Measuring and testing – Gas analysis – Gas chromatography
Patent
1981-11-25
1984-05-08
Kreitman, Stephen A.
Measuring and testing
Gas analysis
Gas chromatography
G01N 2700
Patent
active
044467206
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method and apparatus for detecting the presence of contaminants in a gaseous carrier and has particular but not exclusive reference to the detection of minute quantities of particulate material in a gaseous carrier.
It is known that the presence of very small quantities of particulate material in a gaseous carrier can be detected by impacting the particles on to the surface of a piezo-electric crystal forming part of an electrical oscillator. The resultant increase in mass of the crystal reduces the frequency of oscillation and thereby provides an indication of the presence of particulate material in the carrier and, if required, a measurement of the mass of such material deposited on the crystal.
By suitable treatment of the surface of the crystal, for example by coating the surface with an appropriate substrate, the presence of gaseous contaminants may be detected, such contaminants interacting with the substrate to cause an increase or decrease in the mass of the crystal and hence a decrease or increase in the frequency of oscillation.
Examples of apparatus embodying the techniques just described are to be found in U.S. Pat. No. 3,561,253. That Specification describes various methods of impacting contaminants on to the surface of a piezo-electric crystal forming part of an electric oscillator including the use of jet orifices to impact particulate material on to that surface. A reduction in the frequency of oscillation of the crystal is indicated either by means of a counter fed directly by the oscillator or by beating the output of the oscillator which that of a standard oscillator with a piezo-electric crystal exposed to the same environmental conditions as the first mentioned crystal but not exposed to the impaction of particulate material or other contaminant being detected and/or measured. The beats are counted by a counter to indicate frequency changes.
The use of two crystals has the advantage that the effect of frequency shifts caused by changes in environmental factors, for example temperature, are eliminated.
However, such apparatus is normally operated by skilled personnel who are able to evaluate the counter readings. In addition, it is necessary to clean the surface of the crystal thoroughly between successive measurements and this is a time-consuming and laborious task.
The present invention provides a method and apparatus which allows the aforementioned disadvantages to be substantially avoided.
According to the invention apparatus for detecting the presence of a contaminant in a gaseous carrier comprises a first piezo electric crystal oscillator, carrier gas feed means for enabling contaminant to act upon the said first crystal in a manner which changes the effective mass thereof, measuring means for obtaining a first value related to the total number of oscillations of the crystal during a predetermined period of time, and an arrangement for enabling the first value to be decreased by a second value related to the total number of oscillations of the crystal during a second time period not greater than said predetermined time period.
Preferably the means for producing said values are counters each arranged to produce said values as respective numerical counts.
Desirably the apparatus includes a comparator to compare the numerical count at the end of said second period with a numerical count of preset value for providing an indication as to whether the numerical count at the end of said second period is greater than or less than the preset numerical count.
The apparatus preferably includes a second piezo electric crystal oscillator the output of which drives a counter to determine the said predetermined period of time and said second period of time, the second piezo electric crystal being preferably exposed to substantially the same environmental conditions affecting its stability as said first crystal, except that shielding means are provided so that the second crystal is not exposed to contaminant and the mass of said second crystal remains unchanged.
Testing
REFERENCES:
patent: 3561253 (1971-02-01), Dorman
patent: 3595069 (1971-07-01), Fowler
patent: 3689907 (1972-09-01), Guajardo
patent: 3715911 (1973-02-01), Chuan
Kreitman Stephen A.
The Secretary of State for Social Services in Her Britannic Maje
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