ITMI20131695A1 - DIFFERENTIAL PRESSURE SENSOR WITH TEMPERATURE SENSOR - Google Patents

Description

DESCRIPTION

“DIFFERENTIAL PRESSURE SENSOR WITH DI

TEMPERATURE"

The present invention relates to a differential pressure sensor with a temperature sensor.

In the present application the term 'differential pressure sensor' (in short also 'pressure sensor') indicates a device sensitive to the difference in pressure between two points of a circuit containing a fluid (liquid, gas, vapor, etc.) under pressure and able to generate a signal (typically electrical), which can be continuous or discrete (for example a state, visual or electrical, such as the state of a contact), as a function of this pressure difference. Typically such devices are used to detect the difference in pressure between two points respectively downstream and upstream of an element in a pressurized circuit, such as an oil filter (e.g. in order to detect the degree of clogging of the filter itself indirectly from the pressure drop).

Differential pressure sensors are known which include a piston floating inside a cylinder, the piston dividing the cylinder into two chambers in which the fluid has substantially the same pressure respectively of the aforementioned two points of the circuit under monitoring. The piston is subjected to an axial resulting force whose modulus is a function of the pressure difference, and the position of the piston inside the cylinder depends on this axial force and on an axial elastic force exerted by a spring interposed between the piston and the cylinder. A piston position detection system, typically based on a magnetic principle (e.g. it includes a magnetic sensor or a pair of repulsive / attractive magnets), generates an output signal (which can also consist of a state of a sensor element of pressure) depending on the position of the piston.

In the present application the term 'state' refers to the state of any element within a discrete plurality of possible states (be they states of shape, position, electrical contact, etc.) or to the state of a physical quantity ( e.g. difference in pressure, temperature, etc) above or below (at least) a predetermined value.

Differential pressure sensors (also called 'differential pressure transmitters') are known which are able to detect a plurality, typically a continuum, of values of the aforementioned pressure difference within a working range and to generate an output signal, typically electrical , representative of these recorded values. Typically in such transmitters the piston position detection system is based on a Hall effect magnetic sensor which senses the distance of a permanent magnet fixed to an axial end of the movable piston and generates a corresponding continuous output signal.

Differential pressure sensors of the ON / OFF type are also known (in the present application referred to as 'differential pressure indicators' or in short 'pressure indicators') capable of detecting only the status of the pressure difference above or below a threshold value. set by means of a change of state of its own element according to the position of the piston. In these indicators, the piston position detection system is typically based on a Reed effect magnetic sensor (incorporating an electrical contact), or on a pair of repulsive / attractive magnets which control an electrical contact or a microswitch.

Differential pressure indicators can be of the visual type, where the element that changes state is a mobile visual indicator that has a visible portion based on the position of the indicator, whose position is controlled by a pair of repulsive / attractive magnets ( one on the piston and the other connected to the movable element).

The differential pressure indicators can be of the so-called electric type, where the element that changes state is an electrical contact, or a microswitch, whose closed or open state is controlled by the system for detecting the position of the piston as a function of the position of the piston. .

The Applicant observes that it is advantageous to put the output signal from the piston position detection system in relation to the fluid temperature. For example, there are situations in which the fluid in the circuit is very cold (for example below zero ° C) and therefore very viscous. Consequently, the pressure drop in the aforesaid element (for example the filter) is particularly high, despite the aforesaid element not having a degree of clogging such as to require maintenance. In this situation, in the absence of a control on the temperature of the fluid, the pressure sensor would give a false alarm on the clogging of the filter (or other element). When the fluid temperature increases (for example it reaches or exceeds 30 ° C) reaching the normal operating values, taken as a reference for the calibration of the pressure sensors, its lower viscosity determines a decrease in the corresponding pressure drop below of the preset threshold value.

Differential pressure transmitters equipped with a temperature sensor (for example an NTC thermistor) placed near the Hall effect magnetic sensor are known. However, the Applicant observes that in such known transmitters the temperature sensor, due to its location and the structure of the pressure sensor, is unable to effectively detect the temperature of the fluid in the circuit, since the temperature detected by the temperature sensor is affected. from the external environment to the pressure sensor.

The Applicant observes that by their very nature the differential pressure indicators of the visual type typically do not include the detection of the fluid temperature.

In this context, the Applicant has set itself the problem of developing a differential pressure sensor capable of also detecting the temperature of the fluid in the circuit in a reliable and effective manner, with no or minimal influence from the external environment and / or with a desired accuracy and / or with a desired response speed.

This object, and any others that will result from the present application as a whole, are achieved by a differential pressure sensor according to the following different embodiments, as well as according to the attached claims which represent all further embodiments of the invention, also variously combined with the following embodiments.

