BACKGROUND

In recent years, with emergence of new and advanced technologies and data-based applications, the processing need for data has been growing exponentially. To meet the growing need for data processing and computing, computer chip manufacturers have developed faster processors that consume more power, and server manufactures are packing more processors and components in servers to reduce the overall footprint of servers in a data center and the associated operating cost. The packing density of electronic components in new server designs has grown to the extent that the traditional air cooling process, where cold air stream is blown over the fins of heat sink attached to the components, has reached its limit because of low thermal conductivity of air.

To improve the efficiency of heat dissipation from the sever components, server manufactures and data center operators have started using liquids to transfer heat from these electronic components to external environments (e.g., chillers and ambient). High thermal conductivity of liquids such as water, coolant, or organic liquids enables efficient transfer of heat from the hot electronic components. This has been accomplished using two different approaches, cold plates using conductive fluid such as water and immersion cooling using non-conductive fluid. One of the concerns with the use of liquids in heat exchange devices is that even a small leak of heat exchange fluid can damage the electronic components in the server or put the workers at the risk to exposure to toxic chemicals. A large leak has a potential to damage an entire sever and even a data center and could be catastrophic. To prevent unwarranted damage to the electronic components, server manufactures rely on using leak detector and monitoring system. A typical leak monitoring system consists of a leak sensor that includes two electrically conducting wires (twisted or parallel) that are individually insulated with liquid permeable jacket. A monitoring device constantly monitors the electrical signal (e.g., capacitance and resistance) between these two wires. When a leak occurs, the electrical characteristics of these two wires changes. The monitoring system detects this change, sends an audio visual response, and initiates control processes to take corrective actions such as shutting off the pump for the liquid or electrically isolating the server from the rest of the rack components. The reliability of the leak detector system and its ability to protect the data depend heavily on sensitivity (the amount of leak needed to trigger an alarm), selectivity of the leak detector (avoiding a false positive alarm from high humidity in the server room), and response time (the time it takes to detect a leak).

SUMMARY

A leak detection system includes a flexible leak detector film and a controller. The detector film includes first and second insulators, first and second electrodes adjacent and spaced apart from one another between the first and second insulators, and a bottom ground on one of the insulators opposite the electrodes. The controller has a first input electrically connected to the first electrode, a second input electrically connected to the second electrode, and a ground input electrically connected to the bottom ground. The controller is configured to output a response providing an indication of a dielectric fluid proximate the first or second electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated in and constitute a part of this specification and, together with the description, explain the advantages and principles of the invention. In the drawings,

FIGS. 1A and IB show a parallel plate capacitor and a cross-section showing electric field lines between two plates;

FIGS. 2A, 2B, and 2C show that when the area between two parallel plates is small, the effect of the fringe fields becomes dominant (2A), the field on the bottom side can be shielded using a ground conductor (2B), and a presence of a finger-tip changes the dielectric environment that leads to a change in capacitance (2C);

FIGS. 3A, 3B, and 3C show an experimental setup (3 A), a user interface for controls and parameters (3B), and an example of a qualitative response when a few droplets of liquid were dispensed on the sensor surface (3C);

FIGS. 4A and 4B show the construction of a sensor and response in the user interface to different dielectric fluids for Example I;

FIGS. 5A and 5B show the construction of a sensor and a response in the user interface to different dielectric fluids for Example II;

FIGS. 6A and 6B show the construction of a sensor and response in the user interface to different dielectric fluids for Example III;

FIGS. 7A and 7B show the construction of a sensor and a response in the user interface to different dielectric fluids for Example IV;

FIGS. 8A and 8B show the construction of a sensor and a response in the user interface to different dielectric fluids Example V;

FIGS. 9A and 9B show the construction of a sensor and a response in the user interface to different dielectric fluids for Example VI;

FIGS. 10A and 10B show the construction of a sensor and a response in the user interface to different dielectric fluids for Example VII; and

FIG. 11 is a diagram of leak detector films in use within a fluid-based cooling system. DETAILED DESCRIPTION

This invention discloses a flexible leak monitoring film for detection of leakage of dielectric fluids in heat exchange devices used to transfer heat from electronic components such as processors in servers. The leak detector exploits the principle of capacitive touch sensing to detect the presence of a trace amount of low dielectric fluids. The flexible leak detector film can be wrapped around or integrated with a sleeve or jacket around the heat exchange devices and pipes to contain the leaks in the dielectric fluid. The sensor can also be used in conjunction with other electronic components to alarm the end users about the presence of a leak to prevent unwarranted loss of fluid or exposure of the fluid to individuals.

