EP1299715A1

EP1299715A1 - Device and method for determining the moisture in gases

Info

Publication number: EP1299715A1
Application number: EP01960463A
Authority: EP
Inventors: Martin Rombach
Original assignee: Testo SE and Co KGaA
Current assignee: Testo SE and Co KGaA
Priority date: 2000-07-11
Filing date: 2001-07-04
Publication date: 2003-04-09
Also published as: US20040012912A1; US7013700B2; CN1441900A; BR0112468A; DE10033620A1; CN1225651C; JP4298288B2; WO2002004933A1; JP2004503746A

Abstract

The invention relates to a measuring device for measuring the moisture in gases, comprising the following features: a moisture sensor (10) and an evaluation unit (20) connected thereto to provide a measuring value representing a relative moisture in a gas, and a heating device (30) which is thermally coupled to the moisture sensor (10). Said heating device (30) is adapted to keep the moisture sensor (10) at an operating temperature that is higher than the ambient temperature of the moisture sensor by an at least approximately constant difference in temperature.

Description

description

Device and method for determining the moisture in gases

The present invention relates to a measuring device according to the features of the preamble of claim 1 for determining the moisture in gases.

When measuring moisture in gases, a distinction is made between measuring absolute humidity and relative humidity. The absolute humidity indicates the volume of evaporated liquid per gas volume unit (usually in g / m ³ ). The relative humidity results from the quotient of the absolute humidity and the saturation humidity, the latter indicating the maximum possible volume of evaporated liquid per unit gas volume. The saturation humidity depends on the temperature.

For moisture measurement in gases, it is known to use capacitive moisture sensors which contain a capacitor, the capacitance value of which changes as a function of the humidity of the gas surrounding the sensor. Problems can occur with such sensors, particularly at high humidities.

On the one hand, there is a risk that in high humidity

Moisture precipitates on the sensor when the temperature of the moisture sensor is less than the dew point temperature of the surrounding gas. On the one hand this leads to a falsification of the measured value, and on the other hand it can lead to damage to the humidity sensor if in the vicinity of the

Sensor chemical components are present, which lead to acid or lye formation on the sensor in the presence of water. On the other hand, with conventional capacitive humidity sensors, the measured values drift if they are used for a longer period in environments with high humidity, which leads to falsification of the measurement results.

To avoid these problems, it is known from German Patent 28 51 686 C2 to heat the moisture sensor in order to keep it at a temperature above the temperature of the gas. To determine the relative humidity, in addition to a measured value supplied by the sensor, the ambient temperature and the temperature of the sensor are required, for which additional measurements and measuring devices are required. In addition, a complex control circuit is required in this known device.

Furthermore, it is known from WO 97/02468 to keep a moisture sensor at a constant temperature by heating, which is above the temperature of the gas. The known device is used to determine the absolute humidity in a gas, for which the absolute temperature of the gas is required in addition to a measured value supplied by a humidity sensor. An additional measurement and measuring device is required for this. In addition, the working range is limited by the constant working temperature of the sensor. Moisture measurement is only possible without the risk of moisture forming on the sensor if the temperature of the gas is below the working temperature of the sensor. In order to be able to determine the relative humidity of the gas with this measuring device known from WO 97/02468, a computational correction of the measured value obtained for the absolute humidity is necessary. The aim of the present invention is to provide a humidity sensor for determining the relative moisture in gases available ^• which can be used independent of the gas temperature, and are necessary in the no additional measurements for determining the moisture content.

This goal is achieved by a measuring device according to the features of claim 1.

Thereafter, in addition to a humidity sensor and an evaluation unit connected to the humidity sensor, the measuring device has a heating device thermally coupled to the humidity sensor, which is designed to keep the humidity sensor at an operating temperature that is at least approximately constant differential temperature above an ambient temperature of the humidity sensor.