The invention relates to a differential pressure sensor having a development axis (and a first and second axial end) and comprising:

- a containment body defining a first seat;

- a piston slidably housed in the first seat in order to separate the latter into a first and a second chamber, each chamber being in fluid communication with the outside of the containment body;

- a detection system sensitive to the position of the piston in said first seat and able to generate a first signal representative of said position.

In the present application the term 'axial' is used with reference to said axis of development of the pressure sensor (for example, depending on the context, it can mean along a direction parallel to this axis) and the term 'orthogonal' is used with reference to a generic direction, or a plane, orthogonal to said development axis.

The pressure sensor comprises a temperature sensor positioned externally to said first seat and able to generate a second signal representative of the temperature at which it is found.

Preferably, the containment body comprises a stem projecting axially towards the second end of the pressure sensor and internally defining an axial end portion of the first seat. Preferably the temperature sensor (preferably with a prevalently axial development) is positioned in proximity to said stem so as to be in thermal contact with it for a first axial portion (e.g. a thermal bridge is present between the temperature sensor and said stem). More preferably, the temperature sensor is placed side by side with the stem with respect to a plane orthogonal to said development axis (e.g. for the first axial section the orthogonal section cuts both the temperature sensor and the stem). Preferably the differential pressure sensor comprises a thermal insulation chamber interposed, with respect to an orthogonal plane and for a second axial section (preferably greater than or equal to the axial length of the temperature sensor), between the temperature sensor and an environment external to the sensor pressure at least in correspondence of half (more preferably at least 65%, even more preferably at least 80%) of the perimeter development (preferably continuous in order to avoid thermal bridges) in orthogonal section of the temperature sensor.

Preferably, said thermal insulation chamber is interposed, with respect to an orthogonal plane and for a third axial portion (preferably equal to the axial length of the stem), between the stem and the environment external to the pressure sensor at least in correspondence of half (more preferably at least 65%, even more preferably at least 80%) of the perimeter development (preferably continuous in order to avoid thermal bridges) in orthogonal section of the stem.

According to the Applicant, the aforementioned characteristics, in particular the fact that the temperature sensor is positioned in proximity to (and in thermal contact with) a stem internally defining an axial end portion of the first seat and that there is an interposed thermal insulation chamber. , with respect to an orthogonal plane and for a determined respective axial section, between the temperature sensor and an environment external to the pressure sensor and between the rod and the external environment at least in correspondence with the respective perimeter development in orthogonal section of the sensor of temperature and of the stem, guarantees on the one hand that the temperature sensor (which can have and typically has a certain space occupied) is in good thermal contact with the fluid contained, in use, in the first seat (second chamber), and on the other hand, that it is slightly thermally influenced by the environment outside the pressure sensor. In fact, the insulation chamber also limits the thermal coupling between the external environment and the fluid contained in the axial end portion of the first seat. In particular, the Applicant has experimentally verified that the aforementioned characteristics guarantee that the difference between the temperature of the thermostat and that of the fluid remains contained below 10 ° C (for example 5-6 ° C) and that the response of the temperature sensor to variations in the temperature of the fluid contained in the axial end portion of the first seat it is sufficiently rapid.

In a comparative case in which the temperature sensor is surrounded, for a large part or all of its perimeter development in orthogonal section, by a thermally conductive material (such as for example the brass of which the same body of containment) that extends from an area close to the temperature sensor up to the external surface of the pressure sensor in contact with the external environment, the temperature sensor would be strongly affected by the temperature of the external environment.

Preferably the differential pressure sensor is a differential pressure indicator.

Preferably, the first signal comprises a state, typically electrical, of a first element of the pressure sensor. Preferably said first element is an electrical contact.

Preferably the second signal comprises a state, typically electrical, of a second element of the pressure sensor. Preferably, said second element is an electrical contact or a microswitch.

Preferably the temperature sensor is a thermostat adapted to change its own state, typically electrical, as a function of the state of its own temperature above or below a predetermined temperature. Preferably the thermostat contains an electrical contact capable of changing its electrical state (e.g. open or closed contact) according to the temperature at which it is found. Preferably the thermostat is a bimetallic foil switch (e.g. normally open).

Alternatively, the temperature sensor can be a thermistor.

Preferably, the piston position detection system comprises a first part mounted on said first axial end portion of the piston and a second part disposed externally to said first seat and at least partially in proximity to the first part of the detection system. Preferably, said second part of the detection system is adapted to generate said second signal.

Preferably, the thermal insulation chamber is contiguous, at least in part, to the temperature sensor with respect to the orthogonal plane.

