Gas sensor test and calibration system

A system for testing and calibrating gas sensors with gas stored in a strip of packets. The strip may be fed into a chamber having a defined volume of air. A packet may be punctured with a mechanism to release one or more gases into the volume of air to result in an air-gas mixture. New air may be moved into the chamber to push the mixture out of the chamber to a sensor for testing and/or calibration. Then another packet may be punctured to release one or more gases into the volume of new air in the chamber for another air-gas mixture. This mixture may be moved out of the chamber, in the same manner as the previous mixture, to another sensor for testing and/or calibration. This procedure may be repeated with additional packets on the strip.

BACKGROUND

The invention relates to gas sensors, and particularly to testing and calibration of sensors.

SUMMARY

The invention is a mechanism that provides a stored amount of gas released into a defined volume of air. The gas and air mixture may be transported to a sensor for testing and/or calibration of the sensor.

DESCRIPTION

Gas sensors should be functionally tested and calibrated periodically. In some places, functional testing and some calibration may be a requirement for sensors, for example, personnel sensor badges in a facility where a potential for a presence of a harmful gas exists.

A mechanism that is capable of providing multiple gases in one run for health determination and, in some instances, for calibration of one or more sensors is desired. The present invention may provide a mixture of test gases in air, and to check and also calibrate sensors for CO, H2S, combustible gases, and other gases.

The gases of significant interest may include CO, CH4, and H2S. These three gases should not react with each other at ambient conditions. The three gases may be put together or mixed for incorporation and sealed inside of a blister pack. The pack may be made of low permeability plastics or their metalized derivatives. These blisters may be punctured open at one per run to test and calibrate a sensor module. A number of blister packs may be made and stored in the form of a strip and wound into a roll. Single blisters may be rolled off and punched or punctured open in a manner similar to a way that a cap gun pops small explosive-like and noisy caps. To assure reproducible gas pulses to the sensors, a release of the gases may be released into a fixed amount or volume of air. Then this volume of air, mixed with a determined or fixed amount of released gases, may be transported as a particular air-gas mixture to a gas sensitive area of one or more sensors.

Each single blister pack may be sealed or bonded with a cold working aluminum onto aluminum (for a good seal) or with an adhesive having a long diffusion length. A provision for transport of the gases to the sensors may also be provided.

A generation or providing of gases for checking gas sensors may be effected electrochemically, thermally, or in some other manner. An illustrative example here may include a bubble foil, blister, or similar enclosure for holding and providing the gas. Gas is not necessarily generated but may be stored in a form that makes a release of the gas easy and reproducible.

The present system may provide an alternative to gas generation. Current bump tests may be performed with a gas mixture from a tank (2.5 percent of CH4, 100 ppm CO, 40 ppm H2S, 15 percent of O2). One may provide for a similar gas mixture from a small unit. To fill 1 mL of air with 1 percent of CH4, 100 ppm CO, and 40 ppm H2S, one may need a volume of 10 uL of CH4.

An approach of the present system may include the following items. First, one may make long band of bubble foil with bubbles of about 20 uL out of aluminized polymer like Mylar, and fill them with a gas mixture without overpressure. Second, one may roll the band of bubbles onto a cylinder and pull off one bubble at a time. Third, for each bump test, one may puncture one bubble with a “hammer”, like a cap gun does when it generates a shooting sound from a roll, with chemicals. Fourth, one may let the gas mix with 2 mL of air in an enclosed volume, and pump air through the volume. Fifth, one may pump the gas mixture over the sensors.

A roll for 800 2 mL-pulses could have a width of about one inch and a diameter of about two inches, which does not seem prohibitively large for fixed sensors. Bubble foil appears to be the easiest approach for fixed sensors. This approach may be used for testing portable sensors such as those implemented in personal badges.

An alternative to gas generation may include some of the following items.FIG. 1shows a blister pack or bubble foil10. The bubble foil10may have bubbles13made out of aluminum foil11for a base and have aluminum or an aluminized polymer (e.g., Mylar)12for the bubble side. The aluminum foil11and the aluminum or aluminized polymer12may be joined and sealed by heat or ultrasound at a border14of the bubble to contain a gas15. The bubble13may be filled with a desired gas or gases15before or after the sealing of the border14. A design of the bubble or blister may be one among other approaches, packets or packages for keeping or storing gases until needed for testing and/or calibration of sensors.

