Abstract: The device ascertains existence of a small inkdrop to be ejected from a thermal-inkjet pen. It includes a light source (preferably a nonpoint source with extended threadlike form perpendicular to the light path), detector, and optical element--preferably a cylindrical lens--for passing light from source to detector through an inkdrop probe volume, and for causing that light to be concentrated near the detector. Along a segment of the light path near the detector, the light is concentrated into a cross-section which is much smaller than that of the probe volume and is, within that segment, approximately independent of position along the path.

Abstract: A method and an apparatus allow dynamic tuning of a particle sensor. The particle sensor provides output signals indicating particle detection to a controller, which includes an amplifier whose bandwidth and gain can be adjusted. The bandwidth and the gain of the amplifier are adjusted in accordance with predetermined optimal performance levels under the varying process conditions in which the particle sensor is placed. The optimal signal-to-noise ratio is maintained by adjusting the bandwidth and the gain according to both expected particle velocities and whether a plasma glow is present in the exhaust line for carrying gasses out of a process chamber.

Abstract: An absorption meter determines concentration of chlorophyll a and pheophytin in liquids. Particularly, an immersible absorption meter has a light source, a reflective flow tube, a signal detector, and means for determining the difference between the absorption coefficients at a first wavelength and a second wavelength. The light source may further include a plurality of filters and a selector that selects the filters. The concentration of substances in liquids is determined by measuring the absorption of light at a first wavelength and at a second wavelength, determining the difference between the absorption coefficients for the first and second wavelengths, and converting this data into concentrations.

Abstract: The present invention provides methods and systems for sensing the presence of fluids, such as hydrocarbon-based liquids and/or water, in an ambient environment based upon their light refractive properties. An optical waveguide is deployed in the environment (or a sample chamber disposed within the environment), such that the transmittance of light through the waveguide is attenuated in the presence of the fluid analyte. In one preferred embodiment, the light source is aligned with the waveguide, such that the light entering the waveguide enters one end at an angle relative to the central axis of the waveguide and propagates by internal reflection within the waveguide. Optimal propagation occurs in the absence of the fluid while the presence to the fluid in contact with the waveguide walls causes detectable losses in light propagation due to degradation of the internal reflectance.

Abstract: An obscuration type optical smoke monitor is provided for use particularly in dusty and dirty environments. The smoke monitor comprises a housing enclosing a smoke monitoring chamber; two access ports located opposite each other on the housing; a shunt air flow path located between said two access ports; means to conduct air from said access ports through an optical chamber; a light emitting diode at the first end of said optical chamber, which light emitting diode produces a light beam axially through the center of said optical chamber; a photo transistor at the second end of said optical chamber to receive the light bean and issue an electrical signal proportional to the received light intensity; and an electronic module which provides variable electronic output, generating an analog output essentially proportional of the optical obscuration caused by the smoke.

Abstract: A particle fallout/activity sensor measures relative amounts of dust or other particles which collect on a mirror in an area to be monitored. The sensor includes a sensor module and a data acquisition module, both of which can be operated independently of one another or in combination with one another. The sensor module includes a housing containing the mirror, an LED assembly for illuminating the mirror and an optical detector assembly for detecting light scattered off of the mirror by dust or other particles collected thereon. A microprocessor controls operation of the sensor module's components and displays results of a measurement on an LCD display mounted on the housing. A push button switch is also mounted on the housing which permits manual initiation of a measurement. The housing is constructed of light absorbing material, such as black delrin, which minimizes detection of light by the optical detector assembly other than that scattered by dust or particles on the mirror.

Abstract: In accordance with this invention, a misalignment detector apparatus is provided for a transmissometer with an underfilled reflector. The transmissometer has a transceiver that projects a collimated beam of light of small diameter across a smokestack to a circular reflector of substantially greater diameter than the collimated beam of light so that the reflector is underfilled. The reflector includes apparatus for sensing the position of the collimated beam of light with respect to the center of the reflector and its position on the reflector when it is not on center which allows operator notification of when the beam is on the reflector and when the beam is off the reflector thereby requiring maintenance/realignment.

