DE102015107101A1 - Temperature sensor, inductive conductivity sensor and method of making the same - Google Patents

Abstract

The invention relates to a temperature sensor (10) for determining the temperature of a medium (2) in a container (3), comprising: a temperature element (14) which supplies an electrical signal as a measure of the temperature; and a housing (9) having a housing wall (16), wherein the temperature element (14) in the housing (9) is arranged, wherein the housing (9) for immersion in the medium (2) certain medium-impermeable housing portion (8) wherein the housing section (8) intended for immersion in the medium comprises a region (11) via which the temperature element (14) is connected to the medium (2). The temperature sensor (10) is characterized in that at least the region (11) has a multilayer construction, and the region (11) comprises at least one other physical property, in particular a different thermal conductivity, than the rest of the housing section intended for immersion. The invention further relates to a conductivity sensor (1) and a method for producing such a temperature sensor (10) or such a conductivity sensor (1).

Description

The invention relates to a temperature sensor and an inductive conductivity sensor for determining the temperature or the specific electrical conductivity of a medium in a container. The invention further relates to a method for producing the same.

In general, in process automation for measuring the electrical conductivity of a medium, conductivity sensors are often used which operate according to an inductive or a conductive measuring principle.

A conductive conductivity sensor comprises at least two electrodes, which are immersed in a medium for measurement. To determine the electrical conductivity of the medium, the resistance or conductance of the electrode measuring section in the medium is determined. If the cell constant is known, the conductivity of the medium can be determined from this. For measuring the conductivity of a measuring liquid by means of a conductive conductivity sensor, it is absolutely necessary to bring at least two electrodes into contact with the measuring liquid.

In the inductive principle of the conductivity determination of process media, sensors are used which have a transmitting coil and a receiving coil arranged at a distance from it. By means of the transmitting coil an alternating electromagnetic field is generated which is sensitive to charged particles, e.g. Ions, acts in the liquid medium and causes a corresponding current flow in the medium. As a result of this current flow, an electromagnetic field is produced at the receiving coil, which induces a received signal (induction voltage) in accordance with the Faraday law of induction in the receiving coil. This received signal can be evaluated and used to determine the electrical conductivity of the liquid medium.

Typically, inductive conductivity sensors are constructed as follows: The transmitting and receiving coils are generally designed as toroidal coils and comprise a continuous, through-flow of the medium opening. The coils are arranged in a housing which is immersed in the medium to be measured. Both coils are thus flowed around by medium. Upon excitation of the transmitting coil, a closed current path running within the medium forms, penetrating both the transmitting and the receiving coil. By evaluating the current or voltage signal of the receiving coil in response to the signal of the transmitting coil, the conductivity of the measuring liquid can then be determined. The principle itself is established in industrial process measurement technology and documented in a large number of publications in the patent literature.

The measurement of the specific electrical conductivity is used to control process engineering processes. In food technology, for example, product flows in pipelines are differentiated from cleaning solutions or rinsing water by measuring the specific electrical conductivity. Also, depending on certain media process engineering processes are influenced.

A quick measurement of the specific electrical conductivity is absolutely necessary for this. However, this also requires an accurate and rapid temperature measurement in order to be able to recalculate the conductivity value to a reference temperature. The conductivity is highly temperature dependent. In gases, solutions and electrolytes, for example, the resistance is strongly temperature-dependent, since there the mobility of the ions and the number of charge carriers increases strongly with increasing temperature. As a rule, the charge carrier mobility increases with the temperature and the resistance becomes smaller.

In food technology and biotechnology, there are still demands to thermally sterilize the sensors and to design the sensors so that they are easy to clean (keyword: "hygienic design").

As a matter of principle, inductive conductivity sensors must at least partially consist of electrically insulating material. Usually, plastic is used for this purpose. These plastics require a special approval for use in the food or biotechnology sector.

As mentioned, the accurate and sufficiently fast determination of the temperature is essential for a precise conductivity measurement. The temperature sensor for determining the temperature is accommodated for this purpose either in a plastic housing or in a metal bushing introduced into the plastic housing. The temperature sensor is configured as a wired temperature sensor or as a component on a circuit board. Apart from a thermal compound there is no special thermal connection to the environment.

A plastic housing (often made in one piece) avoids sealing transitions between different material combinations and allows continuous, ie gap-free surfaces. Such enclosures are suitable to meet requirements for hygienic design. The thickness of the plastic significantly affects the temperature response time. This is correspondingly relatively high (> 15 s). But partly large wall thicknesses are needed around for example, to increase the pressure resistance of the housing or to delay vapor diffusion through the housing wall. Plastics that are approved for the food or biotechnology sector, have a poor thermal conductivity.

