Patent Publication Number: US-11661936-B2

Title: Sensors, multiplexed communication techniques, and related systems

Description:
RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. patent application Ser. No. 14/777,510, filed on Sep. 15, 2015, issued as U.S. Pat. No. 10,364,809 on Jul. 30, 2019, which claims benefit of and priority to PCT Application No. PCT/IB2014/059768, filed on Mar. 14, 2014, which claims benefit and priority to U.S. Patent Application No. 61/793,479, filed Mar. 15, 2013, U.S. Patent Application No. 61/805,418, filed Mar. 26, 2013, U.S. Patent Application No. 61/856,566, filed Jul. 19, 2013, and U.S. Patent Application No. 61/880,081, filed Sep. 19, 2013, each of which patent applications is hereby incorporated by reference in its respective entirety as fully as if fully recited in full herein, for all purposes. 
    
    
     BACKGROUND 
     The innovations and related subject matter disclosed herein (collectively referred to as the “disclosure”) pertain to control systems, and more particularly, but not exclusively, to detectors configured to issue an alert or a command to a controller in response to a detected change in state of a given system, with a leak detector configured to respond to a detected leak of a working fluid from a liquid-based heat transfer system being but one example of disclosed detectors and related innovations, and with a flow-rate sensor configured to detect a rate of flow of a fluid through a conduit, for example an optical flow-rate sensor, being but one other example of disclosed sensors and related innovations. Some detectors and control systems are described in relation to cooling systems for electronic devices by way of example. Nonetheless, one or more of the innovations disclosed herein can be suitable for use in a variety of other control-system applications, as will be understood by those of ordinary skill in the art following a review of the present disclosure. 
     Computer system performance and heat dissipation density continue to increase. Consequently, conventional air-cooling is giving way to liquid-cooling in some computer system applications, including, but not exclusively, server and data center applications. Although commercially available liquid cooling systems are considered to be reliable and to provide known and repeatable performance, an automated approach for detecting an unlikely leak might be desirable in some applications. However, commercially available moisture sensors and leak detectors are not compatible with existing control systems for computer systems. 
     Also, approaches for monitoring a rate of flow of a fluid through one or more conduits might be desirable in some applications. For example, a rate of heat transfer through a liquid-to-liquid or an air-to-liquid (or a liquid-to-air) heat exchanger can correspond to a rate of flow of a heat transfer medium (e.g., a liquid coolant) through the heat exchanger. As but one other example, a substantial excursion of fluid flow rate through a conduit can indirectly indicate a leak upstream of the conduit, or a change in heat-transfer performance. 
     However, many commercially available flow-rate sensors are generally considered to be incompatible with existing liquid-cooling systems suitable for computer systems. For example, some known flow-rate sensors are typically too large, too expensive, or both, to be incorporated into liquid-cooling systems suitable for widespread commercialization in connection with cooling systems for computer systems, or other systems. 
     Accordingly, there remains a need for sensors configured to detect a leak from a liquid cooling system. There also remains a need for a monitoring system configured to initiate an alert responsive to a leak detected by the leak detector. A need also remains for a leak detector configured to be compatible with a control system for a computer system or other computing environment. And, there remains a need for flow-rate sensors configured to detect or sense a rate of flow of a working fluid through a conduit, for example, a portion of a flow path through a portion of a liquid-cooling system. There remains a further need for flow sensors to emit a signal responsive to a detected or a sensed flow rate of the working fluid. 
     There also remains a need for such sensors to be compatible with existing communications busses, e.g., by using existing communication protocols or by multiplexing over existing communication busses (e.g., an IPMI bus). 
     SUMMARY 
     Innovations and related subject matter disclosed herein overcome many problems in the prior art and address one or more of the aforementioned, as well as other, needs. This disclosure pertains generally to control systems, including, for example, detectors configured to issue an alert or a command to a controller in response to a detected change in state of a given system. For example, some disclosed detectors are configured to emit a simulated signal (e.g., an electrical signal) as a proxy for a state observed by a sensor, with a simulated fan-tachometer signal being but one example of a proxy signal. 
     Some disclosed detectors are configured to detect a leak of a working fluid from a heat-transfer system. Some disclosed leak detectors are configured to issue an alert or a command to a controller in response to a detected leak of a working fluid from a liquid-based heat transfer system. 