In the measuring device according to the invention, the condensation of liquid on the moisture sensor is reliably prevented in that the temperature of the sensor is kept above the ambient temperature independently of the ambient temperature. A conventional capacitive humidity sensor for determining the relative humidity in a gas can be used as the humidity sensor in the measuring device according to the invention. These sensors have a first and a second electrode with an intermediate moisture-sensitive dielectric, the dielectric constant of which changes as a function of the moisture. In the case of such moisture sensors, suitable evaluation circuits can be used to determine the moisture in the vicinity of the sensor via the capacitance of the capacitor. Due to the increased temperature at the moisture sensor caused by the heating, the measured value determined by the moisture sensor shows a deviation from the actual one Moisture in the gas, the greater the difference in temperature, the greater this deviation. This deviation of the determined value from the actual value can, however, already be taken into account during the adjustment, or the factory adjustment, of the moisture sensor, so that during the use of the measuring device, in addition to the initial value of the moisture sensor, no further measured variables for determining the relative humidity in the gas are required.

Advantageous embodiments of the invention are the subject of the dependent claims.

According to a first embodiment of the invention, it is provided that the heating device for heating the moisture sensor has an electrical heating element, to which a constant electrical power is supplied. The heating element is in particular a heating resistor which is connected to a constant voltage source or to a constant current source. The power converted into heat in the resistor then results as the quotient of the square of the applied voltage and the resistance value, or the product of the square of the current flowing into the heating resistor and the resistance value. Such a heating device, in which an electrical power which is constant over time is converted into heat, makes it possible to keep the moisture sensor thermally coupled to the heating element at a temperature which is above the ambient temperature by an approximately constant differential temperature.

The heated humidity sensor, which is preferably a conventional capacitive humidity sensor, provides a measured value which is dependent on the relative humidity at the humidity sensor, or in its immediate vicinity. The relative humidity in a gas is temperature-dependent, so that the heating of the humidity sensor to a temperature above the gas temperature leads to a deviation between the relative humidity determined at the humidity sensor and the relative humidity prevailing in the other areas of the gas at a lower temperature , In one embodiment of the invention, a correction unit is provided in the evaluation unit, which provides a corrected measured value of the relative humidity from a first measured value determined on the basis of the capacitance of the moisture sensor, this corrected measured value being proportional to the relative humidity in the gas or the relative humidity in corresponds to the gas.

The present invention further relates to a method for determining the relative humidity in a gas by means of a measuring device which has a humidity sensor and an evaluation unit connected to it, and a heating device thermally coupled to the humidity sensor. In the method according to the invention, the humidity sensor is kept at an operating temperature by means of the heating device, which is at least approximately a differential temperature above an ambient temperature of the humidity sensor.

In the method according to the invention, an electrical heating device is used in particular, in which a time-constant electrical power is converted into heat and delivered to the moisture sensor.

According to one embodiment of the method according to the invention, the following method steps are provided for calibrating the measuring device. In a first method step, the moisture sensor with the heating device thermally coupled to it is introduced into a gas at a known first relative humidity and a known temperature, a first adjustment value for the relative humidity being determined by means of the moisture measuring device. The known first relative humidity, which corresponds to the actual humidity of the gas to be measured, represents a first corrected measured value for the first adjustment value supplied by the humidity sensor. In a next method step, the humidity sensor with the heating device becomes a gas with a known second one Relative humidity and the known first temperature, wherein a second adjustment value for the relative humidity is determined by means of the humidity sensor. The known second relative humidity forms a second corrected measured value for the second adjustment value supplied by the humidity sensor. On the basis of the known first and second relative humidity and on the basis of the first and second adjustment value provided by the humidity sensor, parameters are subsequently determined which enable the measurement values supplied by the humidity sensor to be converted into the actual values of the relative humidity.

The present invention is explained in more detail below on the basis of exemplary embodiments in figures. The figures show:

1 block diagram of a measuring device according to the invention according to a first embodiment;

Fig. 2 plan view of a heating element and a moisture sensor, which are glued together; 3 top view of a heating element and a moisture sensor which are in direct contact;

4 shows a front view of an arrangement of heating element and moisture sensor which are arranged next to one another;

5 shows a block diagram of an embodiment of a measuring device according to the invention with a detailed representation of the humidity sensor and the heating device as well as the evaluation unit connected to the humidity sensor;

Fig. 6 graphical representation of the output signal of a moisture sensor and the actual moisture in the gas to be measured.

