Fluid overfill probe with thermal stress prevention

A fluid overfill probe is resistant to failure caused by physical stresses resulting from thermal expansion of probe components. A fluid level detector is connected to circuit components that are mounted on a circuit board located in a housing of the probe. The circuit board is located within a tube that is positioned within, and secured to the housing, and the circuit board is secured to an inner surface of the tube along its edges. The tube has a shape and rigidity sufficient to maintain a gap between the circuit components and the inner surface of the tube such that thermal expansion of probe components result in no physical stress to the circuit components.

FIELD OF THE INVENTION

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.

BACKGROUND OF THE INVENTION

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 probe100is shown schematically inFIG. 1, which shows a partial cross-section diagram of a tanker truck compartment102, which is being filled with a fluid104. The probe100is connected by wires108to an overfill prevention circuit, which is not shown inFIG. 1. Typically, a well106is formed around the top of probe100in order to contain any fluid104that might leak out around the probe100.

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 design200is shown inFIG. 2. In the plane of the optical signal path202, the prism204has a triangular cross section. The optical signal is generated by light source206. When the prism204is surrounded by air, the optical signal is reflected at two interfaces between the prism material and the surrounding air, and redirected toward photodetector208following the path202. The photodetector208generates an electrical output signal that indicates that the optical signal is being detected. This optical signal is directed to components on a printed circuit board that is located in a probe housing210and is surrounded by a potting material212.

The prism204ofFIG. 2is mounted in a prism holder216that has properly positioned holes for receiving each of the light source206and the photodetector208, and a partial cutaway region for receiving the prism204. The prism holder216may comprise an elastomer seal and may have a potting compound218adjacent to it to help seal the internal components from the external environment. The prism holder216helps to maintain the prism, light source and photodetector in an appropriate relative alignment.

When the fluid104in the compartment102rises 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 photodetector208, and the corresponding change in the photodetector output signal is detected by conventional signal processing electronics (not shown inFIG. 2) and used as an overfill warning indicating that loading of the compartment102should be discontinued.

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.

SUMMARY OF THE INVENTION

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.

DETAILED DESCRIPTION

Shown inFIG. 3Ais a cross-sectional side view of an overfill probe310according to an exemplary embodiment of the present invention. The probe components are encased in a housing312of a durable material, preferably a light metal such as aluminum. A prism314is located at a first end of the probe that faces the liquid being monitored in a fluid compartment. Adjacent to the prism is light emitting diode (LED) light source316and a photodetector318. As discussed above, an optical signal generated by the light source316is coupled into the prism314in the direction of the prism surfaces that are contacted by the fluid when the compartment is full. Thus, the signal from the light source is internally reflected and detected by the photodetector when the fluid level is below the position of the probe, due to the large difference in the indexes of refraction of the prism material and the surrounding air. If the fluid level reaches the probe, the refractive index difference is much lower, and the light exits the prism before reaching the photodetector. The change in the output signal from the photodetector is then used by an accompanying overfill control circuit to discontinue filling of the compartment.

The light source316and photodetector318are both electrically connected to printed circuit board (PCB)320, which is located within the probe. The PCB320supports electrical components used in controlling the operation of the probe, and is connected to external circuitry (not shown) via electrical wires322. In the present embodiment, the components are surface-mounted to the PCB320, 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 PCB320is held firmly in the housing to protect it from shock, vibration, fluids and outside contaminants.

The mounting of the PCB320is such that it is separated from the housing312of 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 PCB320a certain level of resistance to thermal stress. Shown inFIG. 3Bis a cross-sectional view of the probe taken along sectional line3B-3B ofFIG. 3A. The PCB320is shown at the center of housing312, and is covered by a layer of protective material324that, in the present embodiment, is silicone. This layer may be poured onto the assembled PCB structure as a viscous liquid that subsequently hardens, and protects the components while providing a conformal coating sufficient to satisfy various regulatory standards when the probe is used in conjunction with hazardous liquids.

Surrounding the silicone-coated PCB is a tube326that is sufficiently rigid to maintain its shape under minor external pressure. The tube326has an interior diameter that is selected relative to the size of the PCB so that the PCB makes a snug fit within the tube326. The tube326is integral with a prism holder325(FIG. 3A) in which the prism314, light source316and photodetector318are mounted in a manner similar as discussed above with regard toFIG. 2. The tube326and the prism holder326may be separate components that are secured together or may be one integral component. In the present embodiment, the tube326and prism holder325comprise a single piece of black nitrile. Thus, the PCB320is located within the tube326and the light source316and photodetector318extend from the PCB320into the prism holder325.

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 holder325. This is a particularly useful feature when the tube is an opaque material, as it facilitates proper alignment of the PCB320in the tube326. As shown inFIG. 3A, a layer327of potting material may also be located adjacent to the prism holder that helps isolate the interior of the probe from the external environment. The portion of the probe extending beyond the potting material layer327is encompassed by a prism protector that prevents damage to the prism while still allowing fluid to make contact with the prism surface. At the opposite end of the tube326a seal331may also be used to close the tube end. The seal may be, for example, a solid or a curable liquid.

As shown inFIG. 3B, the width of the PCB itself is the maximum radial dimension of the PCB assembly when located in the tube326. Thus, while the edges of the PCB320may make contact with an inner surface of the tube326, an air gap328exists on either side of the PCB between the silicone-coated components and the inner tube surface. These air gaps328are created by the rigidity of the tube326that allows it to maintain its shape during assembly of the probe. Once the PCB320is located in the tube326, the tube is sealed, the prism holder being secured to the tube if it is not part of the same integral structure, and the sealing material331being applied at the opposite end. Thus, electrical wires322extend through the seal331at one end and the light source and photodetector extending into prism holder326at the other. The sealed tube is then placed within the housing312, and the space between the tube and the housing is filled with a potting material330, such as a hard-curing epoxy. This secures the tube326relative to the housing312, preventing it from moving within the housing. The potting material may also be selected to protect the tube from external contaminants, vibration and shock.

The formation of air gaps328within 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 gaps328provide a separation between the electrical components on the PCB320and the tube326. Thus, thermal expansion and contraction of the protective layer324and/or of the tube326and potting material330does 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.