The current invention relates to a measuring device for the determination of at least one thermal property of a fluid.

A measuring device according to the invention is described in particular in WO 2015/074833 A1. This measuring device is provided with a first and a second sensor element. The second sensor element is a cantilever that can be used for the determination of a viscosity and the first sensor element is a thermal property sensor, which can measure the volumetric heat capacity and the thermal conductivity.

The thermal property sensor is provided with a heater and two temperature sensors, wherein one temperature sensor has a bigger distance from the heater than the second temperature sensor. The measuring device comprises a support plate where the first and the second sensor element are mounted. For the vibration of the cantilever the support plate is provided with an opening. The heater and the first and second temperature sensor are directly attached to a closed section of the surface of the support.

A further measuring device is disclosed by WO 2016/202538 A1 , especially in Fig. 6. A support plate is provided with an opening, which can also be defined as a window. A heater and a first temperature sensor are placed inside the opening and are distanced from each other.

The problem to be solved is to provide a measuring device which can determine at least one thermal property of a fluid with higher accuracy compared to the state of the art.

The current invention provides a solution for this problem with a measuring device with the features of claim 1

An inventive measuring device for the determination of at least one thermal property of a fluid, especially the volumetric heat capacity and the thermal conductivity, comprises a thermal property sensor and an evaluation unit.

The evaluation unit is adapted to determine the thermal property from a measurement signal determined by the thermal property sensor.

The sensor element comprises a heater, a first and a second temperature sensor.

The thermal sensor element further comprises a mounting plate with an opening.

According to the invention the heater, the first temperature sensor and the second temperature sensor are arranged above or inside the opening.

Above should be defined according to the invention that the aforementioned heater and the two temperature sensors are positioned over or under the opening. The heater and the two temperature sensors can preferably be positioned in such way to the opening that at least a segment of one temperature sensor that at least a segment of the heater and the second temperature sensor or a segment of each of these both elements is basically surrounded by the fluid, so that the accuracy is enhanced compared to WO 2015/074833 A1.

The first temperature sensor is positioned at a first distance from the heater which is at least 20 pm, preferably at least 45 pm, most preferably 50 - 200 pm.

The heater and the second temperature sensor are positioned together. They can be directly attached together.

They can preferably be positioned on a support for holding the heater and the second temperature sensor above or inside the opening. This enhances the stability of these elements. The first temperature sensor is preferably distanced from the support.

Further preferred embodiments of the measuring device are disclosed by the subject-matter of the sub-claims.

The mounting plate can comprises a main surface which extends over a first plane and wherein the opening defines a maximal cross-sectional width of the opening along the first plane and a depth of the opening perpendicular to the first plane, wherein the depth is smaller, preferably at least 3 times smaller, most preferably at least 8 times smaller, than the width of the opening. Therefore the mounting plate cannot be compared with the design of a measuring tube.

It is of advantage that the mounting plate comprises at least one or more closed side surfaces, preferably extending in perpendicular direction to the first plane. This could preferably mean that a plane defined by the side surface can be perpendicular to the first plane. Extending in perpendicular direction can also comprise according to the invention that the side surface extends in an inclined angle to the main surface, but only one vector is perpendicular to the main surface.

The mounting plate can preferably have an ring shape. Ring shape according to the invention can have circular main surface but also rectangular main surface all with a central opening.

The surface of the opening at the first plate 10 is less than 60%, preferably less than 40%, of the main surface of the mounting plate. The main surface of the mounting plate in the first plane is therefore ring shaped and is framing and/or surrounded the surface of the opening.

The mounting plate preferably comprises the said main surface which extends over a first plane and wherein the heater, the first temperature sensor and the second temperature sensor are all positions at said first plane or over a plane parallel to the first plane.

The first and the second temperature sensor, preferably also the heater, extend at least partly parallel to each other. At least the first and the second temperature sensors and preferably also the heater can be wires that extend parallel to each other above or inside the opening. The first and the second temperature sensor are provided as a frame-like structure of the heater, wherein the second temperature sensor is provided in a second frame-like design surrounding the heater and wherein the first temperature is provided in a first frame-like design surrounding the second temperature sensor, wherein the first frame-like design is distanced to the second frame-like design.

The distance between the first frame-like and the second frame like design is preferably uniform at all positions of the frame.

The first and the second frame-like design can have either rectangular, especially square-shaped or circular form.

An advantageous embodiment of the invention is further explained in detail by a drawing. Specific parts of the embodiments, described below, can be understood as separate features that can also be realized in other embodiments of the inventive measurement device. The combination of features described by the embodiment shall not be understood as a limitation for the invention:

Fig. 1 : a top view on a schematically drawing of a first embodiment of a measurement device; and

Fig 2: a top view on a schematically drawing of a second embodiment of a measurement device;

A schematically drawing of a measurement device 1 according to a first embodiment of the invention is shown in Fig. 1 . It comprises a thermal property sensor 2 and an evaluation unit 3.