Abstract: A smoke opacity detector includes a sample chamber; a sample heating block in the sample chamber for heating a sample; a first temperature control for controlling the temperature of the sample heating block to a desired temperature; a measurement stack positioned above the sample heating block for receiving smoke resulting from heating of the sample by the sample heating block; a laser light source for projecting a light beam through the smoke in the measurement stack; a photodetector for detecting the amount of light from the laser light source that passes through the smoke in the measurement stack; a microprocessor for determining the amount of smoke resulting from heating of the sample, in response to the light detected by the photodetector; and a device for selectively eliminating conditions of water vapor in the determination of the amount of smoke by the microprocessor, the device for selectively eliminating including a heating device for heating the measurement stack and a second temperature control fo

Abstract: A structure and an apparatus are provided for use in particle sensor installed to monitor particle level of a process chamber. The process chamber receives process gas from a supply line and removes gas through an exhaust line. The particle sensor's optical components are prevented from contamination by corrosive or coating species in the effluent from the process, by a gas purge line installed in the particle sensor. The gas purge line allows a flow of gas to purge the optical components at a flux not less than the flux of gas being removed from the process chamber in the exhaust line. The flux out of the particle sensor prevents the undesired species from reaching the optical components of the particle sensor from the sampling area where the particle sensor detects the particle level.

Abstract: A method of monitoring a discharge of a liquid comprises the steps of: measuring a presence or absence of a liquid discharged at an optional area of a passage for the liquid during discharging the liquid of a vessel through the passage; comparing a measurement result obtained in said measuring step with a reference measurement result which are to be obtained when a discharge of the liquid is reliably carried out; and determining as to whether a discharge operation for the liquid has been reliably performed. And an apparatus for performing the method.

Abstract: A moisture content measuring apparatus and method wherein a sensor station produces calibration data corresponding to measurements of light at two different wavelengths reflected from a material, and the calibration data is then transferred to a hand held computer. The moisture content of a sample of the material is measured and input to the hand held computer. The process is repeated after changing the wetness of the material, and the hand held computer creates a table correlating reflection data to moisture content. The table is transferred to the sensor station which then continuously generates signals representing moisture content in response to real time reflection data and the table. The cost effectiveness of the sensor stations readily facilitates the use of a dedicated sensor station at each of a plurality of conveyors carrying different materials to be combined into a composite of predetermined proportions by weight.

Abstract: A tank filled with water is covered by a layer of oil containing carbon black. A radiation source preferably a high intensity visible spectrum light is aimed at the bottom surface of the oil/carbon layer. A radiation detector is placed parallel with and immediately above the oil/carbon layer. In operation, the oil/carbon layer will offer an opaque surface thereby preventing the passage of radiation. A bubble rising in the water will momentarily penetrate the oil/carbon layer. If the bubble diameter is greater than the oil/carbon layer thickness, the radiation will pass through the oil/carbon layer. Once the bubble bursts, the oil/carbon layer will immediately heal, thereby sealing off the passage of radiation. The radiation detector will detect any radiation passing through the oil/carbon layer. A passing bubble will be indicated as a discrete blast of radiation. The radiation detector preferably consists of an array of radiation detectors.

Abstract: A multiple printhead cartridge color thermal ink jet printer having apparatus for measuring the offsets between the multiple printhead cartridges each of which includes a nozzle array of one or more columns of nozzles, each column being parallel to the media axis of the printer. The cartridges are individually operated, stationarily and while scanning, to fire drops at an aperture plate interposed between an optical drop detector and the nozzle arrays of the printhead cartridges. The detection of drops passing through the aperture plate and the non-detection of drops blocked by the aperture plate provide information which is utilized in conjunction with the nature of the operation of the cartridges that produced the detected drops to determine the positions of the nozzle arrays of the cartridges relative to each other.

Abstract: A structure and a method use a non-invasive particle monitor to detect particles in a process chamber for a "down sputtering" metal deposition process. In one embodiment, only non-spherical particles are detected using a single laser beam of a predetermined polarization is used, and the phase shift in the polarization due to the passing of a particle through the laser beam is measured. In another embodiment, two closely spaced orthogonally polarized laser beams are used, and the differential intensity of the laser beams is measured when a particle passes through one of the laser beams. In another embodiment, shield tubes for housing optical components are used to prevent coating of the optical components and to prevent deposition to take place outside the shielded area. Internal electric and magnetic fields are used to drive particles through the laser beams for particle detection.