On the other hand, a metal bushing which accommodates the sensor in the housing binds the temperature sensor thermally better to the process, so that a relatively short temperature response time (<10 s) results. However, the seal transition between metal housing and plastic housing presents a potential leakage point for the sensor and creates gaps that do not meet the requirements of a hygienic design.

The invention has for its object to provide a sensor that has a low temperature response time while maintaining a hygienic design.

The object is achieved by a temperature sensor comprising: a temperature element which supplies an electrical signal as a measure of the temperature; and a housing having a housing wall, wherein the temperature element is arranged in the housing, wherein the housing has a medium-impermeable housing section intended for immersion in the medium, wherein the housing section intended for immersion in the medium comprises an area over which the temperature element with the medium in Connection stands. The temperature sensor is characterized in that at least the region over which the temperature element is in thermal communication with the medium has a multilayer construction, and the region comprises a higher thermal conductivity than the rest of the housing section intended for immersion.

Thus, it can be ensured that both a hygienic design is achieved, so also that the temperature of the medium is quickly passed to the temperature sensor and so a low temperature response time can be guaranteed.

In a first advantageous variant, only the area has a multi-layer structure. This represents a cost-efficient, but nevertheless satisfying, embodiment.

In a second advantageous variant, the entire housing section intended for immersion in the medium has a multi-layer structure. This is an easy to manufacture embodiment.

Preferably, the area over which the temperature element is in thermal communication with the medium comprises a ceramic, a metal or a thermally conductive plastic. The multilayer structure comprises at least a first and a second layer, wherein the housing wall forms the first layer and the ceramic, the metal or the thermally conductive plastic forms the second layer.

Ceramics are known as a good conductor of heat; as well as metals. As a metal here is about a stainless steel used. As a thermally conductive plastic is about a plastic with mineral additives used.

In an advantageous embodiment, the ceramic comprises ZrO ₂ , Al ₂ O ₃ or a mixture thereof. These are common ceramics that meet the given requirements.

To achieve intimate contact between ceramic and plastic housing, the ceramic is completely or partially porous. The pores are less than 0.2 μm to comply with common hygiene standards such as "3A" or "EHEDG".

In a preferred embodiment, the housing is made of a plastic, in particular of a thermoplastic. The plastic of the housing differs from the thermally conductive plastic. The plastic of the housing is preferably approved for use in the food or biotechnology sector.

If the multi-layer structure comprises a porous ceramic, the pores are filled directly by the injection-molding material of the plastic of the housing or closed in a subsequent encapsulation step. Thus, the porous ceramic is encapsulated with about a thermoplastic, so that the ceramic permanently fixed and hygienically designed sits in the housing.

The object is further achieved by an inductive conductivity sensor comprising: a transmitting coil which irradiates an input signal into the medium; and a receive coil coupled to the transmit coil via the medium and providing an output signal. The conductivity sensor is characterized in that it comprises a temperature sensor as described above, wherein the housing of the temperature sensor surrounds the transmitting coil and the receiving coil, and the conductivity on the basis of the input signal, the output signal and the temperature can be determined.

The object is further achieved by a method for producing a temperature sensor or an inductive conductivity sensor as described above, comprising the steps of: producing the ceramic, the metal or the thermal conductive plastic; Encasing the ceramic, the metal or the thermally conductive plastic by means of injection molding with a plastic for the design of the housing, wherein this plastic differs from the thermally conductive plastic.

The method preferably further comprises the step of removing the housing section intended for immersion in the medium, in particular a partial area around the ceramic, the metal or the thermally conductive plastic, to a distance, in particular to 0.1 mm, from the height of the Ceramic, metal or thermally conductive plastic. This results in a smooth transition between housing and ceramic, metal or thermally conductive plastic, whereby a hygienic design can be realized.

The invention will be explained in more detail with reference to the following figures. It shows

1 the conductivity sensor according to the invention in a symbolic representation,

2a the temperature sensor according to the invention in cross section,

2 B the temperature sensor according to the invention in cross section in an embodiment, and

2c the temperature sensor according to the invention in cross section in a further embodiment.

The inductive conductivity sensor according to the invention in its entirety has the reference numeral 1 and is in 1 shown. The conductivity sensor 1 is designed for use in process automation.

The conductivity sensor 1 is on a container 3 , about a flange 4 (Generally via a process connection), arranged in which the medium to be measured 2 located. At the container 3 it is about a pipe, for example made of plastic or metal.

The conductivity sensor 1 includes a transmitting coil 6 and a receiver coil 7 in a housing 9 are housed. The housing 9 includes a housing wall 16 , The housing 9 is made of a plastic, in particular a thermoplastic. This plastic is approved for use in the food and biotechnology sector. For example, this is a polyaryletherketone, such as polyetheretherketone (PEEK).

