Abstract:
A pressure transmitter for registering the pressure of a medium. The pressure transmitter includes a transmitter body, a dividing membrane attached to the transmitter body forming thereby a pressure chamber, a first pressure canal and a second pressure canal. The two pressure canals exhibit different hydraulic properties.

Description:
TECHNICAL FIELD 
   The present invention concerns a pressure transmitter, or a pressure sensor with a pressure transmitter, for the registering of a pressure of a medium. 
   BACKGROUND DISCUSSION 
   The pressure transmitters, which utilize a sealed dividing membrane, or diaphragm, are also referred to as pressure mediators or pressure intermediaries. Such pressure transmitters include a pressure transmitter body and a dividing, or separating, membrane. The dividing membrane is attached to the pressure transmitter body to form a pressure chamber between a top surface of the pressure transmitter body and the dividing membrane. Also included is a pressure canal, which is in communication with the pressure chamber and over which a measuring cell is loaded by means of a transfer liquid with the pressure prevailing in the pressure chamber. To the extent that the pressure transmitter is integrated in a pressure sensor, or in a measuring mechanism thereof, as the case may be, the pressure canal can extend out to a measuring cell chamber. In the case of a pressure transmitter that is arranged separated from a pressure sensor, a capillary line is connected to the pressure canal. The capillary line extends out to the pressure sensor. 
   When, during the process of measuring, the pressure of the medium rises rapidly, the transfer liquid is pressed out of the pressure chamber into the pressure canal. In doing this, the transfer liquid flows with a high velocity out of the pressure chamber into the narrow entrance of the pressure canal. Because of the Venturi effect, this can lead to such a decrease of the pressure in the area of the entrance that the dividing membrane is sucked locally onto the membrane bed in the vicinity of the entrance, and the entrance is closed. A measuring of pressure is no longer possible in this state, since communication between the pressure chamber and the pressure measuring cell is broken. When the entrance of the pressure canal converges conically, and this contour is impressed upon the dividing membrane, the above effect is reinforced, for, as the dividing membrane approaches the pressure canal, a kind of annular canal arises between the dividing membrane and the pressure canal in the area of the entrance, wherein the available area for liquid flow gets increasingly smaller with the approach of the dividing membrane. The net effect is a positive feedback of the Venturi effect. 
   SUMMARY OF THE INVENTION 
   It is, consequently, an object of the invention to provide a pressure transmitter of improved dynamic behavior. 
   The object is solved by the pressure transmitter of independent claim  1 . 
   The pressure transmitter of the invention includes: 
   a pressure transmitter body; 
   a dividing membrane attached to the pressure transmitter body to form a pressure chamber between a top surface of the pressure transmitter body and the dividing membrane; 
   a first pressure canal, which extends between a first opening in the pressure chamber and a common pressure transfer path, and a second pressure canal, which extends between a second opening in the pressure chamber and the common pressure transfer path, wherein the second pressure canal exhibits other hydraulic properties than the first pressure canal. 
   The hydraulic properties are the flow resistance and/or the hydraulic capacitance of the first or second pressure canal, as the case may be. 
   The flow resistance refers to the pressure drop at a certain volume flow through the particular canal. Concomitantly, a pressure increase in the medium causes a resistance-dependent flow velocity through the pressure canals. 
   The hydraulic capacitance is a measure of the mass of transfer liquid that can be accommodated in the particular canal as a function of pressure. Due to the compressibility of the transfer liquid, the capacitance is a function of the constant volume of a pressure canal. Additionally, a pressure canal can have a higher capacitance due to a variable volume. A variable volume can, for example, be provided by elastically compressible or deformable bodies, e.g. a filler body or a bellows. The entrance velocity of the transfer liquid into the pressure canals in the case of a pressure increase can depend, consequently, also on the hydraulic capacitance of a given pressure canal. 
   Since the difference between the coefficients of thermal expansion of the usual transmission liquids and the coefficients of thermal expansion of the usual pressure transmitter materials is really quite significant, one will consider a selective volume enlargement for increasing the hydraulic capacitance only in those cases where no great temperature fluctuations are to be expected. This constraint does not apply for a control of the hydraulic resistances. 
