Patent Application
20100078581
This invention relates to fluid control apparatus and, particularly, to optical overfill probes that detect when fluid being transferred into a container has exceeded a desired maximum level and provide a signal used to prevent overfill of the container.
In the art of fluid transfer control, particularly as it applies to the petroleum industry, one of the more common control devices is an overfill sensor for determining when the fluid being transferred into a container, such as a petroleum tanker truck compartment, has exceeded a predetermined level. An output signal from such a probe indicates an overfill risk, and may be used by a fluid transfer controller to discontinue fluid flow into the compartment. In this way, overfilling of the compartment, which is particularly hazardous when dealing with flammable liquids such as gasoline, can be avoided. Such a probe 100 is shown schematically in
One type of overfill probe that is known in the petrochemical industry makes use of an optical signal generated by a light source, such as a light emitting diode, which signal is coupled into a medium having a relatively high index of refraction, such as a glass or translucent plastic. This medium is specially shaped and commonly referred to as a “prism.” The shape of the prism provides multiple surfaces at the interface between the prism material and an external environment, and these surfaces are aligned so as to cause an internal reflection of the optical signal coupled into the prism when the prism is surrounded by air. This internal reflection directs the optical signal toward a photodetector that generates an output signal which indicates that the optical signal is being detected.
A schematic illustration of this prior art probe design 200 is shown in
The prism 204 of
When the fluid 104 in the compartment 102 rises high enough to contact a prism surface at a location where the optical signal is incident, the prism/air interface becomes a prism/fluid interface, and the fluid has an index of refraction much closer to the prism material than does air. According to Snell's law of refraction, (well-known in the art of optical design) the angle of incidence of the optical signal at the prism/fluid interface now results in the transmission of the optical signal through the interface due to the similarity of the relative indices of refraction. As a result, the signal is no longer detected by photodetector 208, and the corresponding change in the photodetector output signal is detected by conventional signal processing electronics (not shown in
Overfill probes of this type may be subjected to a particularly harsh environment. If the compartment contains gasoline or other fuels or harsh chemicals, the probe may be subjected to corrosive vapors. In addition, operating conditions for the compartments often include a wide range of temperature changes. Such changes can put a variety of stresses on the probe that could ultimately lead to its failure. A failure of the probe can cause a false overfill signal to be generated, which prevents fluid from being loaded into the compartment, despite the fact that the compartment may be empty. If this happens, it may be necessary to clean or replace the probe in the field resulting in significant downtime.
In accordance with the present invention, a fluid overfill probe is provided that avoids failure due to physical stresses resulting from thermal expansion of components of the probe. In particular, an internal structure is used that provides the delicate components of the probe with a gap that separates them from surrounding solid materials. The probe includes a fluid level detector that detects when fluid in a container has reached a predetermined level and generates an electrical output signal indicative thereof. Electrical circuit components mounted to a circuit board process the electrical output signal. A housing surrounds the electrical circuit components and the circuit board is fixed in position relative to the housing. However, the mounting of the circuit board is such that a gap exists between the circuit components and surrounding solid materials in the housing.
The fluid level detector may use a light source that generates an optical signal and a photodetector that detects the optical signal and generates a corresponding electrical output signal. The optical signal is coupled by the light source into a prism that has surfaces oriented such that the optical signal is internally reflected within the prism toward the photodetector when no fluid is in contact with the prism surfaces. However, when the fluid level in a container being monitored reaches the prism, and the fluid makes contact with the prism surfaces, the change in relative index of refraction between the interior and exterior of the prism results in the optical signal exiting the prism prior to reaching the photodetector. As such, the electrical signal output by the photodetector also changes, and this signal change is used to take whatever action is desired in response to the fluid level reaching the probe prism.
To create a desired gap, free of solid material, a receptacle may be used that is located within the housing and surrounds the circuit board. The receptacle may be a tube that is fixed in position relative to the housing, for example, by a potting material that surrounds the tube and cures to a rigid state. The circuit board may be positioned adjacent to an inner surface of the tube along its edges. In such a configuration, the edges of the circuit board may make contact with the tube, preventing movement of the circuit board within the tube, but the shape of the tube is such that the desired gap exists between the inner tube surface and the components on the circuit board. Thus, as solid materials change dimension within the probe due to thermal expansion and contraction, there is no physical stress applied to the electrical components by solid materials in the probe housing that surround the circuit board, as the gap is made sufficiently large as to prevent any such contact for the full predetermined operating temperature range of the probe.
