Transmitter with isolation assembly for pressure sensor

Background of the Invention

The present invention betriftt a pressure transmitter for monitoring a pressure in an operating sequence. The invention relates in particular a decoupling or separation unit for a pressure sensor in a Transmitter.

In Druckmeßwertgebern a separating unit used with a separation membrane having a held in a pressure sensor coupling in the transmitter pressure sensor separates from the to be detected operational or working or process fluid. Pressure, of the separating diaphragm to a sensor cavity of the sensor coupling holding the sensor, and transferred to a substantially incompressible isolation fluid. The pressure of the separating fluid reflects the pressure of the process fluid, and the pressure sensor produced in response to the detected pressure an output signal. in the<patcit><text>US Patent No. 4833922</text></patcit> entitled "MODULAR PRESSURE TRANSMITTER", in <patcit><text>US Pat. No. 5094109</text></patcit> entitled "PRESSURE TRANSMITTER WITH STRESS ISOLATION DEPRESSION "and in the <patcit><text>US Pat. No. 5524492</text></patcit> With entitled "PRESSURE TRANSMITTER ISOLATION DIAPHRAGM "are pressure transmitter this type described.

By thermal (volume) expansion of components of the transmitter can Errors in pressure measurements are caused. For example, by Compression and extension or expansion of the separating fluid, typically an oil, Errors are introduced in the pressure measurement. In addition, by thermal expansion of the components of the separation unit, a change caused the measured pressure.

Summary of the Invention

By The present invention is a pressure transducer for detecting the Pressure of a process fluid provided. The transmitter includes a transmitter housing, a Meßwertgeberschaltung and an output connected to the Meßwertgeberschaltung Pressure sensor. defines a sensor coupling in transmitter housing a sensor cavity around the pressure sensor with a separation fluid filled is. The sensor coupling has a first thermal expansion coefficient. A separating membrane is connected to the sensor coupling and seals the sensor cavity from the process fluid from. A Wärmeausgleichs- or -kompensationselement is coupled to the sensor cavity and has a second thermal expansion coefficient on which selected so is that the Volume changes so that by thermal causing volume expansion of the sensor coupling and / or the separating fluid pressure changes be compensated. Various aspects of the invention, the Reducing the separation amount of fluid, using a thin separation membrane and compensating the thermal expansion of the separating fluid and the associated mechanical components.

Brief Description of Drawings

<figref idrefs="S12">1</figref> shows a partial view showing a transmitter with an embodiment a sensor unit of the invention;

<figref idrefs="S13">2</figref> shows a cross-sectional view of the sensor unit of <figref idrefs="S12">1</figref>; and

<figref idrefs="S14">3</figref> shows an exploded perspective view of the sensor unit.

Detailed Description of the Preferred Embodiments

<figref idrefs="S12">1</figref> shows a pressure transducer according to the invention <figref>10</figref> With a transmitter body <figref>12</figref>. a flange <figref>13</figref> and a pressure sensor unit <figref>14</figref>, The transmitter body <figref>12</figref> has a sensor circuit <figref>18</figref> and a Meßwertgeberschaltung <figref>20</figref> on. A pressure sensor <figref>16</figref> is in the unit <figref>14</figref> kept and is connected to the sensor circuit <figref>18</figref> coupled. The pressure sensor<figref>16</figref> speaks to a pressure P1 in a passage or channel <figref>28</figref> and flange <figref>13</figref> at. The sensor circuit<figref>18</figref> is a communication <figref>22</figref> with Meßwertgeberschaltung <figref>20</figref> connected. The Meßwertgeberschaltung <figref>20</figref> transfers to the P1 pressure of a process fluid standing in relationship information over a two-wire communication circuit <figref>26</figref>, This can, for example, a 4 ~ 20 mA current loop or industry standard process control circuit type HART or a Feldbusschleife. The transmitter<figref>10</figref> is the circuit <figref>26</figref> from a control room <figref>27</figref> with a voltage source <figref>29</figref> and a series resistor <figref>31</figref> fully with Voltage supplies.