   The different hydraulic properties of the two pressure canals have the following effect. Because of the different hydraulic properties, it is highly improbable that both pressure canals will have in the case of a rapid pressure increase simultaneously exactly those conditions in the regions of their entrances that the dividing membrane experiences sufficient suction at both locations because of the Venturi effect to seal both canals. When, for instance, the first canal becomes sealed by the Venturi effect, then the pressure in the first canal still rises via the second pressure canal, so that the seal is then removed and further pressure transmission via the first canal can proceed. Theoretically, oscillations between the sealed and open conditions of the entrance of the first canal, or alternating sealing of the entrances of the first and second pressure canals, can happen, but, fundamentally, there is always one canal open, so that a continuous pressure transfer between the pressure chamber and the common pressure transmission path is maintained. 
   The common pressure transmission path assures pressure transfer from the confluence of the first pressure canal with the second pressure canal out to a pressure measuring cell or a hydraulic adjusting member. The common pressure transmission path can, for example, include another pressure canal section in the pressure transmitter body and components connecting therewith, such as a capillary line or a measuring cell chamber. When, for example, the pressure transmitter is integrated in the measuring mechanism of a pressure sensor, then it can be that the confluence of the first pressure canal and the second measuring canal happens first in a measuring cell chamber. In this case, the common pressure transmission path would include essentially only the measuring cell chamber. 
   A different hydraulic resistance can be achieved by variation of the canal length and/or the canal diameter. 
   For example, the first canal and the second canal can each include a bore, especially an axial bore, from the pressure chamber into the pressure transmitter body, with the axial bores then being connected over different paths into the common pressure transmission path. 
   In one embodiment, the axial bore of the first canal is aligned with an axial bore of the common transmission path, i.e., the first bore of the first canal goes directly into the common transmission canal, while a resistance line runs between the laterally displaced, axial bore of the second canal and the axial bore of the common pressure transmission path. The difference in the flow resistance of the first and second canals is essentially determined by the length and the diameter of the resistance line. The resistance line has preferably a smaller diameter than the axial bores of the canals. 
   In another embodiment of the invention, separate resistance lines extend between a bore of the first canal and a bore of the common pressure transmission path and between the bore of the second canal and the bore of the common pressure transmission path, with the resistance lines having different lengths. 
   A resistance line can, for example, be introduced into the pressure transmitter body by a bore, especially a lateral bore, and this bore can, for instance, also serve for the filling of the transfer liquid. Equally, the pressure transmitter body can be built of a plurality, especially two, of fitted portions, wherein the resistance lines are formed in one or in a plurality of the surfaces which become internal following assembly of the portions. This can be done, for example, by appropriate milling or lathe, i.e. turning, work. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be explained on the basis of examples of embodiments shown in the figures, which show as follows: 
       FIG. 1 : a longitudinal section through a first embodiment of a pressure transmitter of the invention; 
       FIG. 2 : a longitudinal section through a second embodiment of a pressure transmitter of the invention; and 
       FIG. 3 : a perspective drawing of a third embodiment of a pressure transmitter of the invention, showing hidden structure with dashed lines. 
   

   DETAILED DESCRIPTION 
   The pressure transmitter shown in  FIG. 1  includes a dividing membrane  1  and a cylindrical pressure transmitter body  2 , on whose top side the dividing membrane  1  is attached to form a pressure chamber. The pressure transmitter body  2  includes a cylindrical membrane-carrying-body  21  and a cylindrical base body  22 , which are fitted together at their mutually facing end surfaces. The membrane carrying body  21  has on its top surface opposite that facing the base body a membrane bed  23 , which is covered by the membrane  1 . Extending from the middle of the membrane bed to the bottom surface of the pressure transmitter body  2  is a continuous, axial, first bore, of which a first section  25  runs in the membrane carrying body  21  and forms a first canal. A second axial bore  26  extends, radially spaced from the first bore, from the membrane bed  23  completely through the membrane carrying body  21  down to the interface between the membrane carrying body and the base body  22 . In the end surface of the base body  22  facing towards the membrane carrying body  21 , a groove  27  has been milled in, communicating with the second bore  26  and extending into a second section  24  of the first bore. The groove  27  and the second bore  26  form together the second canal, with the flow resistance of the second canal being greater than the flow resistance of the first canal. In particular, the flow resistance can be controlled by the cross sectional area of the groove  27 . 