The transmitting coil 6 and the receiver coil 7 For example, are arranged opposite each other on opposite sides of a printed circuit board (not shown). The transmitter and receiver coils designed as rotationally symmetrical toroids ("toroids") 6 respectively. 7 are arranged coaxially one behind the other in this way. The printed circuit board comprises the coils contacting conductor tracks, which the transmitting coil 6 with a driver circuit and the receiver coil 7 connect to a receiving circuit. The driver circuit and the receiving circuit may be components of a sensor circuit arranged on the printed circuit board. The spools 6 . 7 are with a data processing unit 5 , in 1 connected to a transmitter.

The housing 9 forms one the transmitting coil 6 and the receiver coil 7 along its axes of rotation passing through channel 12 , Will the housing 9 in an electrically conductive medium 2 immersed, this surrounds the housing 9 , or one for immersion in the medium 2 certain housing section 8th , and penetrates the channel 12 one, so that in the medium a two coils 6 . 7 penetrating, closed current path 13 can form when the transmitting coil 6 is energized or flows through with an input signal, that is an AC voltage.

The conductivity sensor operates in the manner of a double transformer, wherein the transmitting and the receiving coil 6 . 7 as mentioned at least as far into the medium 6 be introduced that one through the medium 6 extending, the transmitting and the receiving coil 6 . 7 penetrating, closed current path 13 can train. When the transmission coil 6 is excited with an AC signal as an input signal, it generates a magnetic field, the one the coils 6 respectively. 7 passing through the current path 13 whose strength depends on the electrical conductivity of the medium 2 depends. This results in a current path with an ionic line in the medium 2 , Since this alternating electrical current in the medium again causes a variable magnetic field surrounding it, an alternating current in the receiving coil 7 induced as an output signal. This one from the receiving coil 7 supplied as an output signal AC or a corresponding AC voltage is a measure of the electrical conductivity of the medium 2 ,

The conductivity sensor 1 comprises a temperature sensor according to the invention 10 for measuring the temperature of the medium 2 , The data processing unit 5 determines the conductivity of the medium 2 based on the input signal, the output signal and the temperature of the medium 2 , At the temperature sensor 10 It is an electrical or electronic device that provides an electrical signal as a measure of temperature. It is about a thermistor or PTC thermistor as components whose resistance to the Temperature changes. Examples are platinum measuring resistors or ceramic PTC thermistors. Alternatively, a component may be used that directly provides a processable electrical signal, such as a semiconductor temperature sensor that provides approximately a current or voltage proportional to temperature. As further alternatives, a thermocouple or other common temperature measuring element can be used.

As mentioned above is the temperature sensor 10 or the conductivity sensor 1 in one area 11 over the housing section intended for immersion in the medium 8th with the medium to be measured 2 in thermal connection.

The 2a -C show the temperature sensor 10 in a detailed view in cross section.

The temperature sensor 10 includes a temperature element 14 which provides an electrical signal as a measure of the temperature. This is about a thermistor, for example a Pt100 or Pt1000. Via cables 18 This signal, such as resistance values or a voltage, is sent to the transmitter 5 forwarded. The temperature sensor 10 further includes the housing 9 with a housing wall 16 , wherein the temperature element 14 in the case 9 is arranged.

The temperature sensor 10 at least in the area 11 a multi-layer construction. In a first embodiment, only the area 11 a multilayer structure (see 2a and 2 B ). In a second embodiment, the entire housing section intended for immersion in the medium 8th a multilayer structure (see 2c ).

In 2a is the temperature element 14 directly into the housing 9 used, in the context of the invention thus about a platinum or ceramic element. The temperature element 14 thus forms another layer directly 17 ; to the "further layer 17 "See next section. In 2 B and 2c is the temperature element 14 over an (additional) additional layer 17 from the case 9 quasi settled.

The multilayer structure of the housing 9 at least in the field 11 comprises at least a first and a second layer, wherein the housing wall 16 the first layer forms and the ceramic, the metal or the thermally conductive plastic, the second layer 17 forms.

The ceramic is ZrO ₂ or Al ₂ O ₃ . The ceramic is completely or partially porous with pores smaller than 0.2 microns. In particular, the variants with ceramic and metal are in the first embodiment, ie, if only the area 11 the multilayer construction comprises ( 2a and 2 B ), produced.

Alternatively, a thermally conductive plastic may be used. This thermally conductive plastic differs from the plastic of the housing 9 , The thermally conductive plastic is, for example, a plastic based on polyetheretherketone (PEEK), a polyamide (PA), polyethylene terephthalate (PET), polyphthalamide (PPA), polyphenylene sulfide (PPS), polybutylene terephthalate (PBT) or polycarbonate (PC) with the addition of additives, for example on a mineral basis. As a result, thermal conductivities of over 40 W / mK can be achieved (measured according to the "hot-disc method"). These plastics are available, for example, under the brand name "LUVOCOM".