Unless otherwise stated, the same reference symbols in the figures denote the same parts with the same meaning.

Fig. 1 shows a block diagram of an inventive

Measuring device for determining the relative humidity in gases. The measuring device has a humidity sensor 10 and an evaluation unit 20 connected to the humidity sensor 10 for providing a measurement value dependent on a relative humidity in a gas. The evaluation unit 20 has in particular a display, not shown in FIG. 1, in order to output a determined measured value to a user. To heat the moisture sensor 10, a heating device 30 is provided in the measuring device according to the invention, wherein a heating element 32 of the heating device is thermally coupled to the moisture sensor 10. The heating device 30 is designed in such a way that the moisture sensor 10 is kept at an operating temperature which is constant by a th differential temperature is kept above an ambient temperature of the gas to be measured.

This heating means that even at high relative humidities, no liquid condenses on the moisture sensor, which could lead to falsifications of the measurement result and / or damage to the moisture sensor. In addition, the heating prevents the relative humidity in the area of the sensor from reaching high values at which a drift of the measurement signal could occur.

In order to keep the moisture sensor 10 at least approximately independent of the ambient temperature of the gas by the value of the differential temperature above the ambient temperature, the heating device 30 according to FIG. 1 is designed as an electrical heating device in which an electrical power which is constant over time is converted into heat becomes. For this purpose, the heating device 30 has a constant voltage source 34, which is connected to the heating element 32. For converting the electrical energy provided by the voltage source 34 into heat, the heating element 32 preferably has an ohmic resistance or consists of such an ohmic resistance. 2 to 4 show different exemplary embodiments for the thermal coupling of the heating element 32A; 32B; 32C; and the humidity sensor 10A; 10B; IOC.

FIG. 2a shows a side view of a plate-shaped heating element 32A and a plate-shaped moisture sensor 10A, which are connected by means of a heat-conducting adhesive 40, which is introduced between the heating element 32A and the moisture sensor 10A. The contact surface for the adhesive 40 is both the heating element 32A and the moisture sensor 10A selected one of the faces, which has a large Flächenin- ^• holding in relation to the other surfaces of the respective element, so as to achieve a good thermal coupling between the heating element 32A and the humidity sensor 10A.

In the exemplary embodiment according to FIG. 3, which likewise shows a side view of a heating element 32B and a thermally coupled moisture sensor 10B, the heating element 32B and the moisture sensor 10B are likewise joined to one another on side surfaces which are in relation to other side surfaces of the respective element 32B, 10B have a large area to enable good thermal coupling of the heating element 32B and the moisture sensor 10B. A heat-conducting adhesive is dispensed with in the exemplary embodiment according to FIG. 3. Instead, the heating element 32B and the moisture sensor 10B are joined directly, which can be done, for example, by directly applying a heating resistor as the heating element 32B on the sensor 10 in a method using thin-film or thick-film technology.

4 shows a further exemplary embodiment of an arrangement of heating element 32C and moisture sensor IOC in plan view, heating element 32C and moisture sensor IOC being joined to one another on narrower sides, so that the front and rear sides of plate-shaped heating element 32C and plate-shaped heating element 32C Moisture sensor IOC lie approximately in one plane.

The humidity sensor 10 is preferably a conventional capacitive humidity sensor for determining the relative humidity in gases. Such a moisture sensor 10 is shown schematically in a side view in FIG. 5. It has a first plate-shaped electrode 12 and a second plate-shaped electrode rode 14 with an intermediate moisture-sensitive dielectric 16, which form a capacitor 11. The dielectric constant of this dielectric 16 varies with the moisture which is present in the region of the dielectric 16, the first electrode 12 being permeable to moisture so that moisture can penetrate to the dielectric 16. For this purpose, the first electrode 12 preferably consists of a porous material or is formed like a grid. The value of the dielectric constant of the dielectric 16, which value depends on the moisture in the environment, determines the capacitance of this capacitor, which is composed of the first electrode 12, the second electrode 14 and the dielectric 16. The moisture sensor 10 is connected to the evaluation unit 20 for evaluating the dielectric constant or the capacitor capacitance, and thus for determining the relative humidity in the vicinity of the moisture sensor 10.