Preferably the thermal insulation chamber is interposed between the temperature sensor and the external environment in correspondence with substantially the entire perimeter development in orthogonal section of the temperature sensor. In this case a portion of the insulation chamber is on the opposite side (on the orthogonal section and for an axial section) of said stem with respect to the temperature sensor.

Preferably, said thermal insulation chamber is filled with an insulating substance (i.e. having a thermal conductivity lower than 0.2 W m <-1> K <-1>, preferably less than or equal to 0.1 W m <-1> K < -1>, more preferably less than or equal to 0.05 W m <-1> K <-1>), Preferably the insulating substance is a gas such as air (thermal conductivity about 0.03-0.04 W m <-1 > · K <-1>).

Preferably said thermal insulation chamber contains a layer of insulating substance having a minimum thickness, taken on the orthogonal section, greater than or equal to 0.3 mm, more preferably greater than or equal to 0.6 mm.

Preferably the thermal transmittance in the orthogonal plane (i.e. the heat flux in the orthogonal direction by unit surface and temperature difference) of said thermal insulation chamber is less than 100 W / (m <2> * K), more preferably less than or equal to 50 W / (m <2> * K).

Preferably the minimum distance on the orthogonal plane between the temperature sensor and said stem is less than or equal to 4mm, more preferably less than or equal to 3mm for the entire first axial portion.

Preferably the equivalent thermal conductivity (i.e. considering a homogeneous material) of all the material interposed between the temperature sensor and the stem is greater than or equal to 0.1 W m <-1> K <-1>, preferably greater than o equal to 0.2 W m <-1> K <-1>.

Preferably the overall thermal transmittance in the orthogonal plane between said temperature sensor and said stem is greater than 100 W / (m <2> * K), more preferably greater than or equal to 200 W / (m <2> * K), throughout the first stretch.

Preferably the thermal conductivity of the material of said stem is greater than or equal to 5 W m <-1> K <-1>, more preferably greater than or equal to 10 W m <-1> K <-1 >, even more preferably greater than or equal to 50 W · m <-1> · K <-1>.

Preferably the stem is circular in shape on the orthogonal section.

Preferably the thickness of the rod wall on the orthogonal section is less than or equal to 4 mm and / or greater than or equal to 0.5 mm.

Preferably, the axial end portion has a prevalently axial development and is intended to house a first axial end portion of the piston facing the second end of the pressure sensor.

Preferably the axial end portion is circular in shape on the orthogonal section.

In a particularly preferred form, the thermal insulation chamber is interposed, with respect to an orthogonal plane and for the third axial section, between the stem and the external environment at least in correspondence with the entire perimeter development of the stem in orthogonal section.

Preferably said first and / or second and / or third axial portion has an axial length greater than or equal to 3mm, more preferably greater than or equal to 5 mm, even more preferably greater than or equal to 10 mm.

Preferably, the containment body defines a second seat in which said second part of the detection system and said temperature sensor are at least partially housed and in which said isolation chamber develops. Preferably said second seat is a continuous seat. Preferably, said second seat is open towards the second axial end of the pressure sensor. Preferably, said second seat has cylindrical symmetry with an axis of symmetry parallel to said axial direction. Preferably, said stem and said second seat are coaxial (in other words their prevalent barycentric axes of development, e.g. their axes of symmetry, coincide).

Preferably the containment body is a single body. Preferably, the containment body has substantially cylindrical symmetry with an axis of symmetry parallel to said axial direction. Preferably, the containment body comprises a side wall (preferably having a circular orthogonal section) defining the second seat perimeter (with respect to the orthogonal section). Preferably the thickness of said side wall is less than or equal to 10 mm and / or greater than or equal to 0.5 mm. Preferably said stem and said side wall are coaxial. Preferably on each orthogonal section along the entire axial extension of the second seat (possibly with the exception of the bottom of the seat) the stem is separated from the side wall. Preferably on each orthogonal section along the entire axial extension of the second seat, the thermal insulation chamber is interposed between the stem and the side wall. In this way, the formation of thermal bridges with the external environment at the axial end portion of the first seat is advantageously avoided.

Preferably the first part of the piston position detection system comprises (or consists of) a magnet (e.g. permanent), more preferably inserted (axially) into a cavity formed in said first axial end portion of the piston.

Preferably, the second part of the piston position detection system comprises a magnetic sensor arranged in proximity to the first part of the detection system and able to sense the position of the magnet. Preferably, said magnetic sensor is located opposite to said temperature sensor with respect to said stem. Preferably, the magnetic sensor contains an electrical contact adapted to change its electrical state (e.g. open or closed contact) as a function of the mutual distance from the magnet. Preferably the magnetic sensor is a Reed sensor (normally open foil switch which closes in the presence of a sufficient magnetic field).