     Some disclosed detectors are configured to assess one or more aspects of a flow field, e.g., to assess a flow rate. Some disclosed detectors are configured to detect a flow rate of a working fluid through a portion of a heat-transfer system. Some disclosed flow-rate sensors are configured to emit a signal, or to issue an alert or a command to a controller in response to an observed or a detected change in state of a given system. 
     For example, some disclosed flow-rate sensors are configured to emit a signal, or to issue an alert or a command to a controller in response to an observed or a detected rate of flow (or an indicia of a rate of flow) of a working fluid through a liquid-based heat transfer system, as when an observed, detected, or indicated rate of flow exceeds a selected upper threshold flow rate or falls below a selected lower threshold flow rate. Some flow-rate sensors are configured to emit an output signal corresponding to an observed rate of flow (or an observed indicia thereof). 
     By way of example and not limitation, a flow-rate sensor can be configured to emit a simulated fan-tachometer signal (or other proxy signal) proportional to (or, more broadly, corresponding to) an indicia of flow rate observed by the sensor. A controller configured to receive such a simulated fan-tachometer signal can interpret the simulated fan-tachometer signal as corresponding to a predetermined measure of the indicia of flow rate (or measure of the flow rate). In response, the controller can issue a system command in correspondence to the indicia (or flow rate). As but one example, the system command can be a command to transmit an alert to a system administrator and/or a command to increase pump speed, as when the indicated flow rate might not suffice to cool an observed or an anticipated heat load, or to decrease pump speed, as when the indicated flow rate might provide more cooling than necessary based on an observed or an anticipated heat load and continued operation of the pump at a relatively higher speed emits more acoustic noise or consumes more energy than desired. 
     In some embodiments, an emitted signal, or an alert or command, includes a simulated fan-tachometer signal corresponding to a selected fan-rotational-speed as a proxy for an observed state different than a fan-rotational-speed (e.g., a flow rate or a detected leak). For example, an observed operational state can include an operational state of one or more system devices (e.g., a pump in a liquid-cooling system, a heat exchanger in a liquid cooling system, a frequency of an optical signal emitted by an optical emitter, an observed flow rate through one or more portions of a cooling system (e.g., through a segment of a conduit carrying a working fluid), etc.). 
     As but one possible and non-limiting example, a sensor can emit, in response to a detected one of a plurality of observable conditions, a simulated fan-tachometer signal corresponding to a respective fan-rotational speed as a proxy corresponding to the detected condition. For example, a leak detector can emit, in response to a detected leak, a simulated fan-tachometer signal corresponding to a fan-rotational-speed of 500 RPM (revolutions per minute). In turn, the fan rotational speed of 500 RPM can be interpreted by a controller as indicating, for example, that a leak has occurred (or at least has been detected) at a given system location. 
     As another example, a flow-rate sensor can emit, in response to a first observed flow rate (or an observed indicia of such a flow rate), a simulated fan-tachometer signal corresponding to a first fan-rotational-speed and a second fan rotational speed in response to an observed other flow rate (or indicia thereof). For example, the flow-rate sensor can emit a simulated fan-tachometer signal indicative of a selected fan speed proportional to the observed flow rate (or indicia thereof). A controller that receives such a proxy signal can, at least partially responsively to the proxy signal, issue a selected command (e.g., a system command to alter or to maintain a system operational state, a system shut-down command, an administrator alert command) responsive to a given interpretation of the proxy signal. 
     Some controllers are embodied in a computing environment. 
     As used herein, “working fluid” means a fluid used for or capable of absorbing heat from a region having a relatively higher temperature, carrying the absorbed heat (as by advection) from the region having a relatively higher temperature to a region having a relatively lower temperature, and rejecting at least a portion of the absorbed heat to the region having a relatively lower temperature. Although many formulations of working fluids are possible, common formulations include distilled water, ethylene glycol, propylene glycol, and mixtures thereof. 
     Some disclosed leak detectors include a sensor operatively coupled to a leak detector circuit. A leak detector circuit can be configured to deliver a signal having a selected waveform to a monitor circuit during normal operation of the cooling system and to terminate or otherwise interrupt the signal (as by modifying the waveform, for example) when a leak of liquid is detected, as by the sensor. Some disclosed leak detectors are configured to deliver a simulated tachometer signal to a monitor circuit or computing environment. The simulated tachometer signal can be similar to a tachometer signal emitted by a fan during normal operation of the fan until a leak is detected. Upon receiving a signal or other indication of a leak, the leak detector circuit can emit a different signal (or no signal) after a leak is detected. The different signal can be emitted continuously or only while a leak (or moisture or other proxy for a leak) is detected by the sensor. 