In Prints sensor units, an oil is used as the separation fluid, typically. The oil is used to fill a cavity defined by a separation membrane the process fluid is isolated. When the membrane by the pressure exerted by the process fluid pressure Removing or is deflected, the separating fluid is through the diversion under pressure, and a pressure sensor in the sensor unit detects the pressure. This arrangement prevents the corrosive or otherwise adverse process fluid with the pressure sensor comes into direct contact.

&lt;?page 3?&gt;

In of the present invention it is recognized that conventional through typical Sensor units errors are introduced in the pressure measurement. First is for the deflection or displacement of the separation membrane a force for moving the membrane required, whereby the transferred onto the separating fluid Pressure is reduced. This is the actual or the actual pressure Separating the fluid less than the actual pressure of the process fluid. This error contribution can be reduced by a diaphragm and a sensor constructed so be that on the pressure range to be measured only a relatively small diaphragm movement takes place. Secondly, it is desirable to use a separation membrane, wherein the membrane by the adjustment Misprint produced is less than the maximum sensitivity the pressure sensor. For example, a sensor unit with a Separating diaphragm made of stainless steel with a diameter of a Inches and a thickness of 1 mil (mil) a misprint of 0.0001 psi, and a volume change of 1 × 10^-4 inch³ exhibit. Such a pressure sensor which by brazing can be mounted in the unit, is preferred because of its integral a flow lower oil volume is required than with a sensor attached by wire, so that by its use unnecessary oil volume is reduced.

Of the by the compressibility the oil imported Error can be reduced by minimizing the volume of oil, and in that the the cavity for the oil forming walls the unit as rigid as possible be formed. After all one aspect of the present invention by the thermal Volume expansion of the components and the thermal expansion introduced the separation fluid Error. The present invention is in particular a heat compensation element provided in the sensor unit, the thermal volume expansion Dimensional changes caused compensated.

<figref idrefs="S13">2</figref> shows a cross-sectional view of an embodiment of a pressure sensor unit <figref>14</figref>, The sensor unit <figref>14</figref> with a flange <figref>13</figref> a transmitter <figref>10</figref> connected. The sensor unit <figref>14</figref> includes a sensor coupling <figref>42</figref> and an insert member <figref>44</figref> with a low coefficient of thermal expansion to that an inventive heat compensation element forms. A separation membrane<figref>46</figref> accepts a print <figref>91</figref> from passage or channel <figref>28</figref> and defines a membrane cavity <figref>48</figref> in between the separation membrane <figref>46</figref> and a substantially planar or flat surface the sensor coupling <figref>42</figref>, The sensor coupling<figref>42</figref> has Load decoupling areas <figref>50</figref> , which in one embodiment Regions with a reduced thickness in the base of the coupling <figref>42</figref> are, by a load decoupling is achieved. The insert member<figref>44</figref> is at a TIG (tungsten inert gas) welding <figref>54</figref> or other suitable means or agent with the sensor coupling <figref>42</figref> connected. The sensor <figref>16</figref> is at a brazing <figref>56</figref> With a cover <figref>80</figref> connected. The insert member<figref>44</figref> is by brazing or by an adhesive process with the cover <figref>80</figref> connected, comprising the ceramic material with a low coefficient of thermal expansion. An external cavity <figref>58</figref> of The insert element and a lower cavity <figref>60</figref> of the insert member are between the insert member <figref>44</figref> and the sensor coupling <figref>42</figref> educated. An internal cavity <figref>62</figref> the insert member is disposed between the insert member <figref>44</figref> and the pressure sensor <figref>16</figref> educated. A passage <figref>64</figref> combines the cavities <figref>58</figref>. <figref>60</figref> and <figref>62</figref> With the cavity <figref>48</figref>Whereby a sensor cavity <figref>82</figref> educated is that is with an incompressible isolation fluid such. B., filled with an oil, so that the to the membrane <figref>46</figref> pressure applied to the pressure sensor <figref>16</figref> Gets transferred.