Preferably, the differential pressure sensor comprises a positioning element (preferably made of plastic material), distinct from said containment body and housed in said second seat, adapted to support and position the temperature sensor in the pressure sensor. Preferably, the positioning element (in a single body) is also able to support and position the second part of the detection system in the pressure sensor.

Preferably the positioning element is made of a material having thermal conductivity greater than or equal to 0.1 W m <-1> K <-1>, more preferably greater than or equal to 0.2 W m <-1> K <-1>.

Preferably the positioning element comprises a first seat with a prevalently axial development, open towards the first end of the pressure sensor, in which said stem is inserted (preferably in intimate contact).

Preferably, the positioning element comprises a second seat with a prevalently axial development, open towards the second end of the pressure sensor, in which said temperature sensor is inserted. Preferably, the positioning element comprises a third seat with a prevalently axial development, open towards the second end of the pressure sensor, in which said second part of the piston position detection system (e.g. said magnetic sensor) is inserted. Preferably the minimum distance, taken on the orthogonal section, between an external surface of the positioning element and an internal surface of the second seat (e.g. the internal surface of the side wall) is greater than or equal to 0.2mm, more preferably greater than or equal to at 0.5mm.

Preferably the minimum thickness, taken on the orthogonal section, of the portion of the positioning element interposed between the temperature sensor and the stem is less than or equal to 2mm, more preferably less than or equal to 1mm.

Preferably, said thermal insulation chamber is interposed, with respect to an orthogonal plane and for a determined axial section (preferably a prevailing section of the axial length of the positioning element), between the positioning element and the external environment at least in correspondence with half (more preferably at least 65%, even more preferably at least 80%) of the perimeter extension (preferably continuous in order to avoid thermal bridges) in orthogonal section of the positioning element. In a particularly preferred form, the thermal insulation chamber is interposed, with respect to an orthogonal plane and for the determined axial section, between the positioning element and the external environment at least in correspondence with the entire perimeter of the positioning element in orthogonal section.

Preferably, the positioning element comprises, towards the second end of the pressure sensor, a flange with a prevalently orthogonal development and shaped to be inserted in said side wall. More preferably, the flange has orthogonal dimensions (e.g. the diameter) slightly larger than the corresponding orthogonal dimensions of the second seat, in order to form a seal to a resin with said wall.

Preferably the pressure sensor comprises a disk (more preferably with a circular orthogonal plan) adjacent to (more preferably in contact with) said containment element (more preferably said flange) on the side facing the second end of the pressure sensor. Preferably said disk is made of electrically insulating material. Preferably, said disc has two first pairs of holes (axially passing) for housing (in electrical contact) respectively of two pairs of electrical terminals belonging respectively to the piston position detection system (e.g. to the magnetic sensor) and to the temperature sensor. . Preferably, the disc has a second pair of holes (through holes, more preferably arranged on opposite sides of a median axis of the pressure sensor parallel to said axial direction) for housing (in electrical contact) respectively of a pair of electrical wires for the electrical connection of the pressure sensor with an external circuit. Preferably, the disc has (on the side facing the second end of the pressure sensor) electric connection tracks between said first pairs and said second pair of holes, more preferably to electrically connect in series, between said two electric wires, the pair of electrical terminals of the piston position sensing system and the pair of electrical terminals of the temperature sensor. Preferably, the positioning element includes two fourth seats open towards the second end of the pressure sensor for housing the end of a respective electrical wire.

Preferably, the pressure sensor comprises an electrically insulating resin layer which drowns the disc on the side facing the second axial end of the pressure sensor.

Preferably, the pressure sensor comprises at least two electrical terminals and an electrical circuit which connects (typically in series) said electrical terminals with a pair of electrical terminals of the temperature sensor and a pair of electrical terminals of the piston position detection system (in in this way the electrical terminals of the pressure sensor are closed or open depending on the electrical state of the terminals of the temperature sensor and of the detection system).

In a further aspect, the invention relates to an assembly method of the differential pressure sensor of the present invention, comprising inserting (axially) the temperature sensor and the second part of the piston position detection system (e.g. the magnetic sensor) respectively in the second and third seat of the positioning element, place the disc against the positioning element, establish an electrical contact (e.g. by tin soldering) between the first pairs of holes and the respective terminals of the temperature sensor and of the detection system position of the piston, insert (axially) the positioning element in the second seat of the containment body, bend (e.g. for caulking) the axial end edge of the side wall towards the inside of the second seat, insert one end of two electric wires respectively in the two second holes and establish a mutual electrical contact (e.g. for welding a pond).

