Patent Application
20130008256
The invention relates to a method for reducing content of foreign molecules in gaseous or liquid form dissolved in a pressure transfer liquid of a pressure measuring transducer.
Pressure transfer liquids are applied in a large number of pressure measuring transducers in pressure measuring technology, in order to bring a pressure to be measured to a pressure sensor. Liquids, such as e.g. silicone oils, which are as incompressible as possible and have thermal coefficients of expansion as small as possible, are preferably used as pressure transfer liquids.
Typically, pressure measuring transducers have a pressure receiving chamber closed outwardly by an isolating diaphragm; the pressure receiving chamber is connected via a passageway to a pressure measuring chamber, in which the pressure sensor is located. The pressure receiving chamber, the pressure measuring chamber and the passageway are filled with the pressure transfer liquid, which in measurement operation transmits an external pressure to be measured acting on the isolating diaphragm to the pressure sensor.
Examples of this are pressure measuring transducers, in which semiconductor sensors, e.g. silicon chips having doped resistance elements, are applied as pressure sensors. Usually, semiconductor sensors comprise a membrane-like pressure sensor chip, which is supported laterally in the pressure measuring chamber. Semiconductor sensors are, as a rule, very sensitive and are therefore not directly exposed to a medium, whose pressure is to be measured.
Pressure measuring transducers having upstream pressure transfer means are an additional example. In measurement operation, the pressure transfer means has a pressure receiving chamber arranged at a measuring location and closed by an isolating diaphragm; the pressure receiving chamber is connected via a pressure transfer line to the pressure measuring chamber of the pressure measuring transducer; the pressure sensor is located in the pressure measuring chamber; the pressure measuring chamber is arranged remotely from the measuring location.
In such case, it is of special importance for the accuracy of measurement of the pressure measuring transducer that the pressure transfer liquid contains as few foreign molecules dissolved in gaseous or liquid form as possible. Gases and/or liquids dissolved in the pressure transfer liquid acting on the isolating diaphragm, can, under certain circumstances, especially at high temperatures and/or low pressures, lead to gas bubbles forming very suddenly in the liquid, which drastically changes the transfer behavior of the pressure transfer liquid. Depending on the amount of trapped foreign molecules, this results, in certain circumstances, in considerable temperature dependent and/or pressure dependent measurement errors. For instance, water molecules contained in liquid form or in gaseous form as steam in the liquid are especially common and problematic. Due to its vapor pressure increasing exponentially with temperature, water forms steam bubbles at relatively low temperatures very suddenly under certain circumstances.
Currently, to remove foreign molecules dissolved in the pressure transfer liquid, the pressure transfer liquid is regularly pretreated in a vacuum distillation method, before the liquid is introduced, as a rule under vacuum conditions, into the pressure measuring transducer.
Vacuum distillation methods have the disadvantage that the distillation temperature is limited by the thermal stability of the pressure transfer liquid.
Additionally, through the vacuum distillation method, an increasing portion of additional components is withdrawn from the liquid with rising distillation temperature. This unavoidably leads to an increase in the viscosity of the liquid, which is disadvantageous for low loss pressure transfer; the increase in the viscosity of the liquid can likewise only be counteracted by limiting the distillation temperature.
Due to limitations for the distillation temperature, a residual content of foreign molecules, which cannot be made smaller, remains dissolved in the pressure transfer liquid even after the distillation method.
Foreign molecules, such as e.g. water and air are, however, not only contained in the pressure transfer liquid provided for filling, but, can also cling on the inner walls of the measuring transducer to be filled, and find their way into the pressure transfer liquid from there. In order to decrease the number of foreign molecules clinging on the inner walls, the internal spaces are preferably baked before being filled; the internal spaces are heated at high temperature under vacuum for a short time; this loosens the clinging foreign molecules from the inner wall and then they are sucked out via the applied vacuum. However, a residual content of foreign molecules clinging on the inner walls also remains here; this residual content can be released in dissolved form into the pressure transfer liquid after the filling.
It is an object of the invention to provide a method for reducing content of foreign molecules in gaseous or liquid form dissolved in a pressure transfer liquid of a pressure measuring transducer
For this, the invention resides in a method for reducing content of foreign molecules in gaseous or liquid form dissolved in a pressure transfer liquid of a pressure measuring transducer,
In a preferred embodiment, the adsorptive bodies comprise zeolite.
