Measuring and testing – Fluid pressure gauge – With pressure and/or temperature compensation
Reexamination Certificate
1999-02-01
2001-11-06
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
With pressure and/or temperature compensation
Reexamination Certificate
active
06311561
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to the pressure sensing arts. In particulars it is directed to a pressure sensing device which is resistant to corrosive fluids and is easily assembled.
BACKGROUND OF THE INVENTION
Pressure transducers are used in a wide range of applications. In many cases, it is desirable to measure the pressure of fluid media which may be harmful or corrosive to the transducer material, such as water, fuel, oil, acids, bases, solvents, other chemicals, and corrosive gases. There are numerous high-volume applications where a media compatible pressure transducer is highly desired but not available in any currently available technology with satisfactory durability, performance, or price characteristics. There is a need for media compatible pressure sensor packages which have substantial performance and cost advantages over existing technologies and provide new capabilities not previously realized.
Pressure is one of the most commonly measured physical variables. While pressure measuring instruments have been available for many decades, the proliferation of inexpensive solid-state silicon pressure transducers has resulted in tremendous growth in the number and different types of applications of pressure transducers. The most common pressure transducers sensors are also inherently sensitive to temperature. A temperature rise causes the internal fluid to expand. Constrained by the steel diaphragm, the pressure of the fluid rises, producing a false pressure reading. This temperature sensitivity is typically corrected with external passive or active electronic components which add to the cost of the transducer. Fourth, the stainless steel material is not satisfactory for many media applications. Stainless steel will eventually corrode in certain environments with harsh acids and bases present. In some applications, such as in the semiconductor industry and biomedical applications, even if the steel is resistant to the chemical substance in question, minute trace amounts of steel or corrosion products released into the media cannot be tolerated. Also, steel housings add substantially to the weight and size of the transducers.
Solid-state silicon pressure sensors which are not specially packaged for media compatibility are only used with air or other inert gases. Because of the shortcomings of the steel packaged sensors and the conventional silicon sensors, other kinds of packages have been devised. One approach has been to limit media exposure to the more rugged portions of the silicon sensor, allowing the media to contact the silicon diaphragm while isolating the corrosionsensitive metal portions of the sensor. This has been most readily accomplished by allowing media to contact the backside of the silicon diaphragm only. Because differential pressure is often needed, many of these methods involve arranging two pressure sensors together so that the backsides of both are used to measure a differential pressure. U.S. Patents relating to this approach include U.S. Pat. Nos. 4,695,817; 4,763,098; 4,773,269; 4,222,277; 4,287,501; 4,023,562; and 4,790,192. These approaches provide some media compatibility improvements, but are of limited usefulness since silicon corrodes in some acid or base environments. These approaches may add substantially to the sensor cost (especially if two sensors are used for one measurement application), or may be impractical to manufacture and assemble due to the unusual component orientation, assembly, bonding, sealing, and electrical interconnection requirements. The complex assembly of some of these devices is apparent from even a casual examination of the patent drawings. Another approach to exposing the silicon diaphragm only while protecting the metal regions is described in U.S. Pat. Nos. 4,656,454 and 5,184,107. These devices employ an elastomeric seal which contacts the diaphragm and separates the diaphragm and metal interconnect regions. Again, this device provides some improvement over conventional silicon pressure sensors but the elastomeric material also has significant limitations in the chemical environments it can withstand.
Silicon pressure sensors have also been coated with a protective material, such as silicone gel or parylene, to protect the device. This approach is very limited in the types of media in which it is effective, and the coating can also affect the sensor performance. A rubber membrane diaphragm has been used instead of steel for media isolation with a fill fluid. The media compatibility of a rubber device is an improvement over bare silicon but is still limited. Molded diaphragms are disadvantageous from a manufacturing standpoint for the reason that it is difficult to obtain uniform thickness in mass production.
Only a relatively small subset of pressure sensors are designed to withstand exposure to corrosive chemicals for long periods of time. These “media compatible” pressure sensors are protected by a stainless steel housing, and are more expensive than their non-media compatible counterparts, which are typically made from plastic. A stainless steel diaphragm is typically used in the media compatible sensors to provide a barrier between the pressure sensing element and the media. The volume between the steel diaphragm and the pressure sensing element is filled with a fluid, such as silicone oil. When the steel diaphragm deflects due to an externally applied pressure, the fluid transmits that pressure to the internal pressure sensing element, which undergoes a resistance or capacitance change proportional to the pressure.
SUMMARY OF THE INVENTION
In accordance with the present invention, a pressure sensing device is provided. The device includes a housing which has a fluid channel defined therethrough. A circuit board is mounted within the channel and a pressure sensor is attached to the circuit board. The pressure sensor generates a signal in response to a sensed pressure and communicates the signal to the circuit board. A sealing member is disposed on the circuit board and encloses the pressure sensor. A diaphragm is disposed on the sealing member and defines an enclosed chamber between the diaphragm and the circuit board. The pressure sensor is within the enclosed chamber. A pressure transmissive fluid fills the enclosed chamber such that a pressure applied to the diaphragm is transmitted by the pressure transmissive fluid and sensed by the pressure sensor. The circuit board, the sealing member and the diaphragm are compressively held together within the housing such that the enclosed chamber is sealed.
In accordance with another aspect of the present invention, a pressure sensing package for measuring pressure of a medium is provided. A pressure sensing device includes a circuit board, a pressure sensor, a sealing member, a diaphragm, and a pressure transmissive fluid. The pressure sensor is attached and electrically connected to the circuit board. The sealing member is disposed on the circuit board and surrounds the pressure sensor. The diaphragm is disposed on the sealing member and covers the pressure sensor such that a chamber is defined by the diaphragm, the sealing member and the circuit board. The pressure transmissive fluid fills the chamber such that a pressure exerted on the diaphragm is transmitted through the pressure transmissive fluid and sensed by the pressure sensor. A housing has a channel defined therein which allows a medium to flow into the housing. The pressure sensing device is mounted within the channel of the housing such that only the diaphragm is exposed to contact the medium. The pressure sensing device is compressively held together by the housing.
In accordance with another aspect of the present invention, a method of assembling a pressure sensor package is provided. A housing is provided and a circuit board is mounted within the housing. The circuit board has a pressure sensor attached thereon. A sealing member is placed on the circuit board. A diaphragm is placed on a sealing member where the diaphragm, the sealing member and the circuit board form
Bang Christopher A.
Just Marcus S.
Nagy Michael L.
Yang Xiaofeng
Oen William
Rosemount Aerospace Inc.
Thompson Hine LLP
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