Cleaning Method for a Sensor in a Respiratory Gas Analysis Device

Abstract

In a method for operating a respiratory gas analysis device with at least one gas sensor, once a respiratory gas analysis has been carried out, the gas sensor is heated to a temperature that lies above a predefinable temperature threshold value.

Claims

1. A method for operating a respiratory gas analysis device having at least one gas sensor, comprising: after carrying out a respiratory gas analysis, heating the gas sensor to a temperature above a specifiable temperature threshold value.

2. The method as claimed in claim 1, wherein the heating is performed over a specifiable period.

3. The method as claimed in claim 2, wherein the specifiable period is specified depending on a gas concentration determined by the gas sensor during the respiratory gas analysis.

4. The method as claimed in claim 1, wherein the heating is carried out until at least one termination criteria has been met.

5. The method as claimed in claim 4, wherein the at least one termination criteria includes a group of multiple termination criteria has been met.

6. The method as claimed in claim 4, wherein one termination criteria of the at least one termination criteria is a difference between a current signal of the gas sensor and a beginning signal of the gas sensor at the beginning of the respiratory gas analysis, or an absolute value of the difference, falling below a specifiable difference threshold value.

7. The method as claimed in claim 4, wherein one termination criteria of the at least one termination criteria is a quotient of (i) a difference between a current signal of the gas sensor and a beginning signal of the gas sensor at the beginning of the respiratory gas analysis and of (ii) a difference between a signal of the gas sensor at the end of the respiratory gas analysis and the signal of the gas sensor at the beginning of the respiratory gas analysis falling below a specifiable quotient threshold value.

8. The method as claimed in claim 4, wherein one termination criteria of the at least one termination criteria is an absolute value of a gradient of a signal profile of the gas sensor falling below a specifiable gradient threshold value.

9. The method as claimed in claim 4, wherein one termination criteria of the at least one termination criteria is an absolute value of a repeated derivative of a signal profile of the gas sensor falling below a specifiable derivative threshold value.

10. The method as claimed in claim 4, further comprising checking as to whether the at least one termination criterion have been met at time intervals that are each chosen depending on a last determined value of one or more of the at least one termination criteria.

11. The method as claimed in claim 4, wherein the heating is suppressed if the at least one termination criteria has already been met at the end of the respiratory gas analysis.

12. A respiratory gas analysis device comprising: at least one gas sensor, wherein, after carrying out a respiratory gas analysis, the respiratory gas analysis device is configured to heat the at least one gas sensor to a temperature above a specifiable temperature threshold value.

13. The method as claimed in claim 4, wherein the at least one termination criteria is one termination criterion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Exemplary embodiments of the invention are shown in the drawings and will be explained in more detail in the description that follows.

[0019] FIG. 1 shows a flow diagram of an exemplary embodiment of the method according to the invention.

[0020] FIG. 2 shows a signal profile of a gas sensor against time in an exemplary embodiment of the method according to the invention.

[0021] FIG. 3 shows a flow diagram of another exemplary embodiment of the method according to the invention.

EXEMPLARY EMBODIMENTS OF THE INVENTION

[0022] A respiratory gas analysis device has a gas sensor that, in the exemplary embodiments of the invention that follow, can be embodied for example as a nitrogen dioxide sensor. If the intention therefore is to determine the nitrogen monoxide content of the respiratory air, a converter for example integrated into a mouthpiece is located upstream of said gas sensor. In a first exemplary embodiment of the invention, the respiratory gas analysis device is operated by means of a method shown in FIG. 1. After the start 10 of the method, a pre-regeneration 11 is optionally carried out depending on the set operating mode of the respiratory gas analysis device. For this purpose, the gas sensor is heated and purged by means of a purge gas. A respiratory gas analysis 12 is then carried out by introducing respiratory gas into the respiratory gas analysis device and exposing the gas sensor to the gas concentration of nitrogen dioxide to be determined. The gas molecules accumulate on the sensitive layer of the gas sensor and thus change its potential. The sought concentration of nitrogen monoxide or nitrogen dioxide in the respiratory gas sample can be determined on the basis on the change in potential. Once the respiratory gas analysis 12 has ended and thus once the gas exposure has ceased, gas molecules remain on the sensitive layer and therefore contaminate the gas sensor. As a consequence of this, said gas sensor is in a different state after a respiratory gas analysis has been carried out than before the measurement. The sensitivity of the gas sensor drops, this meaning that its measurement accuracy and the number of measurements still possible are also reduced. A period for subsequent heating 14 of the gas sensor is then specified 13 depending on the nitrogen monoxide or nitrogen dioxide concentration determined by means of the sensor during the respiratory gas analysis. If the determined concentration is, for example, greater than 200 ppb, then there is a high need for regeneration and a period of 120 seconds is specified. In contrast, if the concentration is less than 15 ppb, then a period of 10 seconds is sufficient. The subsequent heating 14 represents a post-regeneration of the gas sensor. This involves heating the gas sensor to a temperature of 100° C. and passing a flow of purge gas over it. Once the specified period has elapsed, the post-regeneration measures are ended and the method is ended 15.

