US3194962A - Portable infrared detector comprising a filter wheel - Google Patents

Description

July 13, 1965 H. R. CARLON ETAL 3,194,962

PORTABLE INFRARED DETECTOR COMPRISING A FILTER WHEEL Original Filed March 16. 1961 5 Y w www@ mn@ m Nme m BWM man Rv 0W hv Y B w@ W M United States Patent O 1 Gleim. (Cl. 259-435) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States or America for governmental purposes without the payment to us of any royalty thereon.

This is a division of Serial Number 96,337, tiled March 16, 196i, entitled Fortable Infrared Detector.

This invention relates to a iilter medium for use in infra-red detection devices and to a device which uses said medium. lt is well known that practically all the major countries in the world have in their arsenals supplies oi the so-called nerve gases such as the G-agents. The identity of the G-ageuts is disclosed in the patent to Kramer et al. 2,926,072 (col. 1, lines lll-i7). These nerve gases cannot be detected by the usual senses since tney are odorless, tasteless, and colorless. Several schemes for their detection have been devised. The colorimetric system is exemplified by the above mentioned Kramer patent. An intra-red detection system is shown by Carpenter et al. in Patent 2,930,893.

The instant device is a distinct improvement in the device disclosed by Carpenter et al. A particular object of the present invention is to provide a portable self-contained infra-red gas or aerosol detector with only one moving part.

A further object of this invention is to provide an inexpensive device that is readily adaptable to the detection of a large number of atmospheric contaminants.

A still further object of this invention is to provide a highly sensitive instrument which provides in a matter of seconds a visual and aural alarm in the presence of a nerve gas.

A final object of this invention is to provide a new type absorption band lilter for the far infra-red range that is inexpensive and rugged. These and other objects and advantages of the invention will become apparent as the description proceeds.

in the drawings:

PEG. l is a schematic drawing or the entire device showing the relationship of the optical and electronic componente.

FiG. 2 shows the transmisison spectra of the polyethylene lrn with and without talc (hydrated magnesium silicate) powder applied to the surface of the ilm.

PEG. 3 shows the details of the rotating lter wheel shown schematically in FIG. 1.

FiG. 4 is a cross sectional view taken of line fi--d of FlG. 3.

The complete device as is illustrated by liiG. 1 can be housed in one container. in such cases the path of the infra-red radiation is relatively short and the container must be provided with vent holes so that a sample of the contaminated atmosphere will pass between the intra-red source and the bolometer detector.

rEhe infra-red rays emanate from an infra-red source which may be a Nernst glower, heated wire, or heated cavity. They are reiected by a spherical mirror which concentrates the available energy on the active ake of the thermistor bolometer. ln order to exclude all radiation below about 7.2 microns in wave length an indium antimonide optical filter is placed adjacent to the bolom microns, due to the impregnated talc.

3,194,962 Patented July 13, 1965 ICC eter between the intra-red source and the bolometer.

The rotating lter wheel which is shown in FG. 1 and detailed in FlGS. 3 and 4 will now be described. This wheel consists of a center disk 3 and an outer mounting ring 9 which may be made of any available rigid plastic such as Plexiglas or Lucite. The plastic iilni 5 is a circular piece of polyethylene lm to which has been added a seini-circular piece or sector 7 of polytriiluorornonochloroethylene film which for convenience is called fluorothene film (Kel-YF). The part of the polyethylene disk 5 not covered by the polytritluoromonochloroethylene is impregnated with a mineral powder such as talc (hydrated magnesium silicate) by a method which will be described. There need be no polyethylene substrate under the iiuorotheme film, if so desired.

The infra-red light aiter passing through the optical iilter and the lter wheel which is rotating at 1800 rpm. falls upon the active flake of the thermistor bolometer. This active ilake is in series with the reference lake in the illustrated electronic circuit. The lilter wheel is so designed that the liuorothene sector absorbs infra-red radiation in the 8.5 to 9.1 micron and 10.1 to 10.4 micron ranges while transmitting at 9.8 microns. Radiation through the other wheel sector is attenuated at 9.8 The filter wheel is made to balance optically so that the two sectors transmit equal amounts of energy, although of different wave lengths, and no signal is produced in the detector circuit as the wheel rotates. li a nerve gas is present, some 9.8 micron radiation normally transmitted by the lluorothene wheel sector is absorbed by the gas so that the radiation striking the active flake is then fluctuating at the rate of rotation of the filter wheel, and an alternating current signal is produced. When this condition exists, the weak alt rnating current signal set up in the bolometer circuit is preampliiied and then amplified so that it may operate the alarm, after rectification of the signal to direct current is accomplished. This may be achieved by using synchronous switching as a noise rejection precaution.

The alarm used is a matter of choice since the power from the ampliiier is adequate to actuate a relay which in turn can operate a lamp or a buzzer, not shown in the drawing. Alternatively, the alarm may consist of a (micro) ammeter which is calibrated to give the concentration orp the contaminant.

The filter wheel is prepared as follows. A circular disk of 2 mil polyethylene is cut from the stock and one-half of it is covered with masking tape, front and rear, if it is desired to use a polyethylene substrate under the iluorothene. Otherwise, a semi-circle oi polyethylene would be used. Reagent grade talcum powder is spread over the exposed surface of the disc and rolled into it with a small machined-brass roller, although rolling is not strictly necessary, since scrubbing the lm with a coarse paper is sometimes adequate. The excess talc is rc2-distributed, rerolled, the process being repeated until the desired amount of talc is imbedded in the surface of the plastic ilm. The remaining talc is then wiped from the surface with a fairly coarse paper wiping tissue.

