Calibration device for carbon dioxide sensor

A self-calibrating carbon dioxide sensor that includes a carbon dioxide detector and a carbon dioxide gas generator. In some embodiments, the carbon dioxide gas generator includes a heating element and a carbon dioxide gas releasing solid material in thermal communication with the heating element. The carbon dioxide gas releasing solid material releases carbon dioxide when heated by the heating element. Methods of calibrating a self-calibrating carbon dioxide sensor are also disclosed.

BACKGROUND

This disclosure relates to a calibration device for a carbon dioxide sensor. In particular, this disclosure related to internal generation of a carbon dioxide reference gas to calibrate a carbon dioxide sensor.

Instruments and transducers for the measurement of carbon dioxide (CO2) concentration in air have been described in the patent literature, and also in the form of commercially available products on the market. Many of these instruments are based on the absorption spectra within the infrared wavelength area of electromagnetic radiation of the CO2molecule. Such spectra can be detected and analyzed by spectroscopic instruments according to known technology. By measuring at specific wavelengths where the absorption of CO2deviates from other constituents of air, it is possible to extract an output signal with required sensitivity and specificity.

Other instruments are based on solid electrolyte or an electrochemical cell that uses a catalytic electrode so that the carbon dioxide is either oxidized or reduced with the exchange of electrons. The flow of current due to the oxidation or reduction of the carbon dioxide is then detected as a measure of the concentration of the detected carbon dioxide. However, one problem with carbon dioxide sensors is that they lose sensitivity over time. For example, the working life of an electrochemical cell is determined by the activity of the cell's catalytic electrode that is used to detect carbon dioxide within the sensor. This activity is gradually reduced over time by contaminant gases and poisons such that the sensitivity of the sensor drifts downward.

If the instrument into which the carbon dioxide sensor is built is calibrated regularly, this downward sensitivity drift can be compensated for by adjusting the gain of the carbon dioxide sensor, and any faulty carbon dioxide sensors can be replaced immediately. However, if the instrument is in a difficult position to service, or if calibration of the carbon dioxide sensor is not otherwise freely available, it is often impossible to confirm that the carbon dioxide sensor is functioning correctly. Therefore, as the carbon dioxide sensor reaches the end of its working life, the output of the sensing cell may be insufficient to generate an alarm or other condition. As a result, a situation could arise where toxic levels of gas are present, but the carbon dioxide sensor is incapable of providing the requisite warning.

SUMMARY

A self-calibrating carbon dioxide sensor is disclosed. The sensor includes a carbon dioxide detector and a carbon dioxide gas generator. In one illustrative embodiment, the carbon dioxide gas generator includes a heating element and a carbon dioxide gas releasing solid material adjacent to the heating element. The carbon dioxide gas releasing solid material releases carbon dioxide when heated by the heating element.

In another embodiment, a self-calibrating carbon dioxide sensor includes a sensor housing, a carbon dioxide detector disposed within the sensor housing, and a carbon dioxide gas generator disposed within the senor housing. The carbon dioxide gas generator includes a heating element and a carbon dioxide gas releasing solid material adjacent to the heating element. The carbon dioxide gas releasing solid material releases a known amount of carbon dioxide reference gas when heated by the heating element.

An illustrative method of calibrating a carbon dioxide sensor is also disclosed. The illustrative method includes providing a carbon dioxide detector and a carbon dioxide gas generator adjacent to the carbon dioxide detector. The carbon dioxide gas generator includes a heating element and a carbon dioxide gas releasing solid material adjacent to the heating element. Then the method includes, heating the carbon dioxide gas releasing solid material with the heating element to release a known amount of carbon dioxide gas and detecting the known amount of carbon dioxide gas with the carbon dioxide detector to provide an output value from the carbon dioxide detector. The output value is then calibrated based on the known amount of carbon dioxide gas released.

The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The Figures, and Detailed Description which follow more particularly exemplify these embodiments.

DETAILED DESCRIPTION

This disclosure relates to a calibration device for a carbon dioxide sensor. In particular, this disclosure related to internal generation of a carbon dioxide reference gas to calibrate a carbon dioxide sensor. In many embodiments, a self-calibrating carbon dioxide sensor includes a sensor housing, a carbon dioxide detector disposed within the sensor housing, and a carbon dioxide gas generator disposed within the senor housing. The carbon dioxide gas generator can include a heating element and a carbon dioxide gas releasing solid material adjacent to the heating element. The carbon dioxide gas releasing solid material releases a known amount of carbon dioxide reference gas when heated by the heating element. The sensor can be calibrated based on the known amount of carbon dioxide released by the carbon dioxide gas generator.