Abstract: A system for characterizing the pressure, temperature, movement and flow patterns of a fluid under high pressure within a test cell. The test cell is lined internally with adjustable rock facings. Pressure is measured within the test cell using a device employing pressure-distortable optical fibers. Fluid velocity, flow direction, and filter-cake buildup are measured with laser Doppler velocimetry. The flow pattern of the fluid is viewed using corresponding arrays of transmitting and receiving optical fibers. Temperature of the fluid is estimated using a combination of thermal sensors. The pressure, velocity, viewing and temperature systems are integral to the rock facings of the test cell.

Abstract: A low cost optical sensor using ordinary non-coherent light reads the polymer content of a processed polymer solution and gives a go/no-go signal in response thereto. The system shuts down in response to the no-go signal. There are no moving parts to pick up or be fouled by polymer.

Abstract: An electrostatic precipitator (ESP) intermittent energization controller is included with a CPU that adaptively controls the DC power used to energize the ESP. An opacimeter is mounted to the ESP for detecting the opacity of dust particles flowing through the same. The CPU searches through a table consisting of a predetermined number of charge-pause parameter sets until one charge-pause parameter set is found to cause the difference between the detected opacity level and the preset opacity level to be within a predetermined tolerable range.

Abstract: An optical device is provided for detecting the degree of obscuring moisture or precipitation on the windshield of an automotive vehicle. A source of light, such as a light emitting diode (LED) or a laser diode, is positioned to illuminate the windshield. An optical imager and detector array senses light emitted by the source, either transmitted or reflected by the windshield, and imaged on the array. The light source and detector may be positioned in convenient, out-of-the-way locations inside or outside the windshield. When the windshield is free of obscuring precipitation, light from the source is imaged on a sensor of the array aligned with the optical axis of the imager. When obscuring moisture or precipitation is present on the windshield, light is scattered such that significant portions are imaged on sensors off the optical axis of the imager.

Abstract: A sensor circuit and method for detecting the presence of a substance having a flow that can be turned on and off, such as ink in an ink ejecting print head, has means for turning on and off the flow of the substance at a predetermined frequency. An optical sensor has a light path that is at least partially interrupted by the substance when the flow is turned on. The optical sensor is biased to operate within a range in which the sensor produces an output, even though the light path is only partially interrupted by the substance. An integrator integrates the output of the sensor, and a high gain amplifier amplifies the integrated signal to provide a sensor circuit output signal. When the substance at least partially interrupts the light path, the integrated output signal indicates its presence.

Abstract: A device is provided which passes light of preselected wavelengths through a fluid or vapor to be tested. The light is detected after it has passed through the fluid or vapor. Upon detection, a voltage is created which is proportional in amplitude to the detected light strength for each wavelength of light. These voltages are compared to produce a ratio which represents the condition of the fluid or vapor which is tested. When the ratio is outside of preselected boundaries, control signals are generated which may trigger alarms or otherwise respond to the condition.

Abstract: Changes in the state of a chemical reactant, particularly a resin used for purifying gases, are measured by directing a beam of light through the reactant and detecting changes in the light transmissivity of the reactant as the reactant goes from a first state to a second state.

Abstract: The sensor system monitors solutes in a liquid stream by analyzing the amount of light absorbed in a liquid in a particular wavelength band. The sensor system is arranged for selectability of the type of fiber used for transmission and adjustability of the liquid gap length in the liquid so that individual installations can be arranged with optimum characteristics for sensing particular chemical species in solution by absorption spectroscopy.

Abstract: A device for detecting the marker dye band which is used to monitor the progression of biological macromolecules in gel electrophoresis. This device mounts external to the gel box, and utilizes a single light detector and a pair of AC activated light sources. The light sources produce reflected or transmitted light signals which, when balanced at the detector, cancel. When marker dye is absent the light signals are balanced, and no signal is detected. When marker dye is present at a specific detection point within the gel, the light reflected (or transmitted) is no longer balanced and a signal is detected.

Abstract: A particle sensor which employs the principle that a particle passing through an intense laser beam will scatter light to a photodetector which then generates a measurable signal is provided. The particle sensor uses prisms and a cylindrical lens to compress the laser beam to make it very thin along the axis of particle motion but very wide in the plane perpendicular to particle motion, thereby simultaneously providing high beam intensity for enhanced sensitivity and a large detection area. The optical components of the sensor are mounted on separate sections which allows the optical components to be separately aligned and changed so that the sensor may be easily adapted to various applications.