The measurement device 1 is adapted for the determination of at least one thermal property of a fluid, especially the volumetric heat capacity and the thermal conductivity. This thermal property can be provided as information to a user or it can be used for the further determination of further values, such as the determination of a concentration, a volumetric fraction, a mass fraction and/or a partial pressure of a multi-component mixture, wherein the fluid is said multi component mixture The multi-component mixture can preferably consist of 2-6 components.

Other possibilities are the generation of commands for the regulation of control organs of a process, such as a valve, based on the determined thermal property.

The determined value for the thermal property can be used for numerous other applications.

The fluid mentioned above can be a liquid or preferably a gas or a vapour.

The evaluation unit 2 is adapted to determine the thermal property from a measurement signal determined by the thermal property sensor 3. The thermal property sensor 3 comprises a heater 4, a first and a second temperature sensor 5, 6.

The evaluation unit can use a temperature oscillation technique (TOT) to operate the heater 4. The stimulation can be done by an harmonic oscillation of the heater 4. A phase shift and an amplitude as a measurement signal are measured by the first and the second temperature sensor 1. The phase shift and the amplitude of the first temperature 5 sensor is compared to the phase amplitude shift and the amplitude of the second temperature sensor 6 by applying an algorithm which is provided as a data set at a storage device (not shown) of the evaluation unit 2.

The thermal property sensor 3 comprises a mounting plate 7, also called sensor body, with a square main surface that extends over a first plane 10, which is in Fig. 1 the plane of the drawing sheet. The mounting plate 7 can be monolithic body. The mounting plate is provided with an opening 8, which is positioned in Fig. 1 and Fig. 2 in the centre of the square surface of the mounting plate 7.

The heater, the first temperature sensor and the second temperature sensor can be arranged inside the opening 8. Alternatively these elements can be above the opening. Above can preferably mean that the elements are provided at the main surface of the mounting plate 7 so that the elements 4-6 partly cover the opening with their own sensor bodies.

The sensor boby, the so-called mounting plate 7, comprises a main surface which extends over a first plane 10. The opening 8 defines a maximal cross-sectional width 9 of the opening 8 along the first plane 10 and a depth (not shown) of the opening perpendicular to the first plane 10, wherein the depth is smaller than the width 9 of the opening 8.

The mounting plate 7 comprises at least one or more closed side surfaces 1 1 , preferably extending in perpendicular direction to the first plane 10. In the case of one side surface, the main surface of the mounting plate 8 could be circular or ellipsoidal. Closed means that the side surfaces 1 1 have no openings like the opening 8 of the main surface.

In Fig. 1 a thermal property sensor with free standing wires, representing the heater, the first and the second temperature sensor is shown. The middle wire represents the heater and the other two are the temperature sensors.

In the depicted embodiment of Fig. 1 , one of temperature sensors, the second temperature sensor 6 is attached on the heater and the other temperature sensor, the first temperature sensor 5, has a short distance from the heater preferably 50-200 pm, most preferably 80-120 pm.

The opening 8 is cavity that is formed in the body of the sensor, which is also called mounting plate 7, has rectangular form, especially a squared form with preferred dimensions of 0,2-4 mm ² , most preferably 0, 5-2,0 mm ² .

The working principle of the sensor was considered the temperature oscillation technique (TOT) and the response can be investigated by using Finite Element Method (FEM).

Since the response of each temperature sensor under temperature oscillation technique is an amplitude, the thermal properties (volumetric heat capacity and thermal conductivity) can be derived from the phase difference (response of the so-called iso-amplitude and iso-phase) of the said temperature sensors and a characteristic curve for the sensor element can be derived. It has been surprisingly found that the sensitivity for temperature amplitude and phase on each fluid thermal property is higher with the thermal property sensor shown in Fig. 1 and 2. In addition, one can even identify ambiguity on the response of the sensor, if two iso-curves are crossing each other in more than one position. Moreover, the signal could be used as a graphical method of deriving the thermal conductivity and volumetric heat capacity by finding the cross point of the two iso-curves (amplitude and phase) from the temperature sensors 5 and 6.

The second proposed sensor design considers also the spherical distribution of the heat from a point source. It consists of a small rectangular heater 4’ and two rectangular rings, second temperature sensor 6’ and first temperature sensor 5’. The second sensor 6’ is attached on the heater. One advantage of this design is the spherical distribution of heat that has an exact analytical solution. Another advantage is the very low heat losses to the body of the sensor, which is significant in the wire base design of Fig. 1. The heater 4’ and the second temperature sensor 6’ are also positioned together and the first temperature sensor 5’ is distanced at a distance of preferably 50-200 pm, most preferably 80-120 pm.