The temperature element 14 of the temperature sensor 10 is by means of a thermal adhesive 15 at the further layer 17 , so attached to the ceramic, or one of the thermally conductive plastic. The thickness of this layer is relatively insignificant, since its thermal conductivity is good. This layer is thinly surrounded by the housing wall 16 , which, as already mentioned, is approved for use in the food and biotechnology sector. Thus, the advantages of good thermal conductivity and approval can be combined.

For the production of the temperature sensor 10 or an inductive conductivity sensor 1 In a first step, the ceramic, the metal or the thermally conductive plastic, such as by injection molding, produced. This creates the further layer 17 , In a further step, the ceramic, the metal or the thermally conductive plastic by injection molding with a plastic for the design of the housing 9 molded. In a further step, the intended for immersion in the medium housing section 8th ablated. Preferably, the portion is removed around the ceramic, the metal or the thermally conductive plastic around. It is removed to a distance of about 0.1 mm from the height of the ceramic, metal or thermally conductive plastic. Thus, a hygienic design can be ensured.

LIST OF REFERENCE NUMBERS

1

 conductivity sensor

2

 medium

3

 container

4

 flange

5

 Data processing unit

6

 transmitting coil

7

 receiving coil

8th

for immersion in 2 certain housing section of 1

9

 casing

10

 temperature sensor

11

 Area

12

 channel

13

 current path

14

 temperature element

15

 Thermal Adhesive

16

 housing

17

 Another layer

18

 cables

Claims (11)

Temperature sensor ( 10 ) for determining the temperature of a medium ( 2 ) in a container ( 3 ), comprising - a temperature element ( 14 ), which provides an electrical signal as a measure of the temperature, and - a housing ( 9 ) with a housing wall ( 16 ), wherein the temperature element ( 14 ) in the housing ( 9 ), wherein the housing ( 9 ) one for immersion in the medium ( 2 ) certain medium-impermeable housing section ( 8th ), wherein the intended for immersion in the medium housing section ( 8th ) an area ( 11 ) via which the temperature element ( 14 ) with the medium ( 2 ), characterized in that at least the area ( 11 ) has a multi-layer structure, and the area ( 11 ) comprises a higher thermal conductivity than the rest of the housing section intended for immersion. Temperature sensor ( 10 ) according to claim 1, wherein only the area ( 11 ) has a multi-layer structure. Temperature sensor ( 10 ) according to claim 1, wherein the entire housing section intended for immersion in the medium ( 11 ) has a multi-layer structure. Temperature sensor ( 10 ) according to claim 1, 2 or 3, wherein the region ( 11 ) comprises a ceramic, a metal or a thermally conductive plastic, and the multilayer structure comprises at least a first and a second layer, wherein the housing wall ( 16 ) forms the first layer and the ceramic, the metal or the thermally conductive plastic forms the second layer. Temperature sensor ( 10 ) according to claim 4, wherein the ceramic comprises ZrO ₂ or Al ₂ O ₃ . Temperature sensor ( 10 ) according to claim 4 or 5, wherein the ceramic is wholly or partly porous. Temperature sensor ( 10 ) according to claim 6, wherein the pores are smaller than 0.2 microns. Temperature sensor ( 10 ) according to one of claims 1 to 7, wherein the housing ( 9 ) is made of a plastic, in particular a thermoplastic, wherein this plastic differs from the thermally conductive plastic. Inductive conductivity sensor ( 1 ) for determining the specific electrical conductivity of a medium ( 2 ) in a container ( 3 ), comprising - a transmitting coil ( 6 ), which injects an input signal into the medium, and - one over the medium ( 2 ) with the transmitting coil ( 6 ) coupled receiving coil ( 7 ), which provides an output signal, characterized in that the conductivity sensor ( 1 ) a temperature sensor ( 10 ) according to at least one of claims 1 to 8, wherein the housing ( 9 ) of the temperature sensor ( 10 ) the transmitting coil ( 6 ) and the receiving coil ( 7 ), and the conductivity is determinable based on the input signal, the output signal and the temperature. Method for producing a temperature sensor ( 10 ) or an inductive conductivity sensor ( 1 ) according to at least one of claims 4 to 9, comprising the steps: - producing the ceramic, the metal or the thermally conductive plastic, - encapsulating the ceramic, the metal or the thermally conductive plastic by means of injection molding with a plastic for the design of the housing ( 9 ), wherein this plastic is different from the thermally conductive plastic. The method of claim 10, further comprising the step of: removing the material for immersion in the medium ( 2 ) particular housing section ( 8th ), in particular a portion around the ceramic, the metal or the thermally conductive plastic around, up to a distance, in particular to 0.1 mm, to the height of the ceramic, the metal or the thermally conductive plastic.

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