Abstract: Apparatus and methods for detecting and reporting pollution at an oil facility are disclosed. The technology utilizes a logic control system to read from a pollution sensor and present the sensed data as a pollutant from a leak. The pollution leakage can be further managed and reduced.

Abstract: A plasma processing apparatus includes a microwave output unit, a wave guide tube, a tuner, a demodulation unit, and a calculation unit. The microwave output unit outputs a microwave having power corresponding to setting power while frequency-modulating the microwave in a setting frequency range. The wave guide tube guides the microwave to an antenna of a chamber main body. The tuner is provided in the wave guide tube and adjusts a position of a movable plate. The demodulation unit is provided in the wave guide tube, and acquires travelling wave power and reflected wave power for each frequency. The calculation unit calculates a frequency at which a reflection coefficient, which is calculated on the basis of the travelling wave power and the reflected wave power, for each frequency becomes a minimum point as an absorption frequency.

Abstract: Disclosed herein are advantageous measuring devices, and systems of the present disclosure and adjustment methods/techniques thereof. The present disclosure provides improved measuring devices (e.g., pressure/temperature measuring devices), and improved systems/methods for adjusting one or more features (e.g., offset and/or span) associated with measuring devices. More particularly, the present disclosure provides sealed measuring devices (e.g., sealed signal conditioning devices, such as sealed pressure transducers or transmitters) having adjustment members (e.g., magnet members) that allow a user to adjust the offset and/or span of the sealed measuring devices. The measuring devices include an adjustment member that allows a user to make adjustments to one or more features of the measuring devices. For example, a sealed pressure transducer can include an magnet member that allows a user to make fine (precision) adjustments of output offset and/or span (e.g.

Abstract: A gas alarm device is provided, where a heating control section extends a heating period of time of a heater section if a first determination section determines that electrical characteristics of a sensing section of a gas sensor satisfy a first condition, and continues extension of the heating period of time of the heater section according to a determination result, by a second determination section, of whether or not the electrical characteristics upon lapse of extension of the heating period of time satisfy a second condition, and a gas detection section determines, according to the electrical characteristics upon lapse of extension time, that detection target gas is detected.

Abstract: A calibration method of a detection device and a method for acquiring a calibration file of the detection device are described herein. The acquisition method includes generating the calibration file in an address space accessible by the detection device in a use environment according to a calibration result, and correspondingly storing a unique identification code, the calibration result, and the calibration file of each detection device. When the detection device detects an article, the method includes attempting to acquire the calibration file from itself. If the calibration file can be acquired, the method includes directly loading the calibration file. If the calibration file cannot be acquired, the method includes acquiring the calibration file from the address space accessible by the detection device in the use environment. The solution is applied to the calibration of the detection device on a production line and the use of the detection device.

Abstract: There is disclosed a field calibratable particle sensor solution in a low-cost, very compact form factor. This makes a low-cost sensor more accurate for low-concentration pollution measurements and decreases the cost of pollution measurement systems having a wide geographic coverage. In a related embodiment, the invention illustrates a method and system to remotely and automatically calibrate one or more of the low cost sensors disclosed herein as well as other commercially available sensors (such as optical particle counters, photometers etc.) against a reference instrument (such as a beta attenuation monitor) which may or may not be physically located in the same place as the individual sensors. The method may require minimum (or no) user interaction and the calibration period is adjustable periodically.

Abstract: A method for testing a gas sensor is based on a gas-measuring system with a test gas source and with a pumping device. A predefined quantity of a gas or of a gas mixture is fed in, from the test gas source, to the gas sensor over a predefined time and metered. The response of a measured signal of the gas sensor is determined as a sensor response. Characteristic variables, from which an indicator of the ability of the gas sensor to operate is determined, are determined from the sensor response.

Abstract: In a method for determining the lambda value with a broadband lambda sensor of an internal combustion engine, a measurement current is supplied by the lambda sensor, which measurement current is proportional to the oxygen content or rich gas content in the exhaust gas of the internal combustion engine, and the corresponding proportionality factor is determined by a thrust comparison. The thrust comparison uses the O2 concentration of dry air as reference, independent of an actually present air humidity, and the lambda value is calculated on the basis of a modified Pischinger conversion equation in which a signal supplied by a humidity sensor is taken into account.

