results are to be described.
Even though the equipment as described was developed to measure HF, we would here like to note, that with only a few modifications it can be used for measuring other gases.
All gases can be characterised by their specific absorption spectra. In practice this means that visible light and thermal radiation (infrared radiation) are absorbed at very specific frequencies or wavelengths. In other words, these characteristic frequencies characterise each individual gas. The reason for these characteristic adsorptions is linked with the fact that gas molecules carry out complex "mechanical" oscillations. The molecules can vibrate and rotate and, therefore, these adsorption spectra are usually called vibration-rotation spectra. The complexity of these spectra is very different for different gases. Carbon dioxide, for example, has many thousan
REFERENCES:
patent: 3373651 (1968-03-01), Mack et al.
patent: 3914055 (1975-10-01), Wolga et al.
patent: 4035643 (1977-07-01), Barrett
patent: 4525067 (1985-06-01), Hernandez
Foosn.ae butted.s Trygve
Hagene Jon K.
J.ae butted.ger Tycho
Tschudi Jon
McGraw Vincent P.
Norsk Hydro a.s.
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