The evaluation unit preferably has a resonant circuit (not shown in more detail), the capacitor 11 being part of this resonant circuit and the capacitance of the capacitor 11 being determined on the basis of the resonant frequency of this resonant circuit. To determine the relative humidity from the value of the capacitance of the capacitor or the resonance frequency of the resonant circuit, a conversion or standardization of these values into the values of the relative humidity is necessary. Since manufacturing tolerances occur during the production of the capacitors, the conversion variables or standardization variables differ from moisture sensor to moisture sensor.

In order to eliminate these manufacturing tolerances in the moisture measurement, an individual adjustment or an individual comparison of the determined measured values with the actual measured value is required for each moisture sensor. This Adjustment is carried out at the factory and is independent of the later operating conditions, so that subsequent adjustment of the measuring device due to use is not necessary.

The adjustment of a measuring device before the first use is known and is carried out, for example, by means of a potentiometer in the evaluation circuit, which is set in the factory in order to compensate for tolerances in the capacitor due to production.

According to one embodiment of the invention, it is provided that the evaluation unit has a first evaluation unit 22 which outputs a first measurement signal M 1 which is dependent on the capacitance of the capacitor and thus on the moisture in the region of the moisture sensor 10. With this first measurement signal there are deviations from the actual moisture value, which result from the manufacturing tolerances of the capacitor and from the heating of the moisture sensor with respect to the ambient temperature.

The relative humidity in a gas is temperature-dependent. It results from the quotient of the absolute humidity, which indicates the volume of evaporated liquid per gas volume unit, and the saturation humidity, which indicates the maximum possible volume of evaporated liquid per gas volume unit. It applies here that, given a given absolute humidity of the gas, the relative humidity decreases with increasing temperature. The value for the relative humidity determined on the humidity sensor 10 is therefore always lower in the measuring device according to the invention than the actual value of the relative humidity in the gas. FIG. 6 shows first measured values M1 of the first evaluation unit 22 for different values of the relative humidity between 0% and 100%. The curve marked N represents the actual moisture values or the desired output of the measuring device for these moisture values. As can be seen, the first measured values M 1 deviate from the actual measured values N. These deviations result on the one hand from manufacturing tolerances of the condenser of the moisture sensor and from the heating of the moisture sensor. For the sake of completeness, FIG. 6 shows a curve of measured values MO which are determined with the same moisture sensor without heating.

A correction unit 24 is connected downstream of the first evaluation unit 22 in the measuring device according to FIG. 5, which converts the first measured values M1 into the associated corrected measured values N. Both the curve of the first measured values and the curve of the corrected measured values form a straight line, the curve of the first measured values of the form Ml = ax + b being sufficient while the curve of the actual measured values of the form N = x being sufficient, where x is the parameter for the represents actual relative humidity. All first measured values Ml can be converted into actual measured values (standardized measured values) using the relationships given for Ml and N using the following equation:

N = 1 / a • Ml - b / a = c • Ml - d (1)

In the exemplary embodiment shown in FIG. 6, the factor c = 1.43 and the additive value d = 35.71. The calculation or assignment of the first measured values M1 to the actual measured values N for the relative humidity can be done in. the correction unit in different ways.

According to one embodiment of the invention, it is provided that a computing unit is provided in the correction unit, which calculates the actual measured values N from the first measured values M 1 using equation (1), the actual measured values N being output to a display unit 26. The two parameters of equation (1) are in the

Correction unit saved during factory adjustment. The determination of the two parameters c, d is based on the equation of the straight line of the first measured values Ml. To set up this straight line equation, two measured values for different humidities at a constant temperature are sufficient.