Abstract: Embodiments of the present invention generally relate to methods of controlling gas flow in etching chambers. The methods generally include splitting a single process gas supply source into multiple inputs of separate process chambers, such that each chamber processes substrates under uniform processing conditions. The method generally includes using a mass flow controller as a reference for calibrating a flow ratio controller. A span correction factor may be determined to account for the difference between the actual flow and the measured flow through the flow ratio controller. The span correction factors may be used to determine corrected set points for each channel of the flow controller using equations provided herein. Furthermore, the set points of the flow ratio controller may be made gas-independent using additional equations provided herein.

Abstract: A system and method are disclosed including an automatic sensor excitation voltage adjustment feature, a multi-range concentration feature, and a single calibration feature. The automatic sensor excitation voltage adjustment feature may include a transmitter having an associated microprocessor that provides an initial voltage to an associated sensor. The sensor may include a microprocessor, and as the voltage changes a correction signal may be relayed from the sensor microprocessor to the transmitter microprocessor. The correction signal may be used by the transmitter microprocessor to adjust the voltage applied to the sensor. The multi-range concentration sensor feature may include an amplifier associated with the sensor/microprocessor to create gain settings used to optimize sensor resolution by changing a gain value associated with the sensor. This, in turn, enables a single sensor to be used for a variety of different concentration ranges, as desired by a user.

Abstract: A method for monitoring a nitrogen oxide storage catalytic converter (NSC) in the exhaust gas duct of an internal combustion engine which is operated at least temporarily in a lean fashion, wherein during a lean mode of the internal combustion engine nitrogen oxides from the exhaust gas are stored by the nitrogen oxide storage catalytic converter, wherein during a regeneration phase of the nitrogen oxide storage catalytic converter the internal combustion engine is operated in a rich fashion, and as a result the nitrogen oxides stored in the nitrogen oxide storage catalytic converter are removed, and wherein an exhaust gas component or exhaust gas characteristic variable which is characteristic of the profile of the regeneration is detected during the regeneration phase by means of an exhaust gas probe.

Abstract: A system for calibrating a response to an exhaust-stream NOx level in a motor vehicle is provided. The system comprises a NOx sensor that includes an electrode, a current from the electrode responsive to the exhaust-stream NOx level while a bias voltage is applied to the electrode. The system further comprises a controller configured to interrupt the bias voltage and to adjust a motor-vehicle response to the current based at least partly on an attained voltage of the electrode while the bias voltage is interrupted. Other embodiments provide a method of calibrating a NOx sensor response in terms of gain and offset parameters.

Abstract: A control system and associated methods for an air treatment system. In one aspect, the present invention provides a control system and method for controlling blower speed as a function of separately determined smoke and dust concentrations. In one embodiment, the control system and method provides a variable delayed between changes in motor speed to address undesirable rapid changes between speeds. In another aspect, the present invention provides a system and method for calibrating a sensor to provide more uniform operation over time. In yet another aspect, the present invention provide a system and method for calibrating motor speed to provide more consistent and uniform motor speed over time. The present invention also provides a system and method for tracking filter life by as a function of time, motor speed and/or a sensed variable, such as particulate concentration in the environment.

Abstract: A method for gas analysis can include continuously diverting a portion of drilling fluid into a sample chamber from a drilling fluid stream, creating a bidirectional drilling fluid circulation to liberate gas from the drilling fluid, applying a suction to a gas capturing chamber, and allowing a sample of the liberated fluid to continuously flow through the gas capturing chamber, continuously be filtered, and continuously flow into a gas analyzer. The method can include maintaining the sample chamber in an open configuration or decompressing the gas capturing chamber by closing off one or more sample inlets and sample outlets and exhausting portions of the liberated fluid that do not flow to the gas analyzer. The method can include simultaneously providing analysis data to client devices.

Abstract: A compensated gas detector incorporates first and second pellistors combined with first and second resistors and an intervening switch. Control circuits can close the switch for normal gas detection. The switch can be opened to carry out diagnostic measurements. A compensation coefficient can be established in order to compensate outs from the pellistors due to mechanical damage thereto.

Abstract: A method for the operational test measurement of a lambda probe, particularly for the purpose of calibrating and/or determining the status of the lambda probe, wherein pure air is circulated around a probe head of the lambda probe is disclosed. The method has the following steps: —insertion of the lambda probe into a flow receptacle which is separate from an engine construction, in such a manner that an air stream (L) can be circulated around the probe head; generation of an air stream (L) in the flow receptacle; evaluation of probe measurement signals (S), wherein a calibration value is obtained and/or a heating current of the lambda probe is evaluated. A device for carrying out the method, as well as a system, is also disclosed herein.