A layer of electrically insulating resin is preferably poured into the compartment formed by the side wall and the side of the disc facing the second axial end of the pressure sensor (the resin is advantageously retained by the aforementioned caulking).

Further characteristics and advantages will become clearer from the detailed description of some exemplary but not exclusive embodiments of a differential pressure sensor according to the present invention. This description will be set out below with the help of the accompanying drawings, provided for indicative purposes only and, therefore, not limitative, in which:

Figure 1 shows a partial and schematic perspective view, in section, of a differential pressure sensor according to the present invention; Figure 2 shows a schematic perspective view of the differential pressure sensor of Figure 1 along the section plane II-II;

Figure 3 shows a schematic perspective view of the differential pressure sensor of Figure 1 along the section plane III-III;

Figure 4 shows a partial and schematic perspective view of the differential pressure sensor of Figure 1 in exploded view;

Figure 5 shows a further partial and schematic perspective view of the differential pressure sensor of Figure 1 in exploded view;

- figure 6 shows a perspective view of the positioning element of the pressure sensor in fig.1.

With reference to the attached figures, the reference number 1 indicates a differential pressure sensor according to the present invention. In accordance with a preferred embodiment of the invention, the pressure sensor 1 illustrated in the figures is a differential pressure indicator 1 (i.e. of the ON / OFF type). However, the present invention also extends to differential pressure transmitters, in which for example the magnetic sensor of the piston position detection system has a Hall effect.

The differential pressure indicator has a development axis 2 (the figures show the median axis 2 having the same direction as the development axis) and a first 3 and second axial ends 4 axially opposite.

The indicator includes a containment body 5 defining a first seat 6.

Typically, the containment body, in correspondence with the first seat, is externally equipped with suitable coupling means (for example threads, not shown in detail) with the hydraulic circuit and with suitable O-rings 7 which guarantee the seal.

The indicator comprises a piston 8 slidably housed in the first seat so as to separate the latter in a first 9 and in a second chamber 10. In the example shown a suitable o-ring 11 separates the two chambers in a sealed manner, however in other solutions (not shown, e.g. without O-rings), a modest leakage of fluid between the two chambers is allowed. Each chamber is in fluid communication with the outside of the containment body 5, for example the first chamber being in fluid communication through a first conduit 12 and the second chamber through a second conduit 13, in order to allow the fluid present to pressure in the circuit to invade the two chambers. In use, the position of the piston inside the first seat depends on a resulting axial force generated by the pressure difference in the two chambers and an axial elastic force exerted by a spring 100 interposed between the piston and an internal surface of the first seat. Typically, a calibration dowel 101, coupled to the containment body 5, closes (for example by means of a thread) the first seat at the first axial end 3 (also allowing the fine calibration of the threshold pressure difference). The indicator comprises a first part 14 of a piston position detection system 15, the first part 14 being mounted on a first axial end portion 16 of the piston, facing the second end 4 of the indicator.

The indicator comprises a second part 17 of the detection system 15 disposed externally to said first seat 6 and at least partially in proximity to the first part 14 of the detection system 15, the detection system 15 being sensitive to the position of the piston in the first seat and being adapted to generate a first signal as a function of this position.

The sensor further comprises a temperature sensor 18 positioned externally to the first seat and able to generate a second signal representative of the temperature at which it is found. In accordance with a preferred embodiment of the invention, the temperature sensor 18 exemplarily described is a thermostat 18 adapted to change its state according to the state of its temperature. In this case the second signal consists of the status of the thermostat. Preferably, the thermostat 18 (shown only schematically in the accompanying figures) is a bimetallic foil switch that is normally open (however, depending on the applications it can also be normally closed or in exchange - SPDT with three electrical terminals) and is able to change the state (open or closed) of its contact (identified as the aforementioned second element) according to the temperature at which it is located.

However, the present invention contemplates any temperature sensor, for example a thermistor whose resistance varies continuously with the temperature, for example of the PT100 or NTC or PTC (Negative / Positive Temperature Coefficient) type. In this case the second signal may consist of the resistance value.

Advantageously, the first seat 6 comprises an axial end portion 20 having a prevalently axial development and intended to house the first axial end portion 16 of the piston.

