Abstract:
A portable, calibratible gas detector includes a multi-position gas inflow limiting orifice. When this orifice is in a calibrating position, a source of calibrating gas can be activated to provide a quantity of gas that diffuses into ambient atmosphere flowing through the orifice. The calibration gas can then be sensed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a Continuation-in-Part of U.S. patent application Ser. No. 10/908,737 filed May 24, 2005, now U.S. Pat. No. 7,174,766, entitled “Calibration Device for Carbon Dioxide Sensor.” 

   FIELD OF THE INVENTION 
   The invention pertains to gas detectors. More particularly, the invention pertains to gas detectors which include a self-calibration function. 
   BACKGROUND OF THE INVENTION 
   Gas sensors are known and have been used to sense various gases such as hydrogen, carbon monoxide, carbon dioxide and the like. While known gas sensors are useful and effective for their intended purpose, it has been recognized that at times they can lose sensitivity over time. For example, in connection with electrochemical-type gas sensors, such sensors incorporate a catalytic electrode which is used in connection with detecting gas within the sensor. The activity of that electrode tends to be reduced gradually over time due to contamination and poisoning of the structure. Consequently, the sensitivity of the associated sensor tends to decrease or drift downward. As a result, it is desirable to be able to calibrate such sensors from time to time. 
   Structures which can generate calibration gases are of interest and have been developed. One such structure is disclosed in previously filed U.S. patent application Ser. No. 10/856,363 filed May 28, 2004, now U.S. Pat. No. 7,037,368, for Calibration Device For Gas Sensors. That application has been assigned to the Assignee hereof and is hereby incorporated by reference. Another such structure has been disclosed in U.S. patent application Ser. No. 10/908,737 previously filed on May 24, 2005, now U.S. Pat. No. 7,174,766, entitled “Calibration Device For Carbon Dioxide Sensor”. That application has been assigned to the Assignee hereof and is hereby incorporated by reference. 
   Beyond an apparatus and a method for generating calibration gas, there continues to be a need for gas sensors which can readily and conveniently calibrated. Preferably such units could be substantially self-contained. It would also be preferable if such units could incorporate a self-calibrating capability without significantly adding to the manufacturing costs, size or weight of such units, especially portable gas detectors. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic block diagram of a self-calibrating gas detector in accordance with the invention; 
       FIG. 2  is a front elevational view of a self-contained, portable, gas detector in accordance with the invention; and 
       FIG. 3  is a side elevational view of the detector of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   While embodiments of this invention can take many different forms, specific embodiments thereof are shown in the drawings and will be described herein in detail with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention, as well as the best mode of practicing same, and is not intended to limit the invention to the specific embodiment illustrated. 
   A self-calibrating gas detector which embodies the invention incorporates a flow path with a gas inlet end and an orifice associated therewith. A generator of calibration gas as well as a gas sensor are coupled to the path. Finally, if desired a pump can also be coupled to the path. 
   In accordance with the invention, to carry out a calibration process, the orifice is moved or switched to a state so that it restricts or constricts the inlet of the flow path. This restricts the flow of ambient atmosphere to the sensor. 
   The pump and source of calibration gas can both be activated. The pump, when activated, causes ambient atmosphere to pass a sensing region of the gas sensor at a reduced flow rate relative to normal operational flow past the gas sensor. The activated source of calibration gas generates a known amount of the calibration gas. 
   Those of skill in the art will understand that additional sensors that measure the ambient temperature, humidity, and pressure can be incorporated to further enhance the calibration process since these variables can affect the perceived concentration of a gas. A filter or selectively permeable membrane can also be provided to cover the inlet of the flow path and prevent an inflow of harmful particulates and gasses that might damage the sensor. 
   The calibration gas diffuses into the inflowing ambient atmosphere traversing the path. This mixture in turn flows into or through the sensing region of the gas sensor. The sensor responds thereto and generates an output signal corresponding thereto which can be used for calibration and making a determination as to the performance characteristics of the sensor. 
   In accordance with the invention, a reduced flow rate in a range of 1 to 100 cc/min. is particularly advantageous as it reduces the amount of calibration gas required for the process. This can in turn reduce the size and power consumption of the gas generator. 
   The calibration gas can be generated by any one of a variety of known chemical or electrochemical reactions. For example, a select chemical can be heated until it decomposes. Other forms of gas generation come within the spirit and scope of the invention. Calibration gases can include hydrogen, hydrogen sulfide, carbon dioxide, methane, carbon monoxide and other gases to which the respective sensor will respond. 