Abstract: In a method for correcting the output signal of a broadband lambda probe of an internal combustion engine, the influence of the air humidity on the lambda value determined by the broadband lambda probe is detected and is deducted by means of a compensation model. For this purpose, a measured air humidity is incorporated in the calibration of the broadband lambda probe during an overrun shut-off phase of the internal combustion engine.

Abstract: A gas analyzing system that can perform calibration without being influenced by the deterioration of span gas in a span gas supply line. An open-close device control part that receives a calibration start signal issuing an instruction to start zero calibration and span calibration, and controls an open-close device for a span gas flow path. An open-close device for a zero gas flow path, wherein if the open-close device control part receives a new calibration start signal after a predetermined time has passed since previous calibration was performed, before the span calibration is started, for a predetermined time, the open-close device control part controls the open-close device for the span gas flow path so as to open the open-close device for the span gas flow path, and thereby purges span gas that remains in the span gas flow path.

Abstract: In the calibration of a lambda sensor (26), inaccuracies occur during a fuel cut-off overrun phase depending on the temperature. A method is proposed for correcting an output signal of a lambda sensor (16) of an internal combustion engine (1), having the following steps: detection of a fuel cut-off overrun phase of the internal combustion engine (1), sensing of an exhaust-gas composition by the lambda sensor (16) during the fuel cut-off overrun phase, sensing of a temperature which represents a measure of the intake air of the internal combustion engine (1), calibration of the lambda sensor (16) based on the second temperature.

Abstract: A method of calibrating an apparatus for measuring Radon and/or its progeny in an air sample, includes providing an apparatus for measuring Radon in an air sample that includes a preset offset voltage corresponding to a threshold conductivity value and a preselected reading value for indicating a concentration of Radon and/or its progeny, exposing the apparatus to a calibration area for measuring conductivity of the air therein, comparing the measured conductivity value of the air with the preset offset value for conductivity, and setting a lower threshold conductivity value and corresponding threshold reading value same as the preselected reading value, if the measured conductivity of the air is determined to be lower than the preset offset value for conductivity.

Abstract: The present invention relates to chemical and bioanalytical sensors and sensor systems, particularly to systems incorporating a calibration device for sensors where the calibration device is comprised of an array of micromachined MEMS structures and materials for trapping and retaining calibrant or interferant materials until they are needed and released quantitatively using structure operatively associated with the array.

Abstract: Embodiments of the present invention relate to a gas sensor comprising a flow channel, a concentration modulator positioned in the flow channel, a gas generator in contact with the flow channel, one or more gas detectors positioned downstream of the concentration modulator and gas generator and a pump.

Abstract: The present invention relates to the monitoring of gas concentrations possible in very low ranges (i.e., low ppb and even ppt ranges) and especially use thereof in environmental monitoring, exposure assessment, bomb detection, and health studies. The invention can use a spatial and temporal assessment of gas concentrations that enables the sources of the gas in question to be located and identified which is useful in environmental and health field but can also be applied to other fields an example of which is detecting and locating explosives. This technology can uses small, light weight, and low power components that allow for the monitor to be portable and even worn on a person as a personal monitor. This technology can be used in a stationary monitors as well.

Abstract: A system for calibrating a response to an exhaust-stream NOx level in a motor vehicle is provided. The system comprises a NOx sensor that includes an electrode, a current from the electrode responsive to the exhaust-stream NOx level while a bias voltage is applied to the electrode. The system further comprises a controller configured to interrupt the bias voltage and to adjust a motor-vehicle response to the current based at least partly on an attained voltage of the electrode while the bias voltage is interrupted. Other embodiments provide a method of calibrating a NOx sensor response in terms of gain and offset parameters.

Abstract: A method of calibrating a metabolic analyzer incorporating an oxygen analyzer and a NDIR carbon dioxide analyzer in the field that does not require the use of gas cylinders containing gases of known concentration is described. In calibrating the CO2 detector, at the time of factory setup, the detector output for a gas of a known concentration is measured and stored in the memory of the metabolic analyzer's microprocessor, as is the detector output voltage when the IR source is dimmed by a known percentage. Subsequently, in the field, CO2 levels in ambient air and cell pressure are measured at two different flow rates through the sample chamber and the IR source is again dimmed by the same percentage as had been used at the time of factory setup. Based upon the resulting readings, both the zeroing and span adjustment factors can be computed.