Additionally, the invention includes a first embodiment of the method of the invention, wherein
Additionally, the invention includes a second embodiment of the method of the invention, wherein
Additionally, the invention comprises a third embodiment of the method of the invention, wherein
Additionally, the invention includes a fourth embodiment of the method of the invention, wherein
Additionally, the invention includes an embodiment of the third or the fourth embodiment, wherein the pressure transfer liquid is pretreated in a pretreatment method before filling the pressure measuring transducer with this liquid, in which pretreatment method:
Additionally, the invention comprises a pressure measuring transducer manufactured with the method of the invention, wherein the pressure measuring transducer has
Additionally, the invention comprises a pressure measuring transducer manufactured with the method of the invention, wherein the pressure measuring transducer has
In a further development of the last mentioned pressure measuring transducer, this has
Additionally, the invention comprises a pressure measuring transducer manufactured with the method of the invention, wherein the pressure measuring transducer has
In a first further development of the last mentioned pressure measuring transducer, one of the adsorptive bodies is an adsorptive body that is arranged in a hollow space, which is closed to the environment, that adjoins the pressure receiving chamber on its side lying opposite the isolating diaphragm, and that is embodied as a diaphragm bed for the isolating diaphragm.
In a second further development of the last named pressure measuring transducer, one of the adsorptive bodies is a tubular adsorptive body inserted in a line connecting the pressure receiving chamber to the pressure measuring chamber. In a third further development of the last named pressure measuring transducer, one of the adsorptive bodies is a displacement body inserted in the inner space of the pressure measuring transducer.
In a further development of the last named further development, the adsorptive body inserted as a displacement body is provided with electrical connections, via which a capacitance of the adsorptive body dependent on the state of the pressure transfer liquid soaking the adsorptive body can be measured by means of a capacitance measuring circuit connected thereto.
The method of the invention offers the advantage that the adsorptive bodies permanently remove the foreign molecules from the liquid; the foreign molecules are then no longer available for forming gas bubbles in the pressure measuring transducer.
The invention and further advantages will now be explained in greater detail based on the figures of the drawing, in which seven examples of embodiments are presented. Equal elements are provided with the equal reference characters in the figures. The figures of the drawing show as follows:
The pressure measuring transducer has a pressure transfer means having a pressure receiving chamber 3 closed by an isolating diaphragm 1. Pressure receiving chamber 3 is connected via a pressure transfer line 5 to a pressure measuring chamber 9 enclosed in a measuring transducer housing 7 located remotely from the pressure receiving chamber 3. In pressure measuring chamber 9, a pressure sensor 11, e.g. a semiconductor sensor, is arranged, which in measurement operation serves to measure an external pressure p acting on isolating diaphragm 1. For this, the inner space of the pressure measuring transducer formed by pressure receiving chamber 3, line 5 and pressure measuring chamber 9 is filled with a pressure transfer liquid, which in measurement operation serves to transfer the pressure p acting on isolating diaphragm 1 to pressure measuring chamber 9 and therewith to pressure sensor 11 located therein.
The pressure measuring transducer shown here represents only one example of a pressure measuring transducer, in which the invention is applicable. The invention is also applicable in connection with differently embodied pressure measuring transducers, which have an inner space filled with a pressure transfer liquid for transmitting a pressure to be registered metrologically to a pressure sensor arranged in the inner space.
The pressure transfer liquid is preferably a hydraulic liquid, e.g. a silicone oil, which is as incompressible as possible and which has a thermal coefficient of expansion, which is as small as possible.
Ordinarily marketed, pressure transfer liquids always contain a residual content of foreign molecules, especially water and air, in gaseous or liquid form dissolved therein. This residual content cannot be withdrawn from the liquid due to limitations of the distillation temperature even using a vacuum distillation method. In addition to water molecules, foreign molecules especially well soluble in silicone oils, such as e.g. oxygen, nitrogen and carbon dioxide play an important role here.
According to the invention, the content of foreign molecules in liquid or gaseous form dissolved in the pressure transfer liquid is reduced by bringing the liquid into contact with at least one adsorptive body 13 and, via this contact, foreign molecules dissolved in the liquid are bound to the adsorptive bodies 13 by adsorption. In this way, the adsorbed foreign molecules are withdrawn from the liquid and cannot form gas bubbles in the liquid in measurement operation.
The adsorptive bodies 13 preferably comprise zeolite. Zeolites are alumino-silicates, whose crystal lattice has a cage structure with numerous hollow spaces, which are accessible from all sides through pores.