As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to “a heating element” includes two or more heating elements. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.

As shown inFIG. 1, a self-calibrating carbon dioxide sensor10has a sensor housing12that can substantially surround a carbon dioxide detector22and a carbon dioxide gas generator34. In some embodiments, the sensor housing12can be formed of a metal, but this is not required. In one embodiment, the sensor housing12is formed of nickel plated steel.

In some embodiments, the carbon dioxide detector22is supported by a support plate18disposed within the sensor housing12. In some embodiments, the carbon dioxide detector22is in the form of an electrochemical cell.

The illustrated carbon dioxide detector22includes lower and upper cell plates24and26, a solid electrolyte membrane28, and lower and upper catalyst electrodes30and32. The lower and upper cell plates24and26, may for example, be hydrophobic Teflon™ disks, but this is not required. In this illustrated embodiment, the lower cell plate24is sandwiched between the support plate18and lower catalyst electrode30, the lower catalyst electrode30is sandwiched between the solid electrolyte membrane28and the lower cell plate24, and the upper catalyst electrode32is sandwiched between the solid electrolyte membrane28and the upper cell plate26. The catalyst electrode30and32, may for example, include an element from the group Au, Pt, Pd, Ru, Rh, Ir, Os, Ag, etc., or an alloy or mixture of the elements from this group, or porous elements of the group mixed with carbon black, or porous elements of the group mixed with carbon black and Nafion particles. The solid electrolyte membrane28may be, for example, Nafion or Nafion composite like Nafion/7SiO2-2P2O5—ZrO2, and Nafion/ZrP particles or the Sandia Polymer Electrolyte Alternative (SPEA) for higher temperature applications. Illustrative electrochemical carbon dioxide gas detectors22may be of the type shown in one or more of U.S. Pat. Nos. 4,851,088 and 5,322,612 and U.S. Patent Application Publication No. 2004/0158410, all of which are incorporated by reference herein.

In some embodiments, the carbon dioxide detector22is in the form of an infrared (IR) carbon dioxide sensor. Illustrative IR carbon dioxide sensors are described in U.S. Pat. No. 6,456,943 and U.S. Patent Application Publication Nos. 2002/0157447, 2004/0050142, and 2004/0206906, all of which are incorporated by reference herein.

A carbon dioxide gas generator34or reference gas generator34generates a carbon dioxide reference gas that is provided to the carbon dioxide detector22so that the carbon dioxide detector can be self-calibrated. In many embodiments, the carbon dioxide gas generator34generates a known amount of carbon dioxide within the sensor housing12and the known amount of carbon dioxide is detected by the carbon dioxide detector22.

In some embodiments, the carbon dioxide gas generator34includes a carbon dioxide gas generating chamber36(seeFIG. 2) and a gas diffusion control plate38. In some embodiments, the gas sensor10also includes an active charcoal filter40. The gas diffusion control plate38can separate the carbon dioxide gas generating chamber36and the optional active charcoal filter40from the carbon dioxide detector22and abuts the upper cell plate26on an electrochemical carbon dioxide detector22or abuts an IR sensing chamber of an IR carbon dioxide detector (not shown).

As shown inFIG. 2, the carbon dioxide gas generating chamber36houses a heating element42and a carbon dioxide gas releasing solid material44that is in proximity to, adjacent to, in contact with, or otherwise thermally coupled to, the heating element42. The carbon dioxide gas releasing solid material44, when heated, produces the carbon dioxide reference gas.

In many embodiments, the carbon dioxide gas releasing solid material44includes a Group 2 carbonate material that, when the heating element42is energized, is heated to a known temperature and consequently thermally decomposes the carbon dioxide gas releasing solid material to produces a known amount of carbon dioxide. In some embodiments, the carbon dioxide gas releasing solid material is heated to a temperature in a range from 200 to 1500° C., or from 200 to 1000° C., or from 250 to 500° C., or from 500 to 1500° C., or from 750 to 1250° C. An overpressure of carbon dioxide reference gas then can flow through holes48bin the gas diffusion control plate38directly onto the carbon dioxide detector22.