In one embodiment of the invention, the prism, light source and photodetector are attached, in a predetermined orientation, to a prism holder located within the housing. The prism holder may be secured to the tube and, in one embodiment, the tube and the prism holder are integral parts of a single structure. Thus, the proper orientation of the prism is maintained with regard to the light source and the photodetector, and the circuit board, which connects to the light source and photodetector, extends into the tube, where it is secured in a configuration that leaves a gap adjacent to the circuit components.
Shown in
The light source 316 and photodetector 318 are both electrically connected to printed circuit board (PCB) 320, which is located within the probe. The PCB 320 supports electrical components used in controlling the operation of the probe, and is connected to external circuitry (not shown) via electrical wires 322. In the present embodiment, the components are surface-mounted to the PCB 320, that is, they rely on a solder connection between them and the board for both electrical conductivity and mechanical attachment. This type of mounting has certain advantages, such as a smaller form factor, but also results in a more fragile arrangement due to the direct solder connection between the components and the PCB, and the lack of a protective housing, as is typically present in lead-mounted packages. Thus, in the present embodiment, the PCB 320 is held firmly in the housing to protect it from shock, vibration, fluids and outside contaminants.
The mounting of the PCB 320 is such that it is separated from the housing 312 of the probe by several different interior layers. These layers together combine to protect the PCB in the housing, and are specially arranged to give the PCB 320 a certain level of resistance to thermal stress. Shown in
Surrounding the silicone-coated PCB is a tube 326 that is sufficiently rigid to maintain its shape under minor external pressure. The tube 326 has an interior diameter that is selected relative to the size of the PCB so that the PCB makes a snug fit within the tube 326. The tube 326 is integral with a prism holder 325 (
In one embodiment, the tube and the PCB are sized relative to each other so that the board makes a loose interference fit with the tube. In another embodiment, the inner surface of the tube has grooves that receive the edges of the board and thereby retain the board in a predetermined position and orientation within the tube. This latter embodiment simplifies aligning the board with the tube so that the light source and photodetector engage properly in their intended holes in the prism holder 325. This is a particularly useful feature when the tube is an opaque material, as it facilitates proper alignment of the PCB 320 in the tube 326. As shown in
As shown in
The formation of air gaps 328 within the body of the probe has been found to prevent damage to the components on the PCB that may otherwise be caused by physical stress induced by thermal expansion of components in the probe. In particular, the air gaps 328 provide a separation between the electrical components on the PCB 320 and the tube 326. Thus, thermal expansion and contraction of the protective layer 324 and/or of the tube 326 and potting material 330 does not create physical stress on the circuit components. In contrast, if the protective layer surrounding the electrical components were directly adjacent to, or in direct contact with, a surrounding material, such as a potting compound, the thermal expansion and contraction of the solid materials in the probe could result in physical stress on the circuit components leading to their failure. Since overfill probes such as that described in the present embodiment are intended for use across a wide temperature range (as much as a one hundred degree Celsius temperature change), the present invention provides an overfill probe that prevents thermal stresses that might otherwise result in probe failure.
The present embodiment uses a tube to house the PCB board in a manner that provides gaps around the circuit components. However, those skilled in the art will recognize that a variety of different techniques may be used to provide such gaps. The effect of the tube is to provide a physical isolation between solid materials in the probe body that, if not so isolated, could create physical stresses on circuit components due to relative thermal expansions and contractions. Other means of providing such isolation are considered to be anticipated by the principle of the present invention. Moreover, while the gaps are referred to herein as “air gaps,” the invention contemplates any gap that is free of solid material. Such a gap may be occupied by different gases than air, or even certain fluids having appropriate thermal characteristics.
While the invention has been shown and described with regard to an exemplary embodiment thereof, it will be recognized by those skilled in the art that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