<figref idrefs="S14">3</figref> shows an exploded view of the pressure sensor unit <figref>14</figref>, The ceramic-like cover <figref>80</figref> seals or closes the pressure sensor <figref>16</figref> (in <figref idrefs="S14">3</figref> not shown). The cover<figref>80</figref> is connected to the insert member <figref>44</figref> connected, in which the sensor cavity <figref>82</figref> is formed and at the Receiving the sensor <figref>16</figref> suitable is. The insert member<figref>44</figref> becomes in the coupling <figref>42</figref> added.

at an exemplary embodiment, the separating membrane <figref>42</figref> a diameter of 1 inch, and the cavity <figref>48</figref> is 0.005 inches deep, to an overpressure protection provide, when oil filled is. The passage<figref>64</figref> Has a diameter of 0.020 inches and a length of 0.200 inches. The cross-section the sensor <figref>16</figref> may be rectangular, and between the sensor <figref>16</figref> and the insert member <figref>44</figref> is a gap of 0.008 inch is provided through which the internal cavity <figref>62</figref> of the insert member is formed. The gap through which the cavity<figref>60</figref> educated is, is 0.005 inch, and the intermediate space through the outer cavity <figref>58</figref> of Insert element is formed, is 0.0015. The sensor coupling<figref>42</figref> and the insert member <figref>44</figref> are cylindrical. The sensor coupling<figref>42</figref> can from stainless steel of the type <figref>316</figref> be made, and that insert member <figref>44</figref> consists of low coefficient of thermal expansion made of a material having a lower thermal expansion coefficient than the stainless steel, zeal, such as from Ceramic material, eg. As cordierite, available from Superior Technical Ceramics St. Albans, Vermont or Coors Ceramics Company, Golden, Colorado, available is. The thermal expansion coefficient of cordierite is 2.1 × 10^-6 Inch / inch ° C. The outer cavity<figref>58</figref> of The insert member is an annular space with a Volume formed which increases with the temperature to the thermal Volume expansion of the filled Fluid or other components &lt;?page 4?&gt;the unit <figref>14</figref> to compensate.

A Analysis of this structure indicates that the volume change the cavity and fluid system on a temperature range of -4 ° C to 52 ° C 2.23 x 10^-5 inch³ is. Therefore, the pressure remains above the Pressure relatively constant, because the sensor coupling at a greater rate stretching than the cordierite thereby causing the intermediate space <figref>58</figref> themselves expands and the extension or expansion of the oil fill fluid is compensated.

The herein specific embodiments described are simply preferred Embodiments, and can within the scope of the present invention, numerous made changes will. The invention comprises placing a thermal compensation element in a separation fluid in which is used a pressure sensor to be coupled to a pressure to be detected. The heat compensation element is selected that it is suitable expands at a rate or contracts, which is selected that thermal Volume or space changes in components of the separating fluid or other elements of the sensor unit be compensated. In a preferred embodiment, the total volume is the separating fluid is reduced, and there will be a pressure sensor is used, which does not substantially deform under pressure. The invention can with any pressure sensor type for measuring a static pressure, a gauge pressure, a differential pressure, etc., or other, standing at a pressure related parameters, eg. as for measuring a flow, a level or height, etc., or of parameters be used which are influenced by the expansion of components. The invention is particularly suitable for use in process control instruments, z. B. transmitters, where to be detected process fluid corroding or contaminated is what to injury result of the pressure sensor can. also relate the particular degree, Volume, forms, etc., only a preferred construction and can for precision or other design constraints be suitably modified.

Although the present invention with reference to preferred embodiments has been described for Freelancers can be seen that within the scope of the present invention, changes in the shape or may be made structure and detail.