Abstract: In leak detection, the signal generated by a test gas is superposed by interferences which fade as the vacuum generation in a container proceeds. Depending on the negative slope of the volume signal (MS), a lower indication limit (AG) is calculated. Upon activation of a zero function, the volume signal (MS) is not reduced to zero but only to the level of the indication limit (AG). Any exceeding of the indication limit is identified as a leak. Thus, the maximum sensitivity of the leak detection is guaranteed at any time.

Abstract: A dissolved oxygen measurement system, and a method for calibrating a dissolved oxygen analyzer are provided. The method includes directing a pressurized cleaning fluid, having a known oxygen content, upon a sensing surface of a dissolved oxygen sensor disposed in a fluid. Then, the analyzer waits until the dissolved oxygen sensor response to the pressurized cleaning fluid is relatively stable, and stores a calibration value relative to the dissolved oxygen sensor response to the pressurized cleaning fluid. A dissolved oxygen measurement system is also provided. The system includes a dissolved oxygen sensor disposed in a fluid and a first source of a first compressed cleaning fluid that has a first dissolved oxygen content. A dissolved oxygen analyzer is coupled to the dissolved oxygen sensor and to the first valve and is configured to obtain a first calibration value related to the first dissolved oxygen content.

Abstract: A particulate sampler system includes a diluted exhaust mass flow controller that receives flow from a dilution mass flow controller and a laminar flow element (LFE). The dilution mass flow controller and LFE are fluidly arranged parallel to one another. A controller communicates with the mass flow controllers and the LFE to determine the flow through these devices, command valves in the mass flow controllers and generate data for determining calibration coefficients and correction factors. The diluted exhaust mass flow controller is calibrated and calibration coefficients are generated using a first to fourth order curve fit. Similarly, initial calibration coefficients are generated for the dilution mass flow controller. The diluted exhaust mass flow controller is set at a desired flow point. The dilution mass flow controller is varied between set points corresponding to different dilution ratios at the common, desired set point.

Abstract: Embodiments of the invention provide novel, industrially applicable, and non-obvious apparatus and methods for automatically, routinely, and accurately calibrating a trace detection portal. Embodiments of the apparatus include a calibrant container; a substance (or substances) uniquely identifiable by a trace detection portal as a calibrant; unique placement of the calibrant container's outlet relative to a substance collection port of a sample collection chamber; and/or computer executable instructions that, when executed by a computer processor, cause a consistent release of a measured amount of calibrant into the sample collection chamber upon command and/or at pre-determined time intervals.

Abstract: A sensing system having a zeroing mechanism incorporating a micropump. The micropump may be connected to a gas conditioner for providing zeroing gas to a sensor downstream. The micropump and sensor may be connected to a processor or computer for pump control and receipt of information from the sensor. Upon zeroing and/or baseline correction of the sensor, the pump may be stopped and a sample gas may be fed into the sensor for detection and analysis. Alternatively, the pump may situated downstream of the sensor to draw conditioned gas through the sensor. A valve may be added having a conditioned gas input connected to the conditioner and having a sample input. The valve may have an output connected to the sensor. When the valve is open to the conditioned gas input, it will be closed to the sample input, and vice versa. The system may be for zeroing and/or calibration.

Abstract: In a method for correcting the output signal of a broadband lambda probe of an internal combustion engine, the influence of the air humidity on the lambda value determined by the broadband lambda probe is detected and is deducted by means of a compensation model. For this purpose, a measured air humidity is incorporated in the calibration of the broadband lambda probe during an overrun shut-off phase of the internal combustion engine.

Abstract: A gas sensor apparatus 3 in an air-fuel ratio detection system 1 includes a gas sensor element 4 which outputs a detection signal corresponding to air-fuel ratio, and a gas sensor control circuit 2 which includes a detection section 20 for outputting a first output signal VIP1, a second output signal VIP2, and a third output signal VIP3 in accordance with the detection signal. This detection section 20 outputs the first output signal VIP1 which changes in accordance with the air-fuel ratio at least within a wide first air-fuel ratio zone, the second output signal VIP2 which changes in accordance with the air-fuel ratio within a narrow zone in the vicinity of the stoichiometric ratio, and the third output signal VIP3 which changes in accordance with the air-fuel ratio within a narrow zone in the lean region.