Abstract: An endpoint detector for gas purifiers which contains a detection element comprising a small amount of a highly reactive metal coated on an insulator and exposed to the gas stream being purified. This metal does not react with the gas being purified, but does undergo reaction with the impurity species. As this metal reacts with impurities in the gas stream, its conductivity decreases. The change in conductivity signals that the wave front of impurity-containing gas has reached the sensor, and thus the purifier has reached the end of its useful life. The use of a thin coating of reactive metal as the detection element gives the sensitivity that is required to sense sub-part-per-million levels of impurities. This resistance change sensor may be combined with a thermal measurement to detect the presence of exothermic conditions indicative of a large pulse of impurity gas.

Abstract: System and method for detection and measurement of airborne biological aerosol particles in a gaseous sample are described comprising a source of low energy radiation for irradiating the gaseous sample whereby biological particles in the sample are ionized, a detector for detecting ionized biological particles in the sample including a pair of electrically charged conducting plates disposed in parallel confronting relationship to each other with a preselected space therebetween, a source of electrical power operatively connected to the plates for applying a preselected electrical potential thereacross, and electronics for sensing collisions on the plates of ionized biological particles in the sample.

Abstract: An apparatus for emitting and receiving light comprises an emitter, which consists of a light source (5) and a concave mirror (7), and a receiver which receives light from the emitter and which is connected to analyzing equipment (3). Furthermore, the receiver comprises a concave mirror (9) which is disposed behind the mirror (7) of the emitter and whose diameter is larger than that of the mirror (7) of the emitter and whose focus is located in front of the light source (5). In the focus of the mirror (9) is positioned one end of an optical fibre (19) for transmitting the received light to the analyzing equipment (3).

Abstract: An optical probe apparatus (10) is for testing physical properties of a fluid (F) such as turbidity. The apparatus comprises a probe assembly (16) removably insertable into the fluid. A photometric light source (12) generates light and fiber optic cables (14) transmits light to and from the probe assembly. Light entering the assembly is directed into the fluid, the light being absorbed or scattered by particles in the fluid as it passes therethrough. The amount of remaining light is sensed and a light output from the probe is produced whose intensity is a function of the amount of remaining light. A light detector measures the light output of the probe assembly and produces an indication of the absorbance or turbidity of the fluid.

Abstract: In order to make it possible to measure the light absorption coefficient of various sorts of substances contained in a light scattering material with a high precision, the wavelength of light transmitted through the light scattering material is varied and the coefficient of light absorption is calculated, starting from a ratio of variations in the intensity of the transmitted light to variations in the wavelength.

Abstract: A device for optically detecting a chemical change of fluid comprises a prism placed in fluid to be tested and a layer of color indication material formed on a surface of the prism to thereby detect a chemical change of fluid by means of a transmitted light and a reflected light in the prism.

Abstract: A sample cell for use in measuring the particle size distribution of samples using diffraction-scattering measurement methods has a radiating surface which is beveled to increase the angle of incidence of scattered light which is scattered at a large scattering angle such that it is transmitted through the radiating surface of the sample cell and thereby allowed to impinge upon a detector.

Abstract: An automatic detection system for control of oil spillage into sea waters comprising a plurality of optical fiber cables connected each to a control box and to a plurality of sensors located remotedly in the open sea, and each of the sensors is a part of a closed loop of a light signal, transmitted from the control box and received back by it. Each of the sensors consists of a floating buoy affixed to a permanent location in the open sea, having its center of gravity adjustable and stabilized, so that when it floats, the central part of the sensor rests at sea level. The buoy is provided with a central bore, into which the optical fiber cable's ending is positioned; and a plurality of concentric channels are provided from all sides of the bore, leading sea water from the outer surface of the buoy inwards.

Abstract: The Rolling Average Rate Read Indicator and Method detects the presence of a particular constituent in either a solid, liquid or gas and returns a rapid and accurate measurement of the concentration of the constituent. The Indicator is comprised of a sensor (12) containing a stepper motor (36) driven tape impregnated with a chemical substance reactive to a particular constituent. Discoloration of the tape occurs in the presence of the constituent and this discoloration is converted to an electrical signal which is indicative of the rate of change in the chemical reaction taking place and is linear for some portion of time. The signal is differentiated (16) and the resultant differentiated signal is indicative of the arithmetic mean of a plurality of instantaneous values taken at intervals during the linear portion of the reaction. The extent of the reaction as evidenced by the level of the sensor output signal is continuously monitored and compared (26) to a known value.