   A detector which embodies the invention is also self-checking. For example, a determination can be made whether the flow path through which the ambient atmosphere is to be drawn is clear or whether it is exhibiting the symptoms of blockage. Further, it is also possible to determine if the sampling pump is functioning as expected. 
   In yet another aspect of the invention, the flow path can be restricted by a mechanically movable orifice. The orifice could be moved into a calibration position partly closing the inlet which significantly reduces the gas flow through the flow or sampling path. The orifice can be moved into a normal operational position once the calibration process has been concluded. 
   In another aspect of the invention, the sample pump could be implemented as a peristaltic, diaphragm pump or alternately an electronic pump all without limitation. 
     FIG. 1  illustrates a gas detector  10  in accordance with the invention. The detector  10  incorporates a flow path  12  with an inlet end  14  and an outlet end  16 . In normal operation ambient atmosphere can flow from the inlet end  14  of the flow path  12  through to the outlet end  16  in a direction  20 . 
   The detector  10  can incorporate an orifice  24  which can exhibit at least two different states. In one state the orifice  24  restricts inflowing ambient atmosphere into the flow path  12 . In this state, illustrated in  FIG. 1 , ambient atmosphere can be expected to pass through the flow path  12  at a reduced flow rate relative to normal operation. When not carrying out a calibration function, the orifice  24  can be mechanically slid or rotated from the inlet  14 . 
   Alternately, the orifice  24  can electrically assume a non-constricting state. The non-constricting state can be achieved by means of an electrically actuated transducer which will physically move the orifice  24 . The orifice  24  could also be switched electronically from a constricting state to a non-constricting state as would be understood by those of skill in the art. 
   Detector  10  also incorporates a gas generator  26  and gas sensor  30 . The gas sensor  30  can be implemented with any desired technology. It will be understood that the details of the sensor  30  are not limitations of the present invention. 
   The gas generator  26  can be electrically actuated so as to generate a predetermined quantity of a calibration gas, indicated generally at  32 , in the flow path  12 . Calibration gas  32  diffuses into the stream of ambient atmosphere flowing in the path  12 . A pump  34  can be coupled to outlet  16  to produce a flow of ambient atmosphere and calibration gas  32  through the flow path  12 . 
   Control circuits  40  can be provided to provide electrical signals to actuate the generator  26 , receive inputs from the sensor  30  and to actuate pump  34 . The detector  10  can be carried in a housing  42  which can also contain a power supply, such as one or more batteries as would be understood by those of skill in the art. 
     FIG. 2  is a front elevational view of a detector  10 ′ which embodies the present invention. Elements of the detector  10 ′ of  FIG. 2  which correspond to elements of the detector  10  of  FIG. 1  have been assigned the same identification numeral. As illustrated in  FIG. 2 , in one embodiment of the invention, a movable plate  24   a  can be provided which has an orifice  24   b,  best seen in  FIG. 3 . 
   The plate  24   a  is movable in first and second directions, generally indicated at  44  relative to flow path  12 . In a calibrating or restricting position the plate  24   a  partly closes flow path  12 . Plate  24   a  permits an inflow of ambient atmosphere only through the orifice  24   b.  When in this state, the control electronics  40  can actuate calibration gas generator  26  as well as pump  34  which in turn provides a mixture of ambient atmosphere, drawn through opening  24   b,  and calibration gas  32  from generator  26 . That mixture is presented to gas sensor  30 , via path  12 . Calibration gas  32  can diffuse into or pass by sensor  30  and be detected thereby. 
   Outputs from sensor  30  which are coupled to a control electronics  40  provide an electrical signal indicative of the response of the sensor  30  to the calibration gas  32 . The electronics  40  can carry out either an automatic or a semi-automatic calibration of the sensor  32  in response to the signals received therefrom. 
   The electronics  40  can actuate display  46  to provide a concentration of the sensed calibration gas if desired. Alternatively, the display  46  can provide an indicium of sensed ambient gas, for example, parts per million, in normal operation where the plate  24   b  is located in a non-constricting state as illustrated in  FIGS. 2 and 3 . Power supply  48  can be implemented with rechargeable batteries or replaceable batteries as would be understood by those of skill in the art. 
   In summary, use of the plate  24   a  with opening  24   b  significantly reduces the gas flow through the flow or sampling path  12 . This in turn reduces the amount of calibration gas which is needed to achieve a specific concentration. This in turn can reduce the size and power requirements of gas generator  26  as well as power supply  48 . 
   From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the spirit and scope of the invention. It is to be understood that no limitation with respect to the specific apparatus illustrated herein is intended or should be inferred. It is, of course, intended to cover by the appended claims all such modifications as fall within the scope of the claims.