     For example, some disclosed leak detector circuits are configured to emit a simulated tachometer signal, e.g., a square wave having a duty cycle of about 50% (e.g., a duty cycle ranging from about 45% to about 55%), during normal operation, and to terminate or otherwise interrupt the simulated tachometer signal in response to a detected leak (or moisture or other proxy for a leak, such as a low operating pressure or a low-fluid level internal to the heat-transfer system). Such a leak detector circuit can be compatible with commercially available monitor circuits, firmware and/or software, particularly but not exclusively, monitor circuits, firmware and/or software configured to monitor a rotational speed of a fan using a tachometer signal emitted by the fan. Some monitors (e.g., circuits and/or computing environments) can be based on, by way of example, the Intelligent Platform Management Initiative (IPMI) specification, ver. 1.5/2.0 (described more fully below). 
     In some embodiments, a plurality of sensors or detectors can be operatively coupled to a given communication circuit, and a controller can configured to monitor the given communication circuit. Each respective sensor or detector in the plurality of sensors or detectors can be configured to emit any of a plurality of discrete, simulated signals as respective proxies for a plurality of selected, detectable operational states. For example, the sensors or detectors can emit discrete, simulated fan-tachometer signals corresponding to respective system operational states. Such multiplexing can allow existing communication channels to carry information regarding observed system operational states that differ substantially from the information historically carried by the existing communication channels. 
     By way of example and not limitation, a leak detector can be configured not to emit a simulated fan-tachometer signal in the absence of an observed leak, and to emit (e.g., over a selected communication circuit), responsively to a detected leak, a selected simulated fan-tachometer signal (e.g., a simulated fan-tachometer signal corresponding to a fan-rotational speed of 200 RPM). A controller configured to receive such a simulated fan-tachometer signal can interpret the simulated fan-tachometer signal as corresponding to a predetermined operational state. In response, the controller can issue a system command in correspondence with the operational state. As but one example, the system command can be a command to transmit an alert to a system administrator or a command to shut the system down. 
     As another example, a sensor can be configured to observe an operational state of a centrifugal pump. The sensor can be configured to emit a simulated fan-tachometer signal corresponding to a different fan-rotational speed (e.g., 400 RPM) in response to an observed pump failure (e.g., a pump rotational speed below a selected threshold rotational speed). A controller configured to receive the simulated fan-tachometer signal can issue a system command in response to and corresponding to the indication of a pump failure. The system command can include one or more of a command to transmit an alert to a system administrator, a command to increase a rotational speed of one or more other selected pumps, and a command to shut the system down. 
     Other particular but non-exclusvie examples of multiplexed sensors include sensors configured to observe one or more of a rotational speed of the pump, a static pressure in a fluid within the pump, a temperature of a liquid in the pump, a temperature of a pump component, a flow rate through a conduit, and a number of hours during which a given pump has operated. Each sensor can be configured to emit a selected proxy signal corresponding to an observed operational state of the system. 