Advantageously, the temperature sensor 18 has a prevalent axial development and is positioned in proximity to the stem (and to the axial end portion 20 of the first seat) so as to be in thermal contact with it for a certain axial portion L1 (fig 1). This means, for example, that the temperature sensor 18 is flanked to the stem (and to the axial end portion 20 of the first seat) with respect to a plane (e.g. the section plane III-III of fig. 1) orthogonal to the development axis 2 for the whole axial section L1 (for the whole axial section L1 the orthogonal section cuts both the temperature sensor and the rod). For example, the minimum distance (d1) taken on the whole orthogonal plane between the temperature sensor and the rod is equal to 0.55mm for the whole section L1.

Advantageously, the indicator comprises a thermal insulation chamber 22 interposed, with respect to an orthogonal plane and for a determined axial portion L2, between the thermostat and the environment 19 external to the indicator at least in correspondence with half of the perimeter extension 23 of the thermostat in orthogonal section.

Preferably, as in the example shown, the thermal insulation chamber 22 is interposed between the thermostat and the external environment at the entire perimeter 23 in the orthogonal section of the thermostat.

Preferably, the containment body comprises a stem 26 with a prevalently axial development internally defining the axial end portion 20 of the first seat. For example, the thickness d2 of the rod wall is equal to 1mm.

Preferably, the first part of the system for detecting the position of the piston consists of a permanent magnet 27, inserted axially in a suitable cavity 28 obtained in the first portion 16 of the axial end of the piston.

Preferably, the second part 17 of the piston position detection system 15 comprises a magnetic sensor 29 arranged in proximity to the first part 14 of the detection system 15 and able to sense the position of the magnet. Preferably the magnetic sensor 29 is located on the opposite side to the thermostat 18 with respect to the stem 26 (see Fig. 3). However, the present invention contemplates any alternative arrangement of the magnetic sensor with respect to the temperature sensor, for example mutually arranged at 90 °.

Preferably the magnetic sensor is a Reed sensor containing a double foil switch (made of ferromagnetic material such as an iron-nickel alloy), normally open (but depending on the applications it can also be normally closed or in exchange -SPDT), which closes in the presence of a sufficient magnetic field generated when the permanent magnet is close enough. The foils are typically sealed inside a small glass container filled with inert gas (nitrogen or argon). The ends of the foils opposite the contacts protrude from the container and form the terminals of the Reed sensor contact.

However, it is noted that the present invention also contemplates systems for detecting the position of the piston alternative to the one described here, for example where the first and second part respectively comprise two magnets, or a magnet and a ferromagnetic element, and where the mutual force is exploited. of attraction or repulsion to open or close an electrical contact forming part of the second part of the detection system. Furthermore, the positioning of the second part of the detection system may also differ from that shown here, for example by being able to be positioned axially aligned with the first part of the detection system.

Preferably, the containment body 5 defines a second seat 30 in which the second part 17 of the detection system and the temperature sensor 18 are at least partially housed and in which the insulation chamber 22 develops. Preferably the second seat is a continuous seat and open (as regards the containment body) towards the second axial end 4 of the indicator.

Preferably the differential pressure indicator comprises a positioning element 31 (preferably made of electrically insulating plastic material, such as for example nylon with about 30% glass fiber and thermal conductivity equal to about 0.3 W m <-1> K <-1>), distinct from the containment body and housed in the second seat, adapted to support and position the thermostat 18, and optionally the magnetic sensor 29, in the indicator. Preferably the positioning element 31 comprises a first seat 32 with a prevalently axial development, open towards the first end 3 of the indicator, in which the stem 26 is inserted.

Preferably, the positioning element comprises a second seat 33 with a prevalently axial development, open towards the second end 4 of the indicator, in which the thermostat 18 is inserted.

Preferably, the positioning element comprises a third seat 34 with a prevalently axial development, open towards the second end 4 of the indicator, in which the magnetic sensor 29 is inserted.

Preferably, the containment body 5 comprises a side wall 40 defining the second seat 30 perimeter (with respect to the orthogonal section). For example, the thickness of the side wall is equal to 0.75 mm. Preferably the containment body (as well as the stem, the first and second seat, the side wall, etc.) has substantially cylindrical symmetry with an axis of symmetry parallel to said axial direction. For example, the stem, the second seat and the side wall are coaxial.

Preferably the containment body 5 is a single body, however the present invention contemplates the case in which it consists of two or more pieces. For example (not shown) the second seat can be defined (instead of by the side wall) by a shell forming part of the containment body but distinct from the portion of the containment body which defines the first seat.

Preferably the containment body 5 (or at least its portion that defines the first seat and / or the stem) is made of metal such as brass, aluminum, steel (stainless steel), and their alloys. For example, the material of the stem is brass with a thermal conductivity equal to about 110 W · m <-1> · K <-1>.

In the example shown, the thermal insulation chamber 22 surrounds the thermostat 18 and the stem 26 for their entire perimeter development considered on the orthogonal section and for a certain axial section (respectively L2 and L3).