Abstract: A system and method for determining “real time” air leakage from a duct involves an airflow meter for taking airflow measurements at the inlet and at the outlet of the duct. Also included is a base fan that operates in three different modes to simultaneously vary an enclosure pressure at the inlet and outlet. This is done while a duct fan, inside the duct, is operated to continuously move air through the duct. A comparison of airflow measurements at the inlet, or at the outlet, under the three base fan operational modes will respectively identify whether leaks are present in the duct, upstream or downstream from the duct fan.

Abstract: A method of calibrating a metabolic analyzer incorporating an oxygen analyzer and a NDIR carbon dioxide analyzer in the field that does not require the use of gas cylinders containing gases of known concentration is described. In calibrating the CO2 detector, at the time of factory setup, the detector output for a gas of a known concentration is measured and stored in the memory of the metabolic analyzer's microprocessor, as is the detector output voltage when the IR source is dimmed by a known percentage. Subsequently, in the field, CO2 levels in ambient air and cell pressure are measured at two different flow rates through the sample chamber and the IR source is again dimmed by the same percentage as had been used at the time of factory setup. Based upon the resulting readings, both the zeroing and span adjustment factors can be computed.

Abstract: Method, apparatus, and system are utilized for testing the performance of a gas monitor against predetermined monitor characteristics to determine if performance of the gas monitor is validated in a manner whereby testing gas is directly delivered to the gas sensor.

Abstract: A calibration assembly generates elemental mercury and oxidized mercury for calibrating components of a mercury monitoring system, including making necessary adjustments to efficiencies of a mercury compound converter and an elemental mercury detector. The calibrator generates an elemental mercury sample having a known elemental mercury concentration, [Hg0]1 and combines an oxidizing component with the elemental mercury sample, thereby producing a reduced concentration of elemental mercury [Hg0]2 within the sample. The calibrator measures the concentration of elemental mercury [Hg0]2 within the sample and calculates a difference between the known elemental mercury concentration, [Hg0]1 and the reduced concentration [Hg0]2. The difference between [Hg0]1 and [Hg0]2 is substantially equal to the concentration of oxidized mercury produced by the calibrator.

Abstract: Embodiments of the present invention relate to a gas sensor comprising a flow channel, a concentration modulator positioned in the flow channel, a gas generator in contact with the flow channel, one or more gas detectors positioned downstream of the concentration modulator and gas generator and a pump.

Abstract: A leak test apparatus includes a housing having an interior sized for retaining a plurality of instrument components including a control unit. A sniffer gun is linked to the instrument components, the sniffer gun having an intake point, and a test leak includes a gas reservoir, a constriction, an opening suited for introducing the intake point of said sniffer gun, and a temperature sensor. The temperature sensor is linked, either by means of a wired connection or wirelessly to the control unit for signal transfer.

Abstract: A system and method is provided for obtaining calibration curves for CO and CO2 during laboratory calibration of one or more remote emissions sensing (RES) instruments, and for self-calibration of on-road RES instruments to compensate for changes in background CO and CO2 column densities.

Abstract: System and method for calibrating a sensor comprises a detector capable of detecting electromagnetic radiation at least at a radiation wavelength of interest, an analyzer capable of determining concentration values in response to the detected radiation, and a calibrator capable of automatically calibrating the analyzer in response to at least one concentration value. A method for operating a calibrator comprises receiving an initiation signal capable of initiating the calibrator, automatically obtaining concentration values in response to the initiation signal, and automatically determining a calibration value from the obtained concentration values. In an embodiment, the calibrator automatically determines when a calibration gas equilibrates within a sample chamber.

Abstract: A method for determining the voltage sensitivity of the distance between the mirrors of a Fabry-Perot interferometer in a measuring device, which is intended to measure a predefined gas (CO2), of which at least one absorption maximum or minimum is known. At least two calibration point are defined device specifically using a reference gas (N2) in controlled conditions. A ‘virtual’ signal-control-voltage sensitivity curve, is formed with the aid of the calibration points formed using the reference gas (N2), ratios of the measurement points of the predefined gas (CO2) and of the corresponding values of the reference-gas curve are formed, and at least one voltage value corresponding to the minimum or maximum is defined from the ratios, in which case, on the basis of the wavelengths of the absorption minima or maxima of the gases (CO2, N2) being measured, the voltage sensitivity of the distance between the mirrors can be defined unequivocally.