     Other innovative aspects of this disclosure will become readily apparent to those having ordinary skill in the art from a careful review of the following detailed description (and accompanying drawings), wherein various embodiments of disclosed innovations are shown and described by way of illustration. As will be realized, other and different embodiments of leak detectors and systems incorporating one or more of the disclosed innovations are possible and several disclosed details are capable of being modified in various respects, each without departing from the spirit and scope of the principles disclosed herein. For example, the detailed description set forth below in connection with the appended drawings is intended to describe various embodiments of the disclosed innovations and is not intended to represent the only contemplated embodiments of the innovations disclosed herein. Instead, the detailed description includes specific details for the purpose of providing a comprehensive understanding of the principles disclosed herein. Accordingly the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Unless specified otherwise, the accompanying drawings illustrate aspects of the innovative subject matter described herein. Referring to the drawings, wherein like reference numerals indicate similar parts throughout the several views, several examples of systems incorporating aspects of the presently disclosed principles are illustrated by way of example, and not by way of limitation, wherein: 
         FIG.  1    shows a representative pulse of a square wave emitted by a Hall cell in response to a rotating fan rotor; 
         FIG.  2    shows a representative signal emitted by a fan in a running state, a locked rotor state, and another running state; 
         FIG.  3    shows a representative pin-out for a fan header operatively coupled to a pump; 
         FIG.  4    shows a portion of but one of many leak detector embodiments disclosed herein; 
         FIG.  5    shows a block diagram of a leak detector and a portion of an associated control system in relation to a fluid heat exchange system; 
         FIG.  6    shows a schematic illustration of an embodiment of a circuit configured according to the block diagram shown in  FIG.  5   ; 
         FIG.  7    shows a pinout of a fan header operatively coupled to an embodiment of a leak detector disclosed herein; 
         FIG.  8    shows a schematic illustration of a system including a leak detector disclosed herein; and 
         FIG.  9    shows a schematic illustration of an alternative system including a leak detector disclosed herein; 
         FIG.  10    shows a schematic illustration of a cooling system having an optical flow-rate sensor of the type disclosed herein; 
         FIG.  11    shows a schematic illustration of an optical flow-rate sensor; 
         FIG.  12    shows a schematic illustration of a retainer suitable for the optical flow-rate sensor shown in  FIG.  11   : 
         FIG.  13    shows one possible configuration of a rotational member as disclosed herein; 
         FIG.  14    shows a schematic illustration of an apparatus configured to calibrate a flow-rate sensor of the type shown in  FIG.  13   ; 
         FIG.  15    shows a plot of a calibration of a flow-rate sensor; 
         FIGS.  16 A and  16 B  show respective schematic illustrations of a rotational member of the type shown in  FIG.  13   ; in  FIG.  16 A , a reflector is shown; in  FIG.  16 B , the rotational member has rotated to a position obscuring the reflector shown in  FIG.  16 A  from view; 
         FIG.  17    shows a selected proxy relationship (or correlation) between an observed flow rate of a working fluid (or indicia thereof) and a fan speed indicated by a simulated fan-tachometer signal; and 
         FIG.  18    shows a block diagram of a computing environment suitable for use in combination with systems, methods and apparatus described herein. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes various innovative principles related to control systems by way of reference to specific examples of sensors for such systems. More particularly, but not exclusively, such innovative principles are described in relation to examples of leak detectors configured to detect a leak of a working fluid from a liquid-based heat transfer system (e.g., a liquid-based cooling system for cooling one or more electronic components that dissipate heat during operation), examples of flow-rate sensors configured to observe a flow rate through a liquid-based heat-transfer system, and related systems. Nonetheless, one or more of the disclosed principles can be incorporated in various other control system embodiments to achieve any of a variety of desired control system characteristics. Systems described in relation to particular configurations, applications, or uses, are merely examples of systems incorporating one or more of the innovative principles disclosed herein and are used to illustrate one or more innovative aspects of the disclosed principles. 
     Thus, control systems, sensors, leak detectors, flow-rate sensors, and associated circuits, computing environments, firmware and/or software having attributes that are different from those specific examples discussed herein can embody one or more of the innovative principles, and can be used in applications not described herein in detail, for example, to detect a leak of a fluid (e.g., a liquid, a gas, or a saturated mixture thereof) from, or to observe a local speed of a flow of such a fluid through, a heat-transfer system having any of a variety of flow configurations, such as a contained flow within a fluid conduit or a free-stream flow (e.g., a region of a fluid flow sufficiently spaced from a fluid boundary as not to be influenced by the boundary). Such systems can be configured to transfer heat to or from laser components, light-emitting diodes, chemical reactants undergoing a chemical reaction, photovoltaic cells, solar collectors, power electronic components, electronic components other than microprocessors, photonic integrated circuits, and other electronic modules, as well as a variety of other industrial, military and consumer systems now known or hereafter developed. Accordingly, embodiments of detectors and related control systems not described herein in detail also fall within the scope of this disclosure, as will be appreciated by those of ordinary skill in the art following a review of this disclosure. 
     Overview 
     A wide variety of control systems have been proposed and used. In a general sense, control systems estimate or observe an attribute of a given system under control of the control system. In response to the estimated or observed attribute, a control system can provide an output corresponding to the estimated or observed attribute in order to achieve a desired system response. Controls systems (or portions thereof) disclosed herein can be implemented in a computing environment. As indicated above and explained more fully below, some disclosed systems are configured to detect a leak of a working fluid from, for example, a liquid-based heat-transfer system. Some disclosed systems are configured to transmit an alert or other command in response to a detected leak. 
