# Patent

## Title
Method and device for compensating temperature gradient effects

## Abstract

In an embodiment a method for compensating a temperature gradient effect for gas concentration sensors includes variating a temperature gradient, measuring a variation of gas concentration depending on the variation of the temperature gradient, analysing a dependence of the gas concentration and the temperature gradient for setting up an error correction function and applying the error correction function to correct measured values of the gas concentration.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to German Patent Application No. 102020127455.6, filed on Oct. 19, 2020, which application is hereby incorporated herein by reference.
TECHNICAL FIELD
Special types of gas sensors, e.g., thermal conductivity type sensors can show a temperature gradient dependence. That means that the output signal of the gas sensor not only depends on the gas concentration, e.g., CO2 but also depends on the change rate (gradient) of the temperature.
BACKGROUND
In the state of the art such a behavior is not compensated. Or alternatively, a measurement unit is heated to a constant temperature so that no gradient will occur.
SUMMARY
Embodiments provide a method for compensating temperature gradient effects for gas concentration sensors comprising the following steps:

    
    
        variating a temperature gradient,
        measuring the variation of gas concentration depending on the variation of the temperature gradient,
        analysing the dependence of gas concentration and temperature gradient for setting up an error correction function,
        applying the error correction function to correct measured values of gas concentration.
    
    


In one embodiment the error correction function is a linear function.
In one embodiment the dependence of gas concentration and temperature gradient is analysed by a neuronal network.
Further embodiments provide a device for measuring gas concentrations independent of a temperature gradient comprising:

    
    
        a sensor unit measuring gas concentrations,
        an analysing unit arranged to analyse the dependence of gas concentration on the temperature gradient during calibration of the sensor unit in order to set up an error correction function and to apply the error correction function to measured values of gas concentration after calibration is completed.
    
    


In one embodiment the analysing unit comprises a neuronal network analysing the dependence of gas concentration on the temperature gradient.



BRIEF DESCRIPTION OF THE DRAWINGS
In the following the invention is exemplarily described by using figures. The invention is not limited to the described examples. The figures show:
FIG. 1 shows a temperature vs time curve and a residual error vs temperature curve of a measurement by a CO2 thermal conductivity gas sensor.
FIG. 2 shows the CO2 concentration output signal during a temperature sweep.
FIG. 3 shows similar data like shown in FIG. 2 but measured by a different sensor. The error of the output signal vs the temperature change rate is shown.
FIG. 4 shows the same data like shown in FIG. 2. The green curve shows results after applying the gradient correction.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
As mentioned in the introduction special types of gas sensors, e.g., thermal conductivity type sensors can show a temperature gradient dependence.
FIG. 1 shows such a dependence, for example. The left diagram shows the temperature sweep that is applied to an exemplary sensor. On the right diagram the influence of the temperature to a CO2 concentration output signal (or any another gas concentration output signal) is shown. It is clear that the output signal does not depend on the temperature. Instead it seems to depend on the gradient of the temperature. In embodiments a method is discussed how to compensate for this behavior.
The first step to cancel such a temperature gradient dependence is to estimate the gradient error/drift (error of the gas concentration value in dependence on the temperature gradient) and subtract it from the output signal (value of gas concentration).
FIG. 2 shows the CO2 concentration output signal during a temperature sweep. The gradient error behavior becomes clear. The blue curve shows the temperature that is changed during the experiment. The orange curve shows the gas sensor output signal (gas concentration). If the blue curve is flat the output signal is stable at −1000 ppm (green curve: There is a little temporal drift as well). Note how the sign of the gradient influences the output signal: In case of a negative gradient a negative gradient error occurs; In case of a positive gradient a positive gradient error occurs.
The same data (CO2 output signal) can be plotted vs the temperature gradient. This was done for a different sample in FIG. 3.
The data shown in FIG. 3 are similar to the data in FIG. 2 but measured by a different sensor. The error of the output signal vs the temperature change rate (temperature gradient) is shown.
In FIG. 3 the linear dependence of the gradient error becomes visible. During a calibration procedure the slope “A” of the gradient error is measured. The gas concentration output signal can then be correct by using the following error correction function:

c(CO2 corrected)=c(CO2 measured)−f. 

Herein, “c” is the concentration of CO2 and “f” is the gradient error. The gradient error can be calculated by

f=gradient(T)×A. 

Herein, “gradient(T)” is the temperature gradient.
By applying the error correction function to the measured concentration values a much more stable signal is received. The result can be seen in FIG. 4.
FIG. 4 shows the same data like shown in FIG. 2. The green curve shows results after applying the gradient correction.
It is also possible to use e.g. different or better algorithms to compensate higher order gradient effects. E.g. if the sensor depends on the change of T2. For example, a neuronal network can be used to automatically train for such kind of behaviors.
While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.

## Claims

1. A method for compensating a temperature gradient effect for gas concentration sensors, the method comprising:
variating a temperature gradient;
measuring a variation of gas concentration depending on the variation of the temperature gradient;
analysing a dependence of the gas concentration and the temperature gradient for setting up an error correction function; and
applying the error correction function to correct measured values of the gas concentration.

2. The method of claim 1, wherein the error correction function is a linear function.

3. The method of claim 1, wherein the dependence of the gas concentration and the temperature gradient is analysed by a neuronal network.

4. A device comprising:
a sensor configured to measure gas concentrations; and
an analysing unit configured to:
analyse a dependence of a gas concentration on a temperature gradient during calibration of the sensor in order to set up an error correction function; and
apply the error correction function to measured values of the gas concentration after the calibration is completed,

wherein the device is configured to measure the gas concentrations independent of the temperature gradient.

5. The device of claim 4, wherein the analysing unit comprises a neuronal network configured to analyse the dependence of the gas concentration on the temperature gradient.