In many embodiments, the Group 2 carbonate material includes beryllium carbonate (BeCO3), magnesium carbonate (MgCO3), calcium carbonate (CaCO3), strontium carbonate (SrCO3), barium carbonate (BaCO3), or radium carbonate (RaCO3). In some embodiments, the Group 2 carbonate material includes magnesium carbonate (MgCO3) or calcium carbonate (CaCO3). In other embodiments, the carbon dioxide gas releasing solid material is not a Group 2 carbonate. In these embodiments, the carbon dioxide gas releasing solid material may include copper carbonate (CuCO3).

When the self-calibration gas sensor10is to be calibrated, the heater42is energized to heat the material44to a predetermined temperature and for a predetermined time that causes the material44to release an overpressure of carbon dioxide reference gas which is supplied to the gas detector22. The gas detector22senses the reference gas and generates a reference signal between the lower and upper catalyst electrodes30and32. This signal is used to perform self-calibration of the sensor10. After such self-calibration, the heater42is de-energized so that the overpressure of the reference gas falls to a negligible level.

Self-calibration of the self-calibration gas sensor10can be intermittently repeated as desired. In many embodiments, the carbon dioxide gas releasing solid material can be reheated with the heating element to release a subsequent known amount of carbon dioxide gas. These subsequent known amounts of carbon dioxide gas can be detected and used to recalibrate the output value of the carbon dioxide detector. In some embodiments, a time interval of at least a day, a week, a month, or a year occurs between the heating step and reheating step.

As shown inFIG. 1, the sensor housing12forms a continuous housing that houses the gas detector22and the reference gas generator34. Accordingly, in this construction of the present invention, the gas detector22and the reference gas generator34are not housed in separate and separated housings.

As shown inFIG. 3, a controller50provides an output52based on the carbon dioxide detected by the gas detector22. The controller50may also control the reference gas generator34to calibrate the gas detector22. The output52may be coupled to various devices. For example, the output52may be coupled to an alarm indicator to produce a warning when the level of the detected carbon dioxide gas exceeds a predetermined limit, or the output52may be coupled to an apparatus such as a ventilator to control the effects of the gas being detected. The self-calibration could be pre-programmed to operate on a schedule such as, for example, twice a year or once a year. Calibration could also be initiated through pushing an external button. When self-calibration is in process, the controller50can provide an alarm/warning that self-calibration is being performed, and that the controller50may be out of function momentarily.

In the illustrated embodiment, the lower and upper catalyst electrodes30and32are coupled between the terminals of a voltage source through a resistor54. The junction between the resistor54and the gas detector22is coupled to an amplifier56having a gain controlling element58in a feedback circuit around the amplifier56. The output of the amplifier56is coupled to a processor60that provides the output52. The processor60may also control a switch62to selectively connect a source S to the heater42so as to energize the reference gas generator34.

During normal operation, the processor60provides the output52based on the output of the amplifier56and controls the switch62so that the switch62is open. Thus, the reference gas generator34is de-energized and the output52indicates the level of ambient gas normally being detected by the gas detector22. This ambient gas normally being detected by the gas detector22enters the gas sensor10through one or more suitable holes (not shown) in the sensor housing12, flows through the optional active charcoal filter40, then flows through one or more holes48aof the gas diffusion control plate38into the gas detector22.

During self-calibration, the processor60controls the switch62so that the switch62is closed. Thus, the reference gas generator34is energized to produce the carbon dioxide reference gas and to provide the reference gas to the gas detector22as described above. The processor60receives the output of the amplifier56and may change one or more calibration parameters. Once the calibration procedure is complete, and during subsequent normal gas sensing operation, the processor60may use the one or more calibration parameters to provide a calibrated output52that compensates for any change to sensitivity or other changes to the gas sensor. Accordingly, the self-calibration gas sensor10is calibrated.

The controller50may intermittently repeat the above described calibration as many times as necessary or desired. The time periods between such repeated calibrations may be periodic or aperiodic and may be of any length as desired.

In many embodiments, the circuit50can be mounted as a chip or otherwise on a board or other support within the sensor housing12. The output52may then be run to the exterior of the sensor housing12.

FIG. 1illustrates an embodiment where the sensor housing12forms a continuous housing that houses the gas detector22and the reference gas generator34. However, the gas detector22and the reference gas generator34may instead be housed in separate and separated housings, as desired.