Zeolites are obtainable both as a granular material as well as sintered formed bodies with a relatively freely selectable form. Since zeolites can only adsorb molecules, which can pass through their pores, they are suited especially well to selectively adsorb the disturbing foreign molecules, especially water, oxygen, nitrogen and carbon dioxide, contained in pressure transfer liquids. The adsorption takes place at room temperature so that this method is also suitable for liquids, which have very low thermal stability. Since the exclusively targeted, low molecular weight materials are removed from the liquid by adsorptive bodies 13, the targeted disturbing content of low molecular weight foreign molecules in pressure transfer liquids is reduced through this method without the viscosity of the liquid being detectably increased.
The method of the invention can be executed at different stages of the manufacturing process of the pressure measuring transducer.
In a first variant of the invention, the method is executed in the form of a liquid pretreatment method, to which the liquid is subjected before it is filled into the inner space of the pressure measuring transducer. This variant can be executed, for example, with the apparatus illustrated in
In such case, the pressure transfer liquid is filled into a supply container 15, in which adsorptive bodies 13 are preferably introduced in the form of a fine grained granulate. Preferably arranged in supply container 15 is mixer 17, which mixes the content of supply container 15 at the beginning of the pretreatment method and thereby effects a uniform distribution of the adsorptive bodies 13 in the liquid as indicated in
Preferably, an inner space 19 of supply container 15 remaining above the liquid level is evacuated via a vacuum pump 21 connected thereto via a valve V. In this way, foreign molecules emerging upwardly from the liquid are sucked out and an intake of foreign molecules from the environment is prevented.
Adsorptive bodies 13 remove foreign molecules from the liquid until either dissolved foreign molecules are no longer contained in the liquid or the loading capacity of adsorptive bodies 13 is reached.
The adsorbed foreign molecules are then bound to adsorptive bodies 13 and can accordingly be separated from the liquid together with adsorptive bodies 13. For this, the force of gravity is preferably utilized, which causes the adsorptive bodies 13 to sink to the floor of supply container 15. After adsorptive bodies 13 sink, a layer of adsorptive bodies 13 is located on the floor of supply container 15. The height H of the layer, as indicated by the dashed line in
Alternatively, a filter 27 can be provided in front of outflow 25, in order to prevent adsorptive bodies 13 from leaving supply container 15.
For filling the inner space of the pressure measuring transducer, outflow 25 is preferably connected directly to a filling apparatus (here shown schematically only), via which the liquid freed from the adsorbed foreign molecules is introduced into the inner space of the pressure measuring transducer. The pressure measuring transducer has for this a filling opening 29, via which the inner space to be filled is accessible.
A number of different filling apparatuses and filling methods known from the state of the art can be applied for the filling. Preferably, as shown in
To the extent that adsorptive particles 13 are applied, which have a particle size, which is small compared to the dimensions of the inner space of the pressure measuring transducer to be filled, a second variant of the invention can alternatively be applied, in which the inner space is filled with liquid containing adsorptive bodies 13. In this case, the inner space is preferably directly filled with the liquid containing adsorptive bodies 13 after the introduction of the adsorptive bodies 13 into the liquid. Also, a homogeneous distribution of adsorptive bodies 13 is preferably effected here by mixer 17. In contrast to the earlier described method, the filling here, however, occurs during the mixing procedure or immediately thereafter, so that the adsorptive bodies 13 have no time to settle on the floor of supply container 15. Filter 27 is not used in this case.
The same end result as regards the filled pressure measuring transducer is alternatively also achievable by introducing the adsorptive bodies 13 into the inner space before filling the pressure measuring transducer. The pressure measuring transducer is then subsequently filled with pressure transfer liquid. Also in such case, the adsorptive bodies 13 must naturally have a particle size small compared to the dimensions of the inner space to be filled, in order to assure unhindered pressure transfer through the liquid. Also in this way, adsorptive bodies 13 get into the liquid and bind foreign molecules dissolved in the liquid by adsorption.
In such case, the liquid, with which the inner space of the pressure measuring transducer containing adsorptive bodies 13 is filled, can naturally be supplementally subjected to the pretreatment method described above, in which the liquid, before it is used for filling, is mixed with adsorptive bodies 13, which are separated together with the adsorbed foreign molecules from the liquid before the filling. Then, the pressure measuring transducer is filled with the liquid, which is pretreated using the pretreatment method in this way and is free of adsorptive bodies. This variant offers the advantage that the content of foreign molecules dissolved in the liquid clearly already reduced by the pretreatment method is still further reduced by adsorptive bodies 13 contained in the pressure measuring transducer.