The wiping is repeated several times, With considerable pressure applied, until all talc which is not imbedded is removed. This process produces a surface uniformly impregnated with very line talc particles generally less than 3 microns in diameter. Since the sharpness or" the talc absorption band at 9.8 microns is dependent on the lineness of the particles, a very sharp band results which is deiinitely superior to any which may be obtained through a process wherein some form of a binder such as wax is used to hold the talc particles on the substrate. in addition, the particles become so firmly imbedded in the plastic substrate that no known method short of destrucdiameter, to the wheel.

ZT tion of the film itself, will dislodge them. The iilm may be flexed, folded, doubled, rubbed, immersed in liquids (which are not solvents for polyethylene), and even crumpled in the hand without damage to the talc surface. Furthermore, no protective covering is used or needed for this filter.

In the process just described, if 2 mil polyethylene is saturated with talc particles, an absorption of 80-85% will be effected at 9.8 microns, with absorptions of only about 20% at wavelengths of 8.5 and 10.8 microns. In this particular alarm application it was desired to increase the 9.8 micron absorption and to generally attenuate the tale wheel sector transmission so as to bring the transmission of both halves of the filter wheel to a condition of approximately equal magnitude and thus optically balance the lter wheel. This was accomplished by saturating the other face of the exposed semi-circle or sector and placing over it an additional semi-circle of 4 mil polyethylene that is likewise saturated front and back with talc. Combinations of various thickness of polyethylene used in this manner can yield filter wheels with almost any desired transmission characteristics for the talc sector.

The remaining half of the wheel (it the full polyethylene substrate was used) is unmasked and covered with a piece of half-mil fluorothene (poly-triuoromonochloroethylene). The tluorothene may be held in place temporarily with tape as the mounting ring is prepared. The mounting ring is machined to the outside diameter of the wheel, from Lucite. Since the wheel will be rotated in the instrument, at 1800 rpm., Lucite is chosen because it provides sutiicient rigidity with low mass, as a large mass of the circumference of the wheel cannot be tolerated at this rotational speed. However, the mass is sutiicient to hold the plastic iilm tlat as the wheel rotates. The mounting ring disc is cut out in the center to form a ring having a Width, at a circumference, of approximately 5 mm. The completed Wheel is laid over the ring and attached to it with Duco cement. Since the cement will not adhere permanently to the plastic, a series of circumferential holes are made through the plastic with a hot miniature soldering iron. The cement is then applied through the holes and onto the Lucite ring, forming small rivets of cement which hold the film in place. A second similar mounting ring is then placed over the wheel, providing added rigidity. The wheel is completed through the addition of two small Lucite discs, approximately 2 cm. in

These discs are then centerpunched so that the wheel may be mounted. Additional support is obtained for the plastic disc in this manner.

Obviously this filter-producing process can be extended to the use of other powdered minerals, as has been demonstrated for silica gel, calcium carbonate, potassium perioclate, and various nitrates and sulfates, to name a few. Each of these minerals has a specific absorption in the infra-red band as is set forth in the article by Hunt ct al. in Analytical Chemistry 22:1479-97 (1950).

By this means, one can extend the instant invention to detect other gases or vapors that have an absorption in the infrared comparable to the minerals used. The filter wheel does not have to be made of two sectors but any 5 number of alternating sectors may be used so long as the wheel is optically balanced (in the infra-red band) when it is rotated. Obviously, the use of a greater number of sectors increases the frequency o the alternating current generated in the bolometer circuit. The actual revolutions per minute of the direct current motor and the tiltcr wheel can thus be reduced if it is so desired.

The optical path is not restricted to the straight line path shown in FIG. l but can be made to talte a folded path by means of plane mirrors in order to take up less space.

When using an optical path of twenty-inches, the instrument is capable of detecting as little as 2 micrograms of nerve gas per liter of air in a matter of seconds ie., no more than ten seconds.

2O Instruments utilizing the identical principle described above could be constructed to have stationary filters with rotating optics or any of several related designs.

The 2 microgram per liter sensitivity noted is by no means near the theoretical limit for such a system.

We claim:

Apparatus for the detection of compounds containing a pentavalent phosphorus ester linkage in the atmosphere comprising a source of infrared radiation, a filter wheel made up of alternating sectors of polyethylene film iinpregnatcd with a tine mineral powder and polyethylene illm covered by polytriuoromonoehlorocthylene film, driving means to turn said wheel, detector means located so as to receive said radiation after it passes through said filter wheel, and to receive and respond to variations in 85 the intensity of the infrared reaching it, ampliiier and indicator means connected to said detector means adapted to amplify the variations received and convut it to an aural-visual signal whereby upon passage of samples o said phosphorus compound containing between the infn.-

red source and the detector the normally balanced amount of energy received by the detector is altered by the absorption of some of the infrared radiation by said phosphorus compounds.

References Cited by the Examiner UNITED STATES PATENTS 2,635,649 8/54 Miller Z50-43.5 2,689,801 9/511l EAlelio 117-9 2,775,160 12/56 Foskett et al 25043-5 X 2,930,893 3/60 Carpenter Z50- 43.5 2,953,469 9/60 Fox 117-9 2,954,349 9/60 Jenness SS-l 3,091,690 5/63 McHenry 250`43-5 RALPH G. NILSON, Primary Exam/'11er'. ARCHIE R. BORCHELT, Examiner.

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