The parameters determined for converting the first measured values Ml into the output standardized measured values N are largely independent of the temperature. Thus, in a test of a measuring device according to the invention, an equation for the straight line of the first measured values M1 was created from only two measured values, which were measured at a temperature of 25 ° C. and known relative humidities of 11% and 75%. On the basis of these two measured values, the parameters c and d were determined and stored, the relative humidity of gases, the temperature of which was between -30 ° C. and., Subsequently being determined in the evaluation unit of the moisture sensor using the first measured values M 1 and equation (1) Was 120 ° C, without a significant temperature-related deviation of the output measurement results from the actual values of the relative humidity being observed. To assign the first measured values to the corrected measured values, there is also the possibility of storing a table in the correction unit, which contains first measured values and associated corrected measured values, the associated corrected measured value for each first measured value M 1 that is fed to the correction unit the table is read out.

For the conversion of the first measured values Ml into the standardized measured values N, it is irrelevant whether the deviation of the curve for the first measured values Ml from the curve for the standardized measured values N is due to manufacturing tolerances in the manufacture of the capacitors, by heating the moisture sensor or is caused by these two effects. For the purposes of the invention, therefore, customary evaluation units from known moisture sensors can be provided as evaluation units, in which there is the possibility of individually adjusting the moisture sensor after its manufacture.

LIST OF REFERENCE NUMBERS

10 humidity sensor

10A, 10B, IOC moisture sensor 11 capacitor

12 first electrode

14 second electrode

16 dielectric

20 evaluation unit 22 first evaluation unit

24 correction unit

25 storage unit

26 display unit

30 heating device 32 heating element

32A, 32B, 32C heating element

34 DC voltage source

40 thermally conductive adhesive

Claims

claims

1. Measuring device for moisture measurement in gases, which has the following features:

- A humidity sensor (10) and an evaluation unit (20) connected to it for providing a measurement value dependent on a relative humidity in a gas;

- A heating device (30) thermally coupled to the moisture sensor (10);

That is, the heating device (20) is designed to keep the humidity sensor (10) at an operating temperature that is at least approximately constant differential temperature above an ambient temperature of the humidity sensor.

2. Measuring device according to claim 1, in which the heating device (30) has an electrical heating element (32) to which a constant electrical power is supplied over time.

3. Measuring device according to claim 1 or 2, wherein the heating device (30) has a heating resistor which is connected to a constant voltage source (34).

4. Measuring device according to one of the preceding claims, in which the moisture sensor (10) is a capacitive moisture sensor.

5. Measuring device according to claim 4, wherein the evaluation unit (20) has a first evaluation unit (22) for provision one of the capacitance of the moisture sensor (10) dependent first measurement signal (Ml).

6. Measuring device according to claim 5, in which the evaluation unit (20) is connected downstream of the first evaluation unit

Correction unit (24) for determining a corrected measured value (N) for a first measured value (Ml).

7. Method for determining the relative humidity in a gas, using a measuring device that has a humidity sensor

(10) and an evaluation unit (20) connected to it and a heating device (30) thermally coupled to the humidity sensor, characterized in that the humidity sensor (10) is kept at an operating temperature by means of the heating device (30) which is at least approximately at a differential temperature is above an ambient temperature of the moisture sensor (10).

8. The method according to claim 7, wherein the heating device (30) is an electrical heating device to which a constant electrical power is supplied.

9. The method according to any one of the preceding claims, in which a capacitive humidity sensor is used.

10. The method according to claim 9, wherein the evaluation unit (20) has a correction unit (24) for determining a corrected measured value (N) for a first measured value (Ml) determined on the basis of the capacity of the moisture sensor.

11. The method according to claim 10, in which the corrected measured value (N) is output for a first measured value (M1).

12. The method according to any one of the preceding claims, in which the following method steps are provided for adjusting the measuring device:

- introducing the humidity sensor (10) with the heating device (32) into a gas at a known first relative humidity and a known first temperature, and determining a first adjustment value for the relative humidity by means of the humidity measuring device;

- Introducing the humidity sensor (10) with heating device (32) into a gas at a known second relative humidity and the first temperature and determining a second adjustment value for the relative humidity by means of the measuring device;

- Calculation of parameters (c, d) for converting the first measured values (Ml) into corrected measured values (N) on the basis of the adjustment values;

- Save the parameters (c, d) in the correction unit