In other words, the isolation chamber 22 is interposed, with respect to an orthogonal plane and for a determined axial portion (for example equal to the axial length L3 of the stem), between the stem and the external environment 19 to the indicator in correspondence of the entire perimeter development of the stem in orthogonal section.

It is noted that, with reference to the illustrated example, the thermal insulation chamber 22 has been identified with the space left free inside the second seat 30 and filled with air. However, according to alternative embodiments (not shown), other thermally insulating substances can be included in the thermal insulation chamber, such as for example polyurethane or expanded polystyrene.

For example, the thermal insulation chamber 22 is interposed, with respect to an orthogonal plane and for a determined axial section, between the positioning element 31 and the external environment 19 in correspondence with the entire perimeter development of the positioning element in orthogonal section .

For example, the minimum distance (e.g. at the vertices 25), taken on the orthogonal section, between an external surface of the positioning element and an internal surface of the second seat (e.g. the internal surface of the side wall) is equal to 0.7mm. For example, at this minimum distance the thermal transmittance in the orthogonal plane is equal to about 30 W / (m <2> * K).

In the example illustrated, a portion of the positioning element 31 is interposed between the thermal insulation chamber and the thermostat. However, it is observed that, although not shown, the thermal insulation chamber 22 can also be (at least partially) in direct contact with the outer surface 23 of the thermostat. Furthermore, the thermal insulation chamber (not shown) can be divided into several insulation sub-chambers.

In the example illustrated, a portion of the positioning element 31 is interposed between the thermostat and the stem. For example, said interposed portion has a minimum thickness d1 equal to 0.55 mm for a thermal transmittance in the orthogonal plane equal to approximately 500 W / (m <2 > * K) in order to favor heat exchange.

However, the present invention also contemplates alternative solutions in which for example (not shown) a suitable axial seat open towards the second end 4 and configured for inserting the temperature sensor. In this case, in accordance with the present invention, a portion of the stem would be interposed between the thermostat and the insulation chamber (which preferably surrounds the entire perimeter of the stem). In this situation, a further axial seat can also be obtained in the stem, open towards the second end 4 and configured for the insertion of the magnetic sensor.

Preferably, the positioning element comprises, towards the second end 4 of the indicator, a flange 35 with a prevalently orthogonal development and circular in shape to be inserted in the side wall. Preferably, the flange has a diameter slightly greater than the diameter of the second seat, in order to form a seal for the resin 45 with the side wall.

Preferably the indicator comprises a disk 36 (with a circular orthogonal plan) arranged in contact with the flange of the containment element on the side facing the second end 4 of the indicator. Preferably the disc has two first pairs of holes 37 axially passing through for housing in electrical contact respectively of two pairs of electrical terminals 38, 39 belonging respectively to the magnetic sensor and to the thermostat. Preferably the disc has a second pair of through holes 41 arranged on opposite sides of a median axis 2 of the indicator for housing in electrical contact respectively of a pair of electrical wires 42 for the electrical connection of the indicator with an external circuit. Preferably, on the side facing the second end 4 of the indicator, the disc has suitable electric connection tracks 43 between the first pairs and the second pair of holes, to electrically connect the pair of terminals in series between the two electric wires 42. 38 of the magnetic sensor and the pair of electrical terminals 39 of the thermostat. Preferably the positioning element includes two fourth seats 44 open towards the second end 4 of the indicator for housing the end of a respective electric wire 42.

The present invention also includes alternative embodiments (not shown), in which the pressure sensor comprises more than two electrical terminals, for example four distinct electrical terminals corresponding to the two pairs of electrical terminals belonging respectively to the magnetic sensor and to the thermostat (in this case if the electric circuit 43 is absent), or three separate electrical terminals since one of the three electrical terminals is connected both to the electrical terminal of the magnetic sensor and to the electrical terminal of the thermostat. It is also possible that the pressure sensor is equipped with an electrical connector (not shown) having an appropriate number of pins (greater than or equal to two).

Preferably the indicator comprises a layer of electrically insulating resin 45 which drowns the disc 36 on the side facing the second axial end of the indicator and keeps the disc in position. Preferably the axial end edge 46 of the side wall at the second end 4 of the indicator is folded (e.g. for caulking) towards the second seat 30, in order to contain the resin layer during the casting step.

With reference to the example illustrated, in use the contact between the two electric wires 42 (normally open) closes only when both the contact between the two terminals 39 of the thermostat 18 is closed (temperature of the thermostat above a certain threshold, and therefore temperature of the fluid above a respective threshold), and the contact between the two terminals 38 of the magnetic sensor (position of the piston 8 corresponding to a pressure difference above a given threshold).

