# US 5,349,844 A · System and method for resonant filter mass monitoring > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 5,349,844 A - Document ID: US5349844A - Status: Granted - Kind: A - Application number: 07/944,214 - Filed: 1992-09-11 - Granted: 1994-09-27 - Earliest claimed: 1992-09-11 - Assignees: TRC Companies, Inc. - Inventors: Pedro Lilienfeld - Web page: https://patentgrep.com/patent/US5349844A ## CPC - G01N 5/00: Analysing materials by weighing, e.g. weighing small particles separated from a gas or liquid (G01N9/00 takes precedence {; weighing per se G01G}) - G01G 3/16: measuring variations of frequency of oscillations of the body - G01N 15/0618: {of the filter type (G01N15/0643 takes precedence)} - G01N: INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES (measuring or testing apparatus or processes other than immunoassay, involving enzymes or microorganisms C12M, C12Q) - G01G: WEIGHING (sorting by weighing B07C5/16) ## Abstract A system and method for measuring particulate concentrations. The system includes a substantially planar, peripherally supported filter for capturing the particulates, an oscillator for oscillating the filter substantially perpendicular to its planar surface, and a sensor for measuring the frequency of oscillation. ## Claims ### Claim 1 A system for detecting airborne particulate concentration, comprising: (a) a substantially planar filter; (b) an air sampler for obtaining an ambient air sample and causing said air sample to flow through said filter whereby airborne particulates in said air sample are deposited on said filter; (c) an oscillator for causing oscillatory movement of said filter substantially perpendicular to the plane of said filter; and (d) a detector for detecting said oscillatory movement of said filter. ### Claim 2 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said filter comprises a portion of a filter tape. ### Claim 3 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said filter comprises a membrane. ### Claim 4 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said filter comprises a circular membrane. ### Claim 5 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said filter includes at least one diametric node. ### Claim 6 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said filter includes a plurality of circular nodes. ### Claim 7 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein the periphery of said filter is attached to a fixed support. ### Claim 8 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said filter is a membrane selected from the group consisting essentially of Teflon, polyester, polypropylene, and an acrylic copolymer with nylon substrate. ### Claim 9 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said oscillator means comprises an acoustical drive. ### Claim 10 (depends on 9) A system for detecting airborne particulate concentration according to claim 9, wherein said acoustical drive comprises a piezo-electric harmonic drive. ### Claim 11 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said oscillator means comprises a capacitive drive. ### Claim 12 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said detector comprises a microphone. ### Claim 13 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said detector comprises an optical motion pick-up device. ### Claim 14 (depends on 1) A system for detecting airborne particulate concentration according to claim 1, wherein said detector comprises a capacitive charge device. ### Claim 15 A system for determining airborne particulate concentration, comprising: (a) a substantially planar filter; (b) an air sampler for obtaining an ambient air sample and causing said air sample to flow through said filter at a flow rate (Q) whereby airborne particulates in said air sample are deposited on said filter; (c) an oscillator for causing said filter to oscillate substantially perpendicular to the plane of said filter at a natural resonant frequency (f); (d) a detector for detecting said oscillatory movement of said filter; and (e) particulate concentration determining means for determining the airborne particulate concentration (C.sub.m) of said sample based on the ratio between the initial (f.sub.0) and final (f.sub.1) resonant frequencies of said filter. ### Claim 16 (depends on 15) A system for determining airborne particulate concentration according to claim 15, wherein said concentration is obtained from said initial and final resonant frequencies of said filtration means. ### Claim 17 (depends on 15) A system for determining airborne particulate concentration according to claim 15, wherein said concentration of said sample is determined by equation: ##EQU9## where s is the filter stiffness, Q is the volumetric sampling flow rate, t is the elapsed sampling time period, and f.sub.0 and f.sub.1 are the initial and final resonant frequencies of said filter. ### Claim 18 (depends on 15) A system for determining airborne particulate concentration according to claim 15, wherein said elapsed sampling time period (t) is user selectable. ### Claim 19 (depends on 15) A system for determining airborne particulate concentration according to claim 15, wherein said airborne particulate concentration (C.sub.m) is determined by equation: ##EQU10## where m.sub.0 is the initial mass of the clean filter, Q is the volumetric sampling flow rate, t is the elapsed sampling time period, and f.sub.0, f.sub.1 are the initial and final resonant frequencies of said filter. ### Claim 20 (depends on 15) The system of claim 15 including a fixed support engaging and supporting the periphery of said filter. ### Claim 21 A method for determining airborne particulate concentration, comprising the steps of: (a) obtaining an ambient air sample and providing said sample at a flow rate (Q); (b) collecting airborne particulates in said sample on a substantially planar filter; (c) oscillating said filter in a direction substantially perpendicular to the plane thereof; (d) detecting a natural resonant frequency (f) of said oscillating filter; and, (e) determining the airborne particulate concentration of said sample based on the ratio between initial (f.sub.0) and final (f.sub.1) resonant frequencies of said filter. ### Claim 22 (depends on 21) A method for determining airborne particulate concentration, according to claim 21, wherein said airborne particulate concentration of said sample is obtained from said initial and final resonant frequencies of said. ### Claim 23 (depends on 21) A method for determining airborne particulate concentration, according to claim 21, wherein said concentration of said sample is determined by equation: ##EQU11## where s is the filter stiffness, Q is the volumetric sampling flow rate, t is the elapsed sampling time period, and f.sub.0 and f.sub.1 are the initial and final resonant frequencies of said filter. ### Claim 24 (depends on 20) A method for determining airborne particulate concentration according to claim 20, wherein said airborne particulate concentration (C.sub.m) is determined by equation: ##EQU12## where m.sub.0 is the initial mass of the clean filter, Q is the volumetric sampling flow rate, t is the elapsed sampling time period, and f.sub.0 and f.sub.1 are the initial and final resonant frequencies of said filter. ### Claim 25 (depends on 21) The method of claim 21 wherein said filter is supported by a fixed peripheral support. ### Claim 26 A system for determining airborne particulate concentration comprising a sampling chamber, a filter mounted within said chamber such that an air sample obtained by said chamber is passed through said filter and airborne particles in said sample are collected by said filter, an oscillator for oscillating said filter, and a detector for determining the concentration of particles collected by said filter, said system being characterized in that said filter includes a collection area on which said particles are collected, and said oscillator is arranged to oscillate substantially only the portion of said filter comprising said collection area. ### Claim 27 (depends on 26) The system of claim 26 further characterized in that said collection area is substantially planar. ### Claim 28 (depends on 27) The system of claim 27 further characterized in that said oscillator is arranged to oscillate said collection area in a direction substantially perpendicular to the plane thereof.