As shown in Fig. 2 the heater 4’ is placed in the center of the cavitiy or opening 8. The

temperature sensors 5 and 6 have the form of homocentric rectangles, but they can also have other homocentric forms.

For the second proposed design shown in Fig. 2 two measurement concepts presented good characteristics. Firstly, using the temperature signal of the second temperature sensor 6’ in low excitation frequency, for example 188 [rad/s], both amplitude and phase iso-curves show a relative homogeneous distribution and no ambiguity.

Similarly, the first temperature sensor 5 as an external ring exhibits an even more homogeneous distribution of both iso-curves. The sensitivity of the sensor in both of the measurement concepts is considered comparable and is excellent compared to other sensor designs.

The thermal property sensor proposes therefore a very sensitive determination of the thermal conductivity and the volumetric heat capacity (k, pep) either in a wire based or a ring based design.

As described, the sensor can operate under the TOT-principle but it can by used in any known working principle (steady, TOT, transient).

The rectangular rings based sensor could also have other form, for example, circular. However the rings should preferably be homocentric to each other, meaning they should have the same center. In an optimized design, the heater has the same form, but not the same dimensions, as the rings (circular, rectangular, triangular, ...)

Two are the most favourable primary measurement quantities for the wire based sensor design. The temperature amplitude and phase of temperature sensor for low excitation frequencies from 170-200 [rad/s], most preferably 185-190 [rad/s] and the amplitude and phase difference between the second and the first temperature sensor 6, 5 for middle excitation angular frequencies 500- GOO [rad/s], most preferably 550-570 [rad/s].

Similarly, two are the most favorable primary measurement quantities for the ring based sensor.

The method of a modeling approach can be derived from the article“Measurement of Gas

Thermal Properties Using the Parametric Reduced-Order Modeling Approach" by A. Bardalis, S.Stingelin and A.Pfau, IEEE Sensors Journal, Vol. 16, Issue 12, page 4704-4714, June 2016.

In general the property sensor of Fig. 1 and Fig. 2 can be performed under at least two different angular excitation frequencies, a low angular excitation frequency at around 188 rad/s and a middle angular excitation frequency.

More in general the first angular excitation frequency can be at 150-200 rad/s and the second angular excitation frequency can be at least twice as high, more preferably 2.5-3.2 times higher, as the said first angular excitation frequency

Fig. 3a provides a graph of a primary measurement quantity temperature measured by the second temperature sensor 6 of Fig. 1 . The excitation angular frequency is 188 rad/s.

Fig. 3b provides a graph of a primary measurement quantity temperature amplitude and phase difference between the two temperature sensors 5 and 6 at an excitation angular frequency of 188 rad/s.

From figure 3a the response of the second temperature sensor can be derived, while in the second figure 3b the amplitude and phase difference between the two temperature sensors 5 and 6 of Fig. 1 is illustrated. Both of the primary measurement quantities (amplitude and phase) are dependent on both properties (volumetric heat capacity and the thermal conductivity) while for this range of properties does not show any ambiguity.

Two measurement concepts that presented good results are illustrated in figure 3a and 3b. The contour graphs in figure 3a and 3b present the response of the iso-amplitude and the iso-phase in relation to the fluid thermal properties. They were derived simulating the sensor for different combination of thermal conductivity and volumetric heat capacity. The contour of the graphs is a kind of characteristic curve for the sensor that operates under temperature oscillation technique. This graph exhibits the dependency-sensitivity of each primary measurement quantity

(temperature amplitude and phase) on each fluid thermal property. In addition, ambiguity on the response of the sensor can even be identified, if two iso-curves are crossing each other in more than one position. Moreover, the contour graph could be used as a graphical method of deriving the thermal conductivity and volumetric heat capacity by finding the cross point of the two isocurves (amplitude and phase).

Likewise Fig. 4a shows a primary measurement quantity temperature on the second temperature sensor 6’ of Fig. 2. The excitation angular frequency is 188 rad/s. Fig. 4b depicts the primary measurement quantity temperature amplitude and phase difference between the two temperature sensors 5’ and 6’ of Fig. 2. The excitation angular frequency is 188 rad/s. For the second proposed design of figure 2 two measurement concepts presented good characteristics figure 4a and 4b. Firstly, by using the temperature of the second temperature sensor 6’ in low excitation frequency 188 rad/s both amplitude and phase iso-curves show a relative homogeneous distribution and no ambiguity. Similarly, the first temperature sensor 5’ as an external ring exhibits an even more homogeneous distribution of both iso-curves. The sensitivity of the sensor in both of the measurement concepts is considered comparable.