Abstract: The present invention relates to testing equipment 20 for a fire alarm 1. It includes a testing pole (21), a range spacer (23) connected to the testing pole (21), and a reflective means and/or scattering means situated in the inside of the essentially pot-shaped designed range spacer (23).

Abstract: A leak detector comprises a high vacuum envelope connected with a vacuum pump to bring the vacuum envelope to a pressure sensibly lower than ambient pressure. At least a portion of the envelope is separated from an external environment through a quartz membrane associated with an electrical resistor, which heats the membrane to a sufficient temperature in order to make it permeable to a corresponding test gas (in particular, helium). In operation, an electronic control unit detects variations in the current absorbed by the vacuum pump and consequently signals the test gas diffusion into the chamber and hence the existence of a leak. A sampling line with a sampling pump disposed therein is connected to a probe for the intake of gas directed to expose to the membrane. The gas is discharged to the outside through a discharge line. A system for switching between the gas intake and exhaust is provided, in order to quickly eliminates any test gas trace from said detector.

Abstract: An apparatus and method for purging and zeroing an analyzer. At times, for good analysis of a gas sample or taking a live reading, the analyzer needs to be zero out and/or purge of contaminants. The analyzer can take a live reading, purge and/or zero in any order. The purge function and zero function can automatically be done before, during or after taking a live reading.

Abstract: An apparatus and method for measuring the vapor or gas content of a vacuum chamber (1) by a vapor sensor (21) located in the fore-line of a vacuum system is provide. The gas or vapor content is determined by first providing a zero condition for the sensor using the vacuum pump (25) and them referencing the sensor response to the chamber gas during the evacuation cycle.

Abstract: An apparatus and method for purging and zeroing an analyzer. At times, for good analysis of a gas sample or taking a live reading, the analyzer needs to be zero out and/or purge of contaminants. The analyzer can take a live reading, purge and/or zero in any order. The purge function and zero function can automatically be done before, during or after taking a live reading.

Abstract: An automatic system for calibrating gas sensors comprising a source of SPAN gas and a source of ZERO gas, and a means to control the flow of these gases to sensors. The system schedules and makes all required sensor value corrections and adjustments without human intervention. High resolution monitoring enables life extensions for the sensors, as well as higher accuracy monitoring than would be reasonably expected from low cost sensors.

Abstract: An instrument provides a quantitative measurement of an analyte in a gas. It includes a detection cell that provides an indication of an amount of analyte present in the gas, and a valve having a closed position, a sample gas position, and a calibration gas position. There is a calibration gas inlet orifice in communication with a gas outlet through a first flow path including the valve and the detection cell, and a sample gas inlet orifice in communication with the gas outlet through a second flow path including a scrubber, the valve, and the detection cell. Two orifices of different sizes, a sample gas orifice along the second flow path, and a bypass orifice positioned between the sample gas orifice and the gas outlet, prevent a build up of excessive pressure in the instrument when the valve is in the closed position or the calibration gas position.

Abstract: A closed, gaseous fluid analyzing system includes a gas analyzer measuring cell that operates under substantially stable conditions by controlling both the pressure and flow rate of a plurality of differing gas streams while passing through the analyzer measuring cell. The plurality of gas streams are individually extracted and segregated while passing through the system, with the measuring cell being positioned between a flow controller and pressure regulator that cooperate to control upstream and downstream pressure and flow rate fluctuations in the gas streams. Methods of maintaining both substantially constant pressure and flow rate for the gas streams flowing through the analyzer measuring cell are also set forth.

Abstract: The present invention provides a method and apparatus using a gas concentration sensor for accurately controlling an air fuel ratio in an internal combustion engine, featuring in that before the fuel-vaporized gas purged from the canister enters into the intake manifold whereat the sensor detects the gas concentration of the purged gas, the sensor is adjusted so as to adjust a zero point (or zero output level) of the sensor output. In step 100 of FIG. 7, a judgment is made as to whether the flow rate of air reaches a predetermined level. In step 110, processing for zero-point correction of the gas concentration sensor is performed. Specifically, in a state in which the purge valve 17 is closed, concentration of purge gas is measured by use of the gas concentration sensor 4, and a sensor output S1 at that time is obtained. Subsequently, the sensor output S1 is compared with a correct sensor output S0 in order to obtain a difference &Dgr;S therebetween.