     Some disclosed sensors are configured to be backward compatible with existing control systems. For example, some existing control systems configured to monitor an operational status of a cooling fan for a computer system are configured to emit a signal corresponding to observed fan speeds, or to issue an alert or other command, when an observed fan speed drops below a selected threshold. 
     Taking advantage of an installed base of such existing control systems, some disclosed sensors have a circuit configured to emit a first simulated tachometer signal corresponding to a first observed condition (e.g., similar to a tachometer signal emitted by a normally operating fan) and to emit a different simulated tachometer signal corresponding to a second observed condition. In some instances, the different signal emitted in response to the second observed condition can be similar to a tachometer signal emitted by a failed or failing fan (e.g., a fan operating at an unacceptably low fan speed, or a fan having a locked rotor). 
     Another example of an operational status includes a flow rate through a conduit. Some disclosed sensors emit a simulated fan tachometer signal in correspondence with an observed volumetric (or mass) flow rate (or indicia thereof, such as, for example, a rotational speed of a rotational member within the flow of fluid). 
     An operational status can reflect a presence or absence of a detected leak. Some disclosed leak detectors have a circuit configured to emit a simulated tachometer signal similar to a tachometer signal emitted by a normally operating fan when no leak is detected and to emit a different signal (or no signal) in response to a detected leak. The different signal emitted in response to a detected leak can be similar to a tachometer signal emitted by a failed or failing fan (e.g., a fan operating at an unacceptably low fan speed, or a fan having a locked rotor). 
     Some disclosed systems incorporate a sensor configured to detect or observe an indicia of a change in state of a heat-transfer system. Some indicia pertain to a rate of flow of a working fluid, for example, through a portion of a liquid-based heat-transfer system. Other indicia pertain to a leak of such a working fluid. Some disclosed systems are configured to transmit an alert or other command in response to a threshold condition observed or detected by such a sensor. 
     As but one example, some disclosed flow-rate sensors are configured to observe (or to detect) a frequency at which a rotational member rotates about a selected axis of rotation in response to a passing flow of a working fluid. As will be described more fully below, such a rotational frequency can correspond to a speed (and thus a rate of flow) at which a flow of a selected fluid passes by or over the rotational member. 
     Control Systems 
     By way of introduction, computer systems commonly include one or more axial fans for cooling an electronic component. A rate of heat transfer from an electronic component or from a liquid-to-air heat exchanger (e.g., a radiator) to a stream of air passing over the component or the heat exchanger generally corresponds, in part, to a speed of the air stream. A speed of such an air stream generally corresponds to a rotational speed of the fan. 
     Taking advantage of such a correspondence between a fan&#39;s rotational speed (sometimes expressed in units of “revolutions per minute” or “RPM”, and sometimes referred to as a “fan speed”) and a rate of cooling afforded to an electronic component or a heat exchanger, some computer systems include a control system configured to adjust a fan speed in response to an observed temperature (e.g., a temperature of an electronic component). As an example, some control systems are configured to modulate a duty cycle of, for example, a square wave, and some fans, in turn, are configured to adjust their fan speed in correspondence with the modulated duty cycle. 
     In addition (or alternatively), some computer systems include a control system configured to observe an output signal from a fan. Such an output signal can correspond to a rotational speed of the fan. For example, a fan can include a Hall cell configured to emit a square wave having a frequency corresponding to a rotational speed of a rotating magnetic field generated by a rotating fan rotor. Such an emitted square wave can have a duty cycle of about 50% when the rotor rotates at an approximately constant speed. Since the frequency of the square wave can correspond to the rotational speed of the fan, such a square wave is sometimes referred to as a “tachometer signal.”  FIG.  1    illustrates one pulse from a typical tachometer output having a square wave waveform. As another example,  FIG.  2    shows a representative waveform of a tachometer output for a fan that changes from an operating state (“Running”) having a 50% duty cycle, to a “Locked rotor” state in which no tachometer signal (or a steady-state signal) is emitted because the fan rotor does not rotate, and back to an operating state (“Running”) having a 50% duty cycle. 