Abstract: A gas detection device employs a tunable wavelength laser, modulated about an absorption line of a gas to be detected. By measuring the ratio of the fundamental to the second harmonic of the modulation frequency, the detection is independent of the optical path length. This permits an aimable device in which a laser beam is directed toward a remote surface, reflected thereby, and the reflected beam focused upon a detector. The presence of the gas to be detected in the space between the device and the reflective surface is thereby determined.

Abstract: The invention includes a blood detector disposed on a medical liquid duct adjacent a connection to a duct for circulating blood outside the body. When the medical liquid ceases to flow, blood sediments in the liquid duct and is detected therein. The detector makes it possible to verify that the medical liquid is indeed flowing into the blood. The invention is applicable in numerous blood treatment techniques making use of a circuit for circulating blood outside the body, such as hemofiltration, hemodiafiltration, and plasmapheresis.

Abstract: A sample flow, including a plurality of components which have different types of spectra and are subjected to flocculation, is irradiated with a light of beam including two or more wavelengths. The transmitted light beam is received by a photoelectric converting device, and electric signals corresponding to respective wavelengths of the transmitted light beam are obtained. The correlation coefficient between the electric signal is calculated, from which the flocculation process of the sample flow with regard to the elapsed time can be easily and quickly detected.

Abstract: Methods and apparatus employing optical spectrometry techniques to measure metallic wear debris contamination in used motor oil, even in the presence of carbon particulates. Measurement results based on optical spectrometry as disclosed herein correlate closely with results of laboratory analysis based on atomic spectrometry. Variation in transmittance of a sample of oil as a function of wavelength within the approximate range of 300 nm (violet) to 1300 nm (near-infrared) is measured as an indicator of metallic wear debris contamination. Conveniently, measurements may be taken at just two wavelengths within the range, for example 565 nm (green) and 660 nm (red). The ratio of transmittance at the shorter of the two wavelengths to transmittance at the longer of the two wavelengths is determined as an indication of the amount of metallic wear debris contamination, decreases in the ratio from a ratio of unity indicating increasing levels of contamination.

Abstract: Apparatus for cleaning the interior surface of a window in an inline optical sensors which is subject to contamination and fouling by a product stream in contact therewith. The window surface is wiped by a blade driven by a pneumatic operator, and the amount of air supplied to the operator is adjusted to control the speed at which the blade moves across the window surface. When a cleaning operation is initiated, the blade is wiped across the window surface a predetermined number of times, and measurements utilizing data from the sensor are suspended until the cleaning operation is completed and the data has had time to restabilize.

Abstract: Method and apparatus for determining accumulated film thickness at the inside diameter of a main stream conduit conducting a main stream of a flowing fluid by employing:a transparent shunt conduit to shunt from the main stream a sample stream of the process fluid,a reference light emitter and light detector at a reference section of the shunt where any appreciable film is removed, an upstream sample light emitter and detector opposed thereto,a common source of light so that respective emitters emit light beams of the same intensity, and means for determining concurrently analogs of light received by the detectors, whereby film thickness may be determined for the sample.

Abstract: The apparatus for detecting impurities in a fluid circuit has a body member associated with a fluid conduit or reservoir which defines a detection zone. A magnet is associated with the body member and communicates with the fluid in the detection zone to attract metallic impurities. Fiber optic members extend from opposite sides of the detection zone, with one of the fiber optic members associated with a light source and the other fiber optic member associated with a light sensor. The light is transmitted by the first fiber optic member into the fluid in the detection zone. The light passing through the fluid is transmitted by the second fiber optic member to the light sensor.

Abstract: A nondispersive gas analyzer for use in monitoring the concentration of one or more gases, comprises a body having a cavity having an elliptical reflecting surface, defining a first focus and a second focus, for transmitting radiation between the focuses, a chamber (2) for holding a sample gas to be analyzed, one of the focuses being located within the sample chamber, a chamber (4) for holding an inert gas and a chamber (3) for holding an analyte gas, a radiation source (24) disposed at one of the focuses, and a radiation detector (26, 27) associated with each of the inert and analyte gas chambers for detecting the radiation passing through the sample gas and the inert gas and the radiation passing through the sample gas and the analyte gas located at the other of the focuses.