Claims (10)

CLAIMS 1. Differential pressure sensor (1) having a development axis (2) and a first (3) and second axial end (4) and comprising: - a containment body (5) defining a first seat (6); - a piston (8) slidably housed in the first seat in order to separate the latter into a first (9) and a second chamber (10), each chamber being in fluid communication with the outside of the containment body; - a detection system (15) sensitive to the position of the piston in said first seat and able to generate a first signal representative of said position; - a temperature sensor (18) positioned externally to said first seat and able to generate a second signal representative of the temperature at which it is found, where the containment body (5) comprises a stem (26) projecting axially towards the second end (4) of the pressure sensor and internally defining an axial end portion (20) of the first seat, where the temperature sensor is positioned in proximity of said stem so as to be in thermal contact with it for a first axial portion (L1); and where the pressure sensor comprises a thermal insulation chamber (22) interposed, with respect to an orthogonal plane and for a second axial section (L2), between the temperature sensor and an environment (19) external to the pressure sensor at least in correspondence of half of the perimeter development in orthogonal section of the temperature sensor, said thermal insulation chamber being interposed, with respect to an orthogonal plane and for a third axial section (L3), between the stem and the environment outside the pressure sensor at least at half of the perimeter development in orthogonal section of the stem. 2. Pressure sensor according to claim 1, where the thermal insulation chamber (22) is interposed between the temperature sensor and the external environment in correspondence with substantially the entire peripheral development in orthogonal section of the temperature sensor (18) . Pressure sensor according to claim 1 or 2, wherein said thermal insulation chamber (22) is filled with an insulating substance having thermal conductivity less than or equal to 0.1 W · m <-1> · K <-1>. 4. Pressure sensor according to any one of the preceding claims, where the minimum distance on the orthogonal plane between the temperature sensor (18) and said stem is less than or equal to 4mm for the entire first axial portion (L1) and where the conductivity thermal equivalent of all the material interposed between the temperature sensor and said stem is greater than or equal to 0.1 W · m <-1> · K <-1> for the entire first axial section (L1). 5. Pressure sensor according to any one of the preceding claims, where said thermal insulation chamber is interposed, with respect to an orthogonal plane and for the third axial portion (L3), between the stem and the environment external to the pressure sensor at least in correspondence of the entire perimeter development in orthogonal section of the stem. 6. Pressure sensor according to any one of the preceding claims, where the temperature sensor is a bimetallic foil thermostat able to change its electrical state according to the temperature at which it is found. Pressure sensor according to any one of the preceding claims, where the piston position detection system (15) comprises a first part (14) mounted on a first axial end portion (16) of the piston and a second part (17) ) arranged externally to said first seat and at least partially in proximity to the first part of the detection system, where the first part of the piston position detection system comprises a magnet (27), and where the second part of the detection system of the piston position of the piston comprises a magnetic sensor (29) arranged in proximity to the first part of the detection system, the magnetic sensor being a Reed sensor able to change its electrical state according to the mutual distance from the magnet. 8. Pressure sensor according to any one of the preceding claims, where the containment body (5) defines a second seat (30) in which said detection system (15) and said temperature sensor (18) are at least partially housed and in where said isolation chamber develops, said second seat being open towards the second axial end (4) of the pressure sensor, where the pressure sensor also comprises a positioning element (31), distinct from said containment body and housed in said second seat, adapted to support and position the temperature sensor (18) and a part (17) of the detection system in the pressure sensor, and where said thermal insulation chamber is interposed, with respect to an orthogonal plane and for a determined axial section, between the positioning element and the external environment at least in correspondence of half of the perimeter extension in orthogonal section of the positioning element. Pressure sensor according to any one of the preceding claims, where the pressure sensor comprises at least two electrical terminals (42) and an electrical circuit (43) which connects said electrical terminals of the pressure sensor with a pair of electrical terminals (39) of the temperature sensor and a pair of electrical terminals (38) of the piston position sensing system. 10. Method of assembling the differential pressure sensor (1) according to claims 8 and 9, comprising inserting in the positioning element (31) the temperature sensor (18) and said part (17) of the system for detecting the position of the piston, establish an electrical contact between the respective terminals of the temperature sensor and of said part of the detection system, insert the positioning element in the second seat of the containment body, bend an axial end edge (46) of the containment body at the second end (4) of the pressure sensor towards the inside of the second seat, establish an electrical contact between said electrical terminals (42) of the pressure sensor and said terminals of the temperature sensor and of the detection system, pour one electrically insulating resin layer over said electrical circuit (43).