[0023] FIG. 2 shows the profile of the signal x over time t, during the pre-regeneration 11, the respiratory gas analysis 12 and the heating 14 of the post-regeneration. “Heating” hereinafter is understood to mean both the heating up to a specified temperature and the holding at this temperature so as to thereby achieve a bake-out of the gas sensor. The signal x is measured here as electric potential. A stable starting level of the signal x is reached during the pre-regeneration 11. At time t.sub.1, the signal x increases sharply with the beginning of the respiratory gas analysis 12. At the end of the respiratory gas analysis 12, said signal reaches its maximum at time t.sub.2. Subsequently, it then drops again during the heating 14 of the gas sensor.

[0024] FIG. 3 shows a second exemplary embodiment of the method for operating the respiratory gas analysis device. The start 10 of the method, the possible pre-regeneration 11 and the respiratory gas analysis 12 proceed in the same way as in the first exemplary embodiment. However, the respiratory gas analysis 12 is followed by the determination 20 of one or more features of termination criteria. In a subsequent check 21, it is checked whether all termination criteria assigned to the features have been met. If this is the case, then the heating 14 is suppressed and the method is ended 15 immediately. Otherwise, instead of the specification 13 of a period for the heating, a period until the next check as to whether the termination criteria have been met is selected 22. The selection 22 is made depending on the values of the features determined in step 20. A selected period may be 30 seconds long, for example. In this time, the heating 14 is performed in the same way as in the first exemplary embodiment. Once the specified period has elapsed, a new determination 20 of the features and a new check 21 are performed. This is continued until all termination criteria have been met and the method is ended 15 without any further heating 14. This produces the same profile of the signal x as shown in FIG. 2 for the first exemplary embodiment.

[0025] A first termination criterion is that a difference Δx goes below a difference threshold value. This is determined according to Formula 1:

[00001]$\Delta \text{x=x}\left({\text{t}}_{\text{a}}\right)\text{-x}\left({\text{t}}_{\text{1}}\right)$

Here, x (t.sub.a) is the signal x at the current time t.sub.a when carrying out step 20 and x(t.sub.1) is the signal x at time t.sub.1 at the beginning of the respiratory gas analysis 12.

[0026] A second termination criterion is that a quotient qx goes below a quotient threshold value. The quotient qx is calculated according to Formula 2:

[00002]$\text{qx =}\frac{\text{x}\left({\text{t}}_{\text{a}}\right)-\text{x}\left({\text{t}}_{\text{1}}\right)}{\text{x}\left({\text{t}}_{\text{2}}\right)-\text{x}\left({\text{t}}_{\text{1}}\right)}=\frac{\Delta \text{x}}{\text{x}\left({\text{t}}_{\text{2}}\right)-\text{x}\left({\text{t}}_{\text{1}}\right)}$

Δx, x (t.sub.a) and x(t.sub.1) here have the same definition as in Formula 1. x(t.sub.2) is the signal at time t.sub.2 at the end of the respiratory gas analysis 12.

[0027] A third termination criterion is that an absolute value of a gradient dx/dt of the profile of the signal x over time t goes below a gradient threshold value. As shown in FIG. 2, the absolute value of the negative gradient dx/dt decreases toward the end of the heating 14.

[0028] A fourth termination criterion is that an absolute value of a second derivative d.sup.2x/dt.sup.2 of the signal x with respect to time t goes below a derivative threshold value.

[0029] All features mentioned above that are used to check the termination criteria are low-pass filtered in the present exemplary embodiment so as to thereby filter out high-frequency interference and signal exchanges.

[0030] In a third exemplary embodiment of the method, the second exemplary embodiment is modified such that, during the heating 14, a sharp temperature increase of the gas sensor is carried out before a new determination of the features 20. For this purpose, the temperature in the present case is increased from 100° C. to 130° C. If at least one of the features determined in step 20 reacts with a significant change of at least several percent, then this is taken as an indication that the sensor has not yet sufficiently regenerated and the method should not yet be ended 15 irrespective of the meeting of the termination criteria after the check 21.

[0031] In variants of the second exemplary embodiment and of the third exemplary embodiment, the sequence of determining 20 the features, checking 22 and heating 14 is performed continuously in such a rapid sequence that the selection 22 of a defined period become superfluous.

[0032] In all exemplary embodiments of the method, the respiratory gas analysis device is unavailable for further measurements during the heating 14 of the gas sensor.