Pressure sensing device with moisture filter

Details Pressure sensing device with moisture filter Pressure sensing device with moisture filter

2002-05-06

2004-01-13

Oen, William (Department: 2855)

Measuring and testing

Fluid pressure gauge

Diaphragm

Reexamination Certificate

active

06675654

ABSTRACT:

The invention generally relates to housings or devices with moisture sensitive components, such as e.g. housings with moisture sensitive electronic circuitry or measuring devices with moisture sensitive sensors. This group of devices especially comprises relative pressure sensors.
Relative pressure sensors can be used to measure pressures of media, for example of liquids, gases or vapors, with respect to the currently prevailing atmospheric or ambient pressure, this atmospheric or ambient pressure therefore serving as a reference pressure. In this context, the humidity of the reference air has proven to be a problem, since the humidity can penetrate into the sensor via a reference pressure line and can condense out at temperatures which lie below the dew point. Therefore, there have been extensive efforts to prevent the moisture from penetrating into the sensor.
By way of example, Japanese patent application No. 07110364 has disclosed a capacitive relative pressure sensor with a base body and a diaphragm which is connected, along its edge region, in a pressure-tight manner to the base body, so as to form a reference pressure chamber. The reference air is introduced into the reference pressure chamber through a reference pressure line and a bore. In the reference pressure line there is a water absorber which is intended to dry the reference air.
This solution is unsatisfactory to the extent that the moisture content in the absorber is enriched. When the absorber is saturated, or when the temperature rises and some of the bound water can be desorbed again, moisture passes into the sensor.
The European patent application which bears the publication number 974 825 A2 follows a different approach. The structure of the relative pressure sensor is generally as described above, but a hydrophobic filter element is used instead of the absorber, this element being held at a temperature which the temperature inside the sensor never falls below and is preferably significantly colder than the temperatures in the interior of the sensor. In this way, moisture condenses out at the filter element when the filter temperature falls below the dew point of the warmer ambient air. Therefore, the air which enters the interior of the sensor only has a water content which corresponds to a relative atmospheric humidity of 100% at the filter temperature. However, since the temperature inside the sensor never falls below the filter temperature and is generally in fact above the filter temperature, there is no possibility of the moisture condensing in the interior of the sensor, since the dew point is not reached.
The required cooling of the filter element is achieved, for example in sensors for the food industry, by keeping the filter element in thermal contact with the cold process medium via the sensor housing. The device described is advantageous to the extent that the saturation problems which arise with an absorber do not occur.
On the other hand, the need to control the temperature of the filter element entails significant design limitations which are unacceptable for certain applications.
Moreover, the user is subject to restrictions in terms of operation and maintenance of the pressure-measuring device. By way of example, if the filter element were to be heated, during cleaning operations, to temperatures which lie above the normal operating temperature in the interior of the sensor, it would be possible for air with a high level of humidity to penetrate into the sensor, and this moisture could then condense at normal operating temperature.
For similar reasons, the above filter elements or absorbers are unsuitable for other devices with moisture sensitive components which require a gas exchange with the ambient for the purpose of cooling or pressure adjustment. This especially applies, if changes of the ambient temperature with associated changes of the reative humidity must be expected. Such moisture sensitive components may be, for instance, electronic circuits.
The present invention is therefore based on the object of providing a device with a filter element which overcomes the problems described.
According to the invention, the object is achieved by the relative pressure sensor according to the independent patent claim 1, and by the device according to independent patent claim 8. Further advantages and aspects of the invention are given in the dependent claims, the description and the drawings.
The device comprises a housing which defines a chamber in its interior which comprises at least one aperture through which the chamber is in fluid communication with the environment of the housing by means of a gas exchange path; and a filter element disposed in the gas exchange path, said the filter element comprising a hydrophobic and/or hydrophobicized, nanoporous material.
An especially preferred embodiment the chamber comprises an inlet aperture and an outlet aperture, through which the chamber is in fluid communication with the environment of the chamber by means of respective gas exchange paths, wherein a respective filter element is provided in both gas exchange paths, said filterelements comprising a hydrophobic and/or hydrophobicized, nanoporous material
The device is especially suitble as housing for electronic circuitry. Optionally, an airflow can be guided through the chamber by means of a conventional ventilator, therby enabling an effective heat exchange.
The relative pressure sensor according to the invention for capturing a measured pressure with respect to a reference pressure comprises:
a sensor element having
a base body and
a measurement diaphragm which, along its edge region, is connected in a pressure-tight manner to the base body so as to form a reference pressure chamber,
the measurement diaphragm having a first diaphragm surface, which faces away from the reference pressure chamber and can be exposed to the measured pressure, and a second diaphragm surface, which faces the reference pressure chamber;
a reference pressure path which extends between a surface which can be exposed to the reference pressure and an opening in the reference pressure chamber, with the result that the second diaphragm surface can be exposed to the reference pressure; and
a filter element which is arranged in the reference pressure path;
wherein
the filter element comprises a hydrophobic and/or hydrophobicized, nanoporous material.
The nanoporous material is preferably arranged as a layer on a porous support material or is embedded in the matrix of a porous support material, the support material serving in particular to ensure the required mechanical stability.
The nanoporous material preferably comprises an inorganic material, in particular a ceramic material, Al
2
O
3
or TiO
2
being preferred. ZrO2, SiO
2
, aluminosilicates, aluminum beryllium silicates; apatite, cordierite, mullite, zeolite, SiC and Si
3
N
4
, carbon, Vycor Glass and their mixtures are in principle also suitable.
In this context, the term “nanoporous” refers to a pore size distribution whose maximum, based on the pore frequency, lies at a pore diameter of less than 4 nm, preferably less than 2.5 nm, more preferably between 0.4 and 2 nm, even more preferably between 0.5 and 1.5 nm, and particularly preferably between 0.7 and 1 nm. In a particularly preferred exemplary embodiment, the distribution maximum is approximately 0.9 nm.
The preferred production process using the sol gel process can be used to achieve a sufficiently narrow pore size distribution which ensures a uniform filter action. The maximum pore diameter should preferably be no more than 10 nm, more preferably no more than 5 nm, even more preferably no more than 2 nm.
The terms microporous and mesoporous are also customarily used in filtration technology to describe layers with pore sizes in the nanometer range. According to this technology, what are known as mesoporous layers have pore diameters of between 2 nm and 50 nm, while what are known as microporous layers have pore diameters of less than 2 nm. In the context of these definitions, the nanoporous material used

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