Alternatively to the introduction of adsorptive bodies 13 into the inner space of the pressure measuring transducer, adsorptive bodies 13 can also be introduced in one or more hollow spaces, serving here to accommodate adsorptive bodies likewise formed as granulate and connected to the inner space of the pressure measuring transducer to be filled with the pressure transfer liquid. The hollow spaces are closed to the inner space by a partition, such as e.g. a metal grate, which is permeable to the liquid but is impermeable to adsorptive bodies 13. The foreign molecules dissolved in the liquid located in the hollow space(s) are also here bound to adsorptive bodies 13 through adsorption, and are thus permanently removed from the liquid.
Since adsorptive bodies 13, in such case, remain in the hollow spaces, they bring about no degradation of the pressure transmitting properties of the liquid in the inner space. This variant is also especially applicable in pressure measuring transducers, which have inner spaces with small dimensions. Since adsorptive particles 13 do not penetrate into the adjoining inner space, the particle size of adsorptive bodies 13 here does not need to be small compared to the dimensions of the inner space of the pressure transducer.
The hollow space(s) can be arranged at different locations in the pressure measuring transducer.
Thus, for example, as shown in
The hollow spaces 57, 59 containing adsorptive bodies 13 are arranged in the interior of a connection piece 61 of the pressure measuring transducer. Connection piece 61 connects measuring transducer housing 7 to the upstream pressure transfer means and a portion of line 5 extends through connection piece 61.
Hollow space 57 directly borders the portion of line 5 and is separated from this by a partition 63, which is permeable to the liquid and impermeable to adsorptive bodies 13.
Hollow space 59 is integrated in a region of filling opening 29 adjoining line 5.
This offers the advantage that insert 69 with adsorptive bodies 13 can be introduced into filling opening 29 before filling the inner space of the pressure transfer means. Therewith the option is provided to heat the inner space and the adsorptive bodies 13 under vacuum before the filling, and then to fill the inner space under vacuum via insert 69. In this way, adsorptive bodies 13 have their maximum adsorption capacity at the beginning of the filling procedure. A further advantage is that the entire amount of liquid to be filled flows over adsorptive bodies 13 into the inner space, and, in such case, foreign molecules dissolved in the liquid are adsorbed by the adsorptive bodies 13.
Adsorptive bodies 13, which remain in the inner space of the pressure measuring transducer, respectively in the adjoining hollow spaces 43, 49, 57, 59, during measurement operation offer the additional advantage that they can also adsorb foreign molecules that initially cling to the inner walls of the inner space or hollow spaces 43, 49, 57, 59 and, under certain circumstances, only detach after a long time and then enter the liquid.
Alternatively or supplementally to the granular adsorptive bodies 13 described above, also individual adsorptive bodies with greater dimensions, e.g. adsorptive bodies embodied as one piece, molded parts, can be applied in the pressure measuring transducer. These adsorptive bodies are preferably sintered zeolite inlays, which, for their respective location of use in the inner space of the pressure measuring transducer and/or in an adjoining hollow space opened to the inner space but otherwise outwardly closed, can be manufactured to accurately fit in the pressure measuring transducer.
Additionally provided in
Preferably, adsorptive body 79 is provided with electrical connections 81, via which electrical capacitance of adsorptive body 79 can be measured by a capacitance measuring circuit 83 connected thereto. Since adsorptive bodies 79 have an extremely porous structure, adsorptive body 79 is saturated after the filling of the pressure transfer liquid. Correspondingly the capacitance of adsorptive body 79 is a measure for the capacitance of the pressure transfer liquid, which, in turn, delivers information concerning the state of the pressure transfer liquid. Correspondingly, the state of the pressure transfer liquid can be monitored based on the measured capacitance of adsorptive body 79.
Via the capacitance measurement, it is recognizable directly after the filling of the pressure measuring transducer whether, as regards the adsorption of the foreign molecules by adsorptive body 79, a stable equilibrium state has been achieved. Moreover, based on the capacitance measurement, an increase in the content of foreign molecules, especially air and moisture, arising in the pressure transfer liquid in the area of adsorptive body 79 during the operational period of the pressure measuring transducer can be detected. Such a subsequent increase can be caused e.g. by damage to isolating diaphragm 1 or an unsealed site in the pressure measuring transducer.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102010003708.7 | Apr 2010 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/052908 | 2/28/2011 | WO | 00 | 9/24/2012 |