   The second section  24  of the continuous, first bore forms the first section of the common pressure transmission path, which is then continued in the embodiment by a capillary line  3 . 
   In a variant of this embodiment, instead of groove  27 , a groove is provided in the end surface of the membrane carrying body  21  facing away from the membrane bed  23 . This alternate groove extends between the first section  25  of the first axial bore and the second axial bore  26 . This variant is advantageous in the case where the base body is rotationally symmetric, because no angular alignment is then required, when the membrane carrying body  21  is fitted with the base body  22 . 
     FIG. 2  shows a embodiment of the invention having essentially the same structure as the first embodiment. A pressure transmitter body  102  has, thus, again a membrane carrying body  121  and a base body  122 , which are fitted together, as above described. Moreover, a first continuous, axial bore is provided having a first section  125  through the membrane carrying body  121  and a second section  124  through the base body  122 , as well as a second axial bore  126 , which is connected with the continuous, first axial bore by way of a groove  127  in the end surface of the base body  122  facing towards the membrane carrying body  121 . However, in this case, the continuous, first axial bore is not in the middle of the membrane bed  123 , but, instead, extends likewise eccentrically. The first and the second axial bores are at equal distances from the central axis of the pressure transmitter body  102 , it being understood that they could also be at different distances. The displacement of the first bore away from the central axis means that the pressure- and temperature-dependent membrane stroke above the first bore differs less from the membrane stroke over the second bore. Naturally, also this embodiment can have, instead of the groove  127  in the end surface of the base body  122 , a corresponding groove in the neighboring end surface of the membrane carrying body  121 . 
     FIG. 3  shows finally an embodiment of a pressure transmitter having a resistance ratio which is simple to tune during manufacture. To this end, the pressure transmitter includes a pressure transmitter body  202 , which again includes a base body  222  and a membrane carrying body  221 , which are fitted together at mutually adjoining end surfaces. In one or both of the mutually adjoining end surfaces is formed an annular groove  227 , which is coaxial with the pressure transmitter body  202  and extends along the circumference of a circle. The groove, or the unit formed by two aligned grooves, serves as a resistance line. Extending through the base body  222  is a lower axial bore  224 , which communicates with the annular groove  227 , i.e. the axial bore is located on the radius of the annular groove and intersects with the groove. The lower axial bore  224  forms a first section of the common pressure transmission path, which is then continued in this embodiment by a capillary line  203 . 
   On the membrane-containing top surface of the membrane carrying body  221  is formed membrane bed  223 , which is covered by a dividing membrane, omitted here for reasons of clarity. Extending from the membrane bed  223  through the membrane carrying body are two upper axial bores  225 ,  226 , both of which communicate with the annular groove. The exact arrangement of the two bores with respect to one another is not critical, although it appears appropriate for the purpose that they have a large separation from one another. The maximum separation is achieved when the two bores are exactly opposite one another, referenced to the annular groove. Now, when during measuring operation the medium pressure acting on the dividing membrane rises, the transfer liquid must reach the lower axial bore  224  by way of the two upper axial bores  225 ,  226  and the annular groove  227 . The effective flow resistances for a first pressure canal and for a second pressure canal, respectively, now are the result of contributions from the flow resistance of the upper axial bores and the resistance of the shortest connection to the lower bore  224  by way of the annular groove  227 . 
   By selection of the azimuth angle between the base body  222  and the membrane carrying body  221 , the resistance ratio can, therefore, be tuned during the assembly of the pressure transmitter body. In the drawing of  FIG. 3 , for example, the first upper bore  225  is aligned with the lower bore  224 , while the second upper bore is displaced with respect to the lower bore by 180°. Here, the difference between the flow resistances is maximum. If the membrane carrying body is rotated by 90° with respect to the drawn position, then both upper bores are rotated by 90° with respect to the lower bore, and the flow resistances would be identical, assuming that equal diameters are present in the upper bores. Between these extremes, the resistance ratio can be determined by choice of the azimuth angle between the membrane carrying body  221  and the base body  222 . By choosing the flow cross section of the annular groove, the maximum possible resistance difference between the two canals becomes predetermined. 
   In the above-described examples of embodiments, axial bores of always equal diameters have been used on an introductory basis. Naturally, the bores can also be of different diameters, in order to achieve different resistances. In the same vain, the bores can, as well, deviate from the axial direction.