As indicated inFIG. 2, the foil10may be rolled up into a coil16and installed in a mechanism or source20on a coil or spool holder17. The foil may be fed from coil16through a container21via slots18and19, respectively. The slots18and19are such so as to provide somewhat air tight integrity to container21with the foil10moving through the slots. Roller24may guide the foil10through slot18. Roller25may guide a punctured foil10out of slot19to be wound as a coil26on a wind-up spool26. Container21may have an input port22and an output port23for air to enter and for a gas air mixture to exit, respectively.

For each bump test, one bubble13may be punctured and emptied into a defined volume28of container21. There may be a plate29situated to keep foil14flat against an inside surface31of container21. A device32may have a pointed object33attached and facing plate29. Device32may be rotated about a hinge or anchor34towards the plate29. During this rotation, pointed object33may puncture a bubble13formed by foil12, or other material, by going through a hole35of plate29as the bubble passes by the hole. During this puncture of bubble13, a certain or defined amount of a particular gas15may be released into the chamber28. Upon the release of the gas, air may be pumped through port22, for instance with an air mover45, into the volume28and the certain or defined volume of air with the bubble13of gas15may be pushed out of volume28through port23to one or more gas sensors30to be tested (e.g., a health check) and/or calibrated.

Permeability and chemical resistance of the bubble13may be noted for preservation of the gas15. For instance, an all-metal enclosure should keep CH4with traces of CO and H2S inside each bubble13. A desired permeability of the material containing the bubble or blister13may about 10−15/cm2. Aluminum may be reported to be stable in conjunction with H2S. However, a question is whether the aluminum may deplete some of the H2S. Depending on layout and circumstances of the setup and testing, the answer may be no. However, if the answer is assumed to be yes, H2S might be generated separately and/or provided outside of bubble13. Or bubble13may be made with other materials. One may further note whether H2S is safe and compatible, and whether H2S is sufficiently inexpensive for use with the present system in testing and/or calibration of sensors. The answer may depend on an application of the system or source20.

A gas transport37from a gas source20to a sensor30may be noted inFIG. 3. Fixed and portable sensors may require different approaches. For fixed gas sensors, there may be transport of gases15through or from the ambient36to the sensor30by diffusion and/or convection. There may be some space available and full automatization may be desired. For portable gas sensors, transport of gases15from the ambient36to the sensor may often be effected by pumping. Space may be at a premium and human action possible or desirable for effecting transport.

A flow design for fixed gas sensors might include reproducible gas pulses by an active transport, as diffusion may appear unacceptably slow and convection be unpredictable. Heat convection for gas transport38is shown by a diagram of a setup inFIG. 4. A heating element39may be placed proximate to the output port of gas source20to heat the gas15which will rise to follow the path of the gas transport38of the ambient36. The gas15may eventually reach gas sensor30to be sensed. Heat convection appears easy but not necessarily optimal for reproducibility.

A pumping a gas pulse may result in the highest reproducibility for calibration. For example, a pump41may be included in a fixed gas sensor arrangement as shown inFIG. 5. Pump41may take air from the ambient36and push the air into a gas container42via a channel43. Container42may be for receiving gas15from a gas source20. Gas15may be pushed from container42by air from pump41through a channel44to a gas sensor30for detection. A fixed volume of air may be determined and pumped according to a volume or amount of gas released into container42. The overall system with the pump may relax space requirements for a gas source20(which could fit in a flameproof housing for safety purposes). Further, the present pump system may be economical if the pump41is inexpensive. An example of such a pump may be a Mesopump™ produced in a 10,000 unit volume (by a company like Honeywell). A number of mesopump units may be connected together for increased capacity. A mesopump arrangement may have a capacity to provide 12 mL/min at about 40 uW. It may have a size of about 10×10×2 mm and operate at a temperature up to 100° C. The pump may provide a 3 kPa pressure difference or differential.

A flow design may be provided for portable gas sensors. The design may be more complex for portables than for fixed sensors. Reproducible gas pulses may be provided by an active transport in a portable design. It may be noted that dominant gases could include combustibles, H2S, and the like. An existing pump41may be utilized for gas transport from a gas source20. Or, one may let a manual operator press a button that drives pumping (which may be difficult in that the sensor or sensors should feel the gas). An integration of a source20may need a stronger interaction with the design of a sensor system. One may instead begin with fixed sensors rather than portable sensors, and make frequent visits to them unnecessary.

Although the invention has been described with respect to at least one illustrative example, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.