A non-conductive fluid and a conductive fluid, one or both, can be detected proximate one or more electrodes of the leak detector films. Conductive and non-conductive are relative terms. A liquid that is deemed non-conductive for one application might be conductive enough to short electronic components in other applications. Therefore, whether a fluid is deemed conductive or non-conductive depends upon the requirements of a particular application including the leak detector films described herein. The table below provides the conductivities of some relevant liquids. For embodiments of this invention and as indicated, these fluids are designated as conductive or non-conductive as well.

The detected fluids are proximate when the fluids are within the fringe field around the electrodes and need not be, but can be, in physical contact with the electrodes. A bottom ground electrode in the leak detector films is used to reduce/eliminate potential interference in the leak detection due to unwarranted change in the fringe field. A top ground electrode can optionally be used in the leak detector films to increase the signal strength for detected fluids. A controller is electrically connected, wired or wirelessly, with the leak detector film and is configured to detect via the leak detector film the presence of fluids, indicating a possible fluid leak. The controller can provide in a user interface a visual indication of the detected fluid, or different visual indications for different detected fluids, and possibly generate an alarm indicating leakage of the detected fluid. The alarm can be displayed locally on the user interface and optionally transmitted and displayed, or otherwise indicated, remotely from the source of the leak, for example within the control room of a data center. The alarm could also provide an indication or identification of the component and location where the leak was detected. Basic Principle

The principle behind detection of fluid leaks, in particular dielectric fluid leaks, is based on the change in capacitance between two conductors separated by a fixed distance between them. This principle is illustrated in FIG. 1A showing a parallel plate capacitor and FIG. IB showing a cross-section with electric field lines between two plates. In many practical cases, two parallel plates are used where the area of parallel plates is significantly larger than the area around the edges and the effect of edges is neglected. The capacitance between two parallel conductors with area A separated by a dielectric material with dielectric constant e _t with thickness d between two plates (L,W » d) is given by € = & e A/ά , where ¾ is the permittivity of free space (8.85 x 10 ¹² Farad/meter). The electric field generated by two parallel pates is uniform between the plates and it is non-uniform around the edges. Table 1 provides the definition of parameters for the parallel plate capacitor of FIG. 1A. _

The capacitance between the two plates depends on the dielectric constant of the material between the plates and is almost insensitive to small changes in the fringe fields around the edges. If the area of the plate is smaller compared to the area of edges (FIG. 2A), then the uniform electric filed between two plates has smaller contribution to the capacitance than the fringe fields around the edges. These fields have been exploited to make capacitive touch sensors, as illustrated with the configuration in FIG. 2B where the field on the bottom side is shielded using a ground conductor. The principle means that a small change in the dielectric environment introduced by the presence of a finger-tip (FIG. 2C) causes change in the capacitance between two conductors. The change in the capacitance is converted into an electrical signal using circuit boards similar to the DI736A product from Texas Instruments, Dallas, TX or LC717A30UJGEVK product from On Semiconductor, Phoenix, AZ. This principle has been used to build different sensors such as touch sensors, proximity sensors, and liquid level sensors. Embodiments of this invention exploit the same principle to detect the presence of low dielectric constant liquids.

Sensor Test Set Up

FIGS. 3A, 3B, and 3C show an experimental setup, a user interface for viewing and entering controls and parameters, and an example of qualitative response when a few droplets of liquid were dispensed on the sensor surface. Capacitive sensors with different architectures were prepared using different materials and structures as described below in the Examples. A Touch Sensor Evaluation Kit (available as LC717A30UJGEVK, from On Semiconductor, Phoenix, AZ) in the Liquid Level Detection Mode, as represented by sensor tape 2, was used to detect the change in capacitance in the presence of dielectric liquid. One of the electrodes forming the capacitor (sensor tape 2) was connected to the Ci _n input of a main controller 4 of Touch Sensor Evaluation Kit LC717A30UJGEVK. The other electrode was connected to the Ca™ input of main controller 4. The ground plate was connected to the ground of main controller 4. The main controller 4 includes a USB controller 6 that was connected to a computing device 8 (e.g., laptop or tablet computer) running the user interface as shown in FIGS. 3B and 3C.

The circuit parameters (e.g. gains and liquid settings) were adjusted until a stable response was observed in the user interface (FIG. 3C) when the electrodes were dry. These circuit parameters were adjusted until there were no errors (system error, calibration error) and noise levels were low as indicated by noise level in the user interface. Then droplets of liquid were dispensed on to the electrodes to mimic leaks. MiliQ water from in-house MilliQ system, isopropyl alcohol from Sigma Aldrich and NOVEC fluid 649 from 3M Company were used as three dielectric liquids with respective dielectric constants of 80, 18 and 1.8. For each sensor architecture, the electronic parameters were optimized to obtain a stable response in the user interface with dry electrodes, and the sensor response was recorded in the user interface by capturing an image of the user interface. Liquid droplets were then dispensed over the electrodes, and the response in the user interface was recorded. For the next liquid, the process was repeated, if needed, by adjusting the electronic parameters again.

EXAMPLES

Different conducting films were laminated with an insulating base substrate to fabricate a sensitive detector for dielectric fluids. An off-the-shelf electronics board and associated graphical user interface that processes the input from capacitive touch sensor were used to convert the change in capacitance in the presence of a dielectric liquid into a qualitative visual response (in the user interface) from these sensors when droplet of fluids with different dielectric constants were dispensed.

The following abbreviations are used herein: mm = millimeters; cm = centimeters. Example I

Two ~2mm wide, 52cm long strips of copper coated KAPTON film (Dupont Electronics, Inc. Corp., Wilmington, DE) were laminated, ~2mm apart, on to a 1cm wide polyimide film tape (available as Polyimide Film Tape #5143HC 2319-05, from 3M Company, St Paul MN) so that the KAPTON film sides of the strips are attached to the adhesive side of the polyimide tape. A 3mm wide conductive film (available as Conductive Tape #1181HD 0008, from 3M Company, St Paul, MN) was attached on the back side of the polyimide tape so that it is centered between two conducting strips. Al.Ocm wide woven porous film (available as ANCI Claf, #558523-Blue, from IX Nippon ANCI. Inc, Kennesaw. GA) was laminated over the conducting electrode strips ensuring that the electrodes were covered by it. A 1.27cm wide 3M conductive tape (available as #1181HD 7251, from 3M Company, St Paul, MN) was gently applied over the woven mesh to perform as a ground shield on this side of the sensor. The construction of the sensor is shown in FIG. 4A and includes, arranged as shown, a bottom electrode 11, a second insulator 12, electrodes 13, a first insulator 14, and atop ground 15. Table 2 provides the parameters for the configuration of Example I.

One of the two electrodes of the sensor was connected to the C _m input of main controller 4 and the other electrode was connected to the Ca™ input. The top and the bottom grounds were connected together and then connected to ground of main controller 4. The gains of main controller 4 were adjusted using the user interface until a stable response with highest sensitivity was observed with no liquid on the sensor. Small droplets of liquid dielectric liquid were then applied over the C _m electrode next to the top ground. The response in the user interface was then recorded. The process was then repeated with the remaining two liquids, and the response to different fluids from the sensor is shown in FIG. 4B.

Example II

Two ~3mm wide, 52cm long strips of conductive film (available as Conductive Tape #1181HD 0008 from 3M Company, St Paul, MN) were laminated, ~1.5mm apart, on to a 7.2cm wide KAPTON film so that the adhesive sides of the conductive tape strips were attached to the KAPTON film. A 3mm wide conductive film (available as Conductive Tape #1181HD 0008 from 3M Company, St Paul, MN)) was attached on the back side of the KAPTON film so that it is centered between two conducting strips. A1.27cm wide 3M polyimide film (available as #5413HC 2319-05 from 3M Company, St Paul, MN) was laminated over the conducting electrode strips ensuring that the electrodes were covered by it. A 1.27cm wide conductive film (available as Conductive Tape #1181HD 7251, from 3M Company, St Paul, MN) was gently applied over the polyimide film acting to perform as a ground shield on this side of the sensor. The construction of the sensor is shown in FIG. 5 A and includes, arranged as shown, a bottom ground 21, a second insulator 22, electrodes 23, a first insulator 24, and a top ground 25. Table 3 provides the parameters for the configuration of Example II.

One of the two electrodes of the sensor was connected to the Cm input of main controller 4 and the other electrode was connected to the Ca™ input. The top and the bottom grounds were connected together and then connected to the ground of main controller 4. The gains of main controller 4 were adjusted in the user interface until a stable response with highest sensitivity was observed with no liquid on the sensor. Small droplets of liquid dielectric liquid were then applied over the Cm electrode next to the top ground. The response in the user interface was then recorded. The process was then repeated with the remaining two liquids, and the response to different fluids from the sensor is shown in FIG. 5B.

Example III

Three ~3mm wide, 66cm long strips of conductive film (available as Conductive Tape #3314-07, from 3M Company, St. Paul, MN) were laminated, -l.Omm apart, onto a 5.0cm wide KAPTON film so that the adhesive sides of the conductive tape strips were attached to the KAPTON film. A 2.54cm wide polyimide film (available as Polyimide Film #5413HD 7324-07, from 3M Company, St. Paul, MN) was laminated over the conducting electrode strips ensuring that the electrodes were at the center and covered by it. A 2.54cm wide conductive film (available as Conductive Tape #1181HD 3314-07 from 3M Company, St. Paul, MN) was gently applied over the polyimide film to serve as a ground shield on this side of the sensor. The construction of the sensor is shown in FIG. 6A and includes, arranged as shown, a bottom ground 31, a second insulator 32, electrodes 33, and a first insulator 34. Table 4 provides the parameters for the configuration of Example III.

Two outer electrodes of the sensor were connected together and then to the Ca™ input of main controller 4 and the central electrode was connected to the Ci _n input. The bottom ground was connected to the ground of main controller 4. The gains of main controller 4 were adjusted, using the user interface, until a stable response with highest sensitivity was observed with no liquid on the sensor. Small droplets of liquid dielectric liquid were then applied over the Ci _n electrode next to the top ground. The response in the user interface was then recorded. The process was then repeated with the remaining two liquids, and the response to different fluids from the sensor is shown in FIG. 6B.

Example IV

Three ~3mm wide, 66cm long strips of conductive fdm (available as Conductive Tape #3314-07, from 3M Company, St. Paul MN) were laminated, ~ 1 0mm apart, on to a 5.0cm wide KAPTON fdm so that the adhesive sides of the conductive tape were attached to the KAPTON fdm. A 2.54cm wide 3M polyimide fdm (available as Polyimide Film #5413HD 7324-07 from 3M Company, St. Paul, MN) was laminated over the conducting electrode strips ensuring that the electrodes were at the center and covered by it. A 2.54cm wide conductive tape (available as Conductive Tape #1181HD 3314-07, from 3M Company, St. Paul, MN) was gently applied over the polyimide fdm to serve as a ground shield on this side of the sensor. The construction of the sensor is shown in FIG. 7A and includes, arranged as shown, a bottom ground 41, a second insulator 42, electrodes 43, and a first insulator 44. Table 5 provides the parameters for the configuration of Example IV. Two outer electrodes of the sensor were connected together with the bottom ground and then to the ground of main controller 4 and the central electrode was connected to the C _m input. The gains of main controller 4 were adjusted, using the user interface, until a stable response with highest sensitivity was observed with no liquid on the sensor. Small droplets of liquid dielectric liquid were then applied over the C _m electrode. The response in the user interface was then recorded. The process was then repeated with the remaining two liquids, and the response to different fluids from the sensor is shown in FIG. 7B.

Example V

Two ~lmm wide, 52cm long strips of copper coated KAPTON fdm were laminated, ~2mm apart, on to a 2.54cm wide 3M polyimide fdm tape (available as Polyimide Film #5143HD 7324-07, from 3M Company, St. Paul, MN) so that the KAPTON sides of the strips are attached to the adhesive side of the polyimide tape. A 1.27cm wide 3M polyimide fdm (#5413HD 2319-05) was applied over the conducting electrode strips ensuring that the electrodes were covered by it. A 2.54cm wide conductive fdm (available as Conductive Tape #1181HD 3314-7, from 3M Company, St. Paul, MN) was gently applied on the opposite side of the electrodes to serve as a ground shield on this side of the sensor. The construction of the sensor is shown in FIG. 8A and includes, arranged as shown, a bottom ground 51, a second insulator 52, electrodes 53, and a first insulator 54. Table 6 provides the parameters for the configuration of Example V.

One of the two electrodes of the sensor was connected to the Cm input of main controller 4 and the other electrode was connected to the Ca™ input. The bottom ground was connected to the ground of main controller 4. The gains of main controller 4 were adjusted, using the user interface, until a stable response with highest sensitivity was observed with no liquid on the sensor. Small droplets of liquid dielectric liquid were then applied over the area between the electrodes next to the top ground. The response in the user interface was then recorded. The process was then repeated with the remaining two liquids, and the response to different fluids from the sensor is shown in FIG. 8B. Example VI

Two ~lmm wide, 91cm long strips of copper coated KAPTON fdm were laminated,

~ 1.5mm apart, on to a 1.27cm wide polyimide fdm tape (available as Polyimide Film #5413HD 2319-05, from 3M Company, St Paul, MN) so that the KAPTON sides of the strips are attached to the adhesive side of the polyimide tape. A 6mm wide porous mesh (available as 55gsm Resin Bonded Carded Polyester fdm style # ADL 2 from Fitesa North America, Green Bay, WI) was applied over the conducting electrode strips ensuring that the electrodes were covered by it. A 1.27cm wide conductive tape (available as Conductive Tape #1181HD 3314-7, from 3M Company, St. Paul, MN) was gently applied on the opposite side of the 3M polyimide tape to provide a ground shield on this side of the sensor. The construction of the sensor is shown in FIG. 9A and includes, arranged as shown, a bottom ground 61, a second insulator 62, KAPTON fdm 63, electrodes 64, and a first insulator 65. Table 7 provides the parameters for the configuration of Example VI.

One of the two electrodes of the sensor was connected to the C _m input of main controller 4 and the other electrode was connected to the Ca™ input. The bottom ground was connected to the ground of main controller 4. The gains of main controller 4 were adjusted, using the user interface, until a stable response with highest sensitivity was observed with no liquid on the sensor. Small droplets of liquid dielectric liquid were then applied on the mesh over between the electrodes. The response in the user interface was then recorded. The process was then repeated with the remaining two liquids, and the response to different fluids from the sensor is shown in FIG. 9B.

Example VII

Two ~lmm wide, 91cm long strips of copper coated KAPTON film were laminated,

~ 1.5mm apart, on to a 1.27cm wide conductive tape (available as Conductive Tape #1181HD 3314-07, from 3M Company, St Paul MN) so that the KAPTON film sides of the strips are attached to the adhesive side of the conductive tape. A 6mm wide porous mesh (available as 55gsm Resin Bonded Carded Polyester film style # ADL 2 from Fitesa North America, Green Bay, WI) film was applied over the conducting electrode strips ensuring that the electrodes were covered by it. The construction of the sensor is shown in FIG. 10A and includes, arranged as shown, a bottom ground 71, an adhesive 72, a second insulator 73, electrodes 74, and a first insulator 75. In this construction the adhesive layer 72 is part of the conductive tape, and second insulator 73 is the KAPTON fdm that is the part of the copper coated KAPTON fdm. Table 8 provides the parameters for the configuration of Example VII.

One of the two electrodes of the sensor was connected to the Ci _n input of main controller 4 and the other electrode was connected to the Ca™ input. The bottom ground was connected to the ground of main controller 4. The gains of main controller 4 were adjusted, using the user interface, until a stable response with highest sensitivity was observed with no liquid on the sensor. Small droplets of liquid dielectric liquid were then applied over the mesh between two electrodes. The response in the user interface was then recorded. The process was then repeated with the remaining two liquids, and the response to different fluids from the sensor is shown in FIG. 10B.

Fluid-Based Cooling System

FIG. 11 is a diagram of leak detector fdms in use within a fluid-based cooling system. A fluid-based cooling system includes a tank 80 containing a cooling dielectric fluid 81 and a pipe 82 coupled to tank 80 for providing fluid communication with tank 80 at a gasket 83 in the tank. A leak detector fdm 84 is wrapped around and in physical contact with pipe 82 for detecting cooling fluid leaks from pipe 82. A leak detector fdm 85 is placed on tank 80 in physical contact with gasket 83 for detecting cooling fluid leaks from gasket 83. A leak detector fdm 86 is wrapped around and in physical contact with an exterior surface of tank 80 for detecting cooling fluid leaks from tank 80. Leak detector fdms 84, 85, and 86 can be implemented with any of the leak detector fdms described herein. The leak detector fdms can be arranged in physical contact with the components of a fluid-based cooling in any particular configuration for use in detecting leaks of cooling fluid. A fluid-based cooling system for a data center, for example, would typically contain many tanks and associated pipes for use in cooling electronic components, and only one tank and pipe are shown FIG. 11 for illustrative purposes.