Abstract: For measuring the flow rate of a thin stream of molten materials such as that of glass, the diameter of the thin stream is measured, as is the velocity. The velocity is measured on the basis of the measurement of the time separating the successive appearance of an emission sequence emitted at first and second points on the path of the molten material. A correlation is then established between the sequences and the time interval corresponding to the passage of the same irregularities at the two selected points identified by this correlation.

Abstract: According to the present invention, the accuracy of particle size histograms produced from data relating to an impedance variation (Coutler Counter Method) caused by particles passing between electrodes disposed on either side of an orifice through which the particles pass is improved by excluding false data from the data used in producing the histograms, the false data resulting from more than one particle being resident in the orifice at a given data collection time. The preferred embodiment employs a light detecting method and apparatus to detect false readings and to provide correction information used to correct data obtained by the Coulter Counter Method and apparatus.

Abstract: An apparatus is provided for indicating how dirty an air filter is that is disposed in an air stream for removing dirt therefrom. The apparatus includes an arrangement in the form of a light unit for measuring how dirty the air filter is. The apparatus also includes an indicator that is connected to the light unit.

Abstract: An apparatus is disclosed for counting and determining at least one leucocytic sub-population, comprising a case for injecting a flow of sample to be analyzed inside a measurement tank through which a light beam passes, comprising internal and external nozzles for sleeving the sample by means of a pressurized liquid. Electrodes for the resistivity measurements of the leucocytic sub-population are component elements of the injection case. Units for collecting and processing the signals delivered by the optical sensor and the electrodes are also provided.

Abstract: A narrow passband, reflective optical filter at a wavelength L, has a container filled with a pressurized gas, the pressurized gas having a ground state transition corresponding to the wavelength L and transparent to all other wavelengths. The container has at least one optically transparent major planar surface serving as the face thereof, whereby incoming broadband light is absorbed by the gas, except at its resonance wavelength L, which is specularly reflected. The optically transparent face of the container optionally has a broadband anti-reflective coating to improve its out-of-band rejection of unwanted wavelengths of the light signal.

Abstract: A particle counter operates by the opacity method and has a light barrier or electric eye. The light beam of the light barrier or electric eye penetrates a measuring channel for a fluid transverse to the passage of the fluid through the channel. An electronic analysis assembly is connected in series following the receiver of the light barrier or electric eye. The light emitting surface of the light barrier or electric eye is formed by the end surface of a light-emitting fiber. The light inlet or receiving surface is formed by the end surface of a receiving fiber. The end surface of light-emitting fiber lies in a first boundary surface of the measuring channel. The end surface of the receiving fiber is concentric to the light-emitting fiber end surface and is located in a second boundary surface of the measuring channel parallel to the first boundary surface.

Abstract: A continuous flow condensation particle counter employs a working fluid to saturate a gas stream, and then condense onto particles carried in the gas stream as the stream is directed through a condenser and cooled below the supersaturation point. A porous saturation block is positioned beneath the condenser, and receives droplets of working fluid condensate and water as the droplets descend from the condenser by gravity. The saturation block is constructed of a hydrophobic material, whereby it allows the working fluid to pass through but retains water within a collection reservoir centered below the condenser. A vacuum pump and a periodically actuated solenoid valve are employed to introduce a partial vacuum to the reservoir and thereby evacuate accumulated water. The preferred working fluid is a perfluorinated hydrocarbon fluid that is substantially odorless, chemically inert and non-flammable. Water is immiscible in the working fluid, and thus readily separable.

Abstract: The Airborne Infrared Transmissometer measures the infrared transmission between a test target aircraft and a detection system. A transmitter is attached to an aircraft and emits a calibrated infrared beam. A ground-based, shipborne, or airborne receiver measures the transmitted signals in the exact same path between the target aircraft and the detection system. Data recording equipment can be attached to both the transmitter and receiver to measure the associated outputs.

Abstract: In a smoke detection system, smoke density in a sampling chamber is measured by flashing a strobe light through the chamber and sensing light flux emitted from the chamber and comparing it with light flux from the strobe light itself. The measurements are performed by peak detectors which load sample-and-hold circuits to provide steady signals. The two signals are combined in mathematical manner to compensate for zero-offset and rate error between the two signals. The combined and corrected output is used to actuate a visual alarm signal, such as a segmented bargraph display to indicate air pollution. The bargraph has programming pins for tapping off each individual bargraph segment to achieve plural preset alarm thresholds.