     In general, a control system can be configured to transmit an alert or other command in response to an observed signal exceeding a selected upper threshold or falling below a selected lower threshold. Some control systems are configured to resume monitoring the observed signal after transmitting the alert or other command Other control systems (sometimes referred to in the art as a “latching system”) are configured to continuously transmit an alert or other command. 
     Some existing control systems are configured to observe a tachometer signal emitted by a rotating fan and to emit a signal or otherwise initiate a system command (e.g., send an “alert”, or initiate a system shut-down) in response to a selected change in state of a tachometer signal. A selected change of state of a tachometer signal can include a drop in frequency below a selected threshold (e.g., corresponding to an unacceptably low fan speed), a cessation of a tachometer signal or an emission of steady-state tachometer signal, as when a fan rotor stops rotating. In relation to  FIG.  2   , such a control system can be configured to emit a signal or otherwise initiate a system command if an observed signal indicates that a fan is in a “locked rotor” state. 
     Some suitable control systems configured to monitor fan speed are based on the Intelligent Platform Management Initiative (IPMI) specification, ver. 1.5/2.0. Generally, IPMI is a message-based, hardware-level interface specification. An IPMI subsystem can operate independently of an operating system of a computer incorporating the IPMI subsystem, allowing a system administrator to manage the computer independently of the operating system (e.g., before the operating system boots, or when the computer is powered down). A Baseboard Management Controller (BMC) can include a specialized microcontroller configured to manage an interface between the system management software and computer system hardware. 
     Among many features, an IPMI subsystem can monitor a status of various operating parameters, including, for example, system temperatures, fan speeds, chassis intrusion, etc. In some instances, an IPMI subsystem can be configured to monitor a tachometer signal emitted by one or more fans and, when the tachometer signal indicates a fan speed below a selected threshold, the subsystem can emit an alert or other command. 
     Computer systems incorporating such control systems for fans commonly include a plurality of electrical connectors, with each being configured to operatively couple a fan to a corresponding plurality of circuits configured, respectively, to power, control and monitor the fan. For example, such an electrical connector can have four electrical couplers corresponding respectively to (A) a power supply circuit configured to convey an electrical current for powering the fan motor; (B) an electrical ground; (C) a pulse-width modulation circuit configured to convey a pulse-width modulation signal (sometimes referred to as a “PWM signal”) for controlling the fan; and (D) a sense circuit configured to convey a tachometer signal corresponding to a fan speed (sometimes referred to in the art more generally as a frequency generator signal, or an “FG” signal). Such an electrical connector is sometimes referred to in the art as a “header” or a “fan header”.  FIG.  3    shows a typical pinout for a header with annotations reflecting use of the header in conjunction with a pump. 
     Leak Detectors 
     A leak detector circuit can be configured to respond to a leak (e.g., moisture or another selected proxy for a leak) of a working fluid detected by a sensor. For example, an innovative leak detector circuit can be configured to emit a first waveform in the absence of a detected leak and to emit a second waveform responsive to a detected leak. Any suitable sensor configured to detect a leak (or other proxy for a leak, e.g., moisture, presence of a working fluid at a position external to a heat-transfer system, a low pressure in the heat-transfer system, a low fluid level in the heat-transfer system) can be used in connection with such an electrical circuit. 
     As but one of many possible examples of leak-detection sensors, a leak-detection sensor  5  can have a first leak-detection wire  10  and a second leak-detection wire  20 , as shown in  FIG.  4   . The first and the second leak-detection wires  10 ,  20  can comprise respective exposed traces on a printed circuit board. As shown in  FIG.  4   , the first leak-detection wire  10  can extend from a power plane, V 1 . The second leak-detection wire  20  can extend generally parallel to and spaced apart from the first leak-detection wire  10 . A region in which the first and the second wires  10 ,  20  are coextensive can define a leak-sensitive region  25  of the sensor. 
     A leak can be detected when an open circuit between the first and the second leak-detection wires  10 ,  20  is closed. For example, a drop  30  of a leaked liquid can span a gap between the first and the second leak-detection wires  10 ,  20  within the leak-sensitive region  25  of the sensor  5 , electrically coupling the first and the second leak-detection wires to each other. 
     When the circuit between such first and second leak-detection wires  10 ,  20  is closed, the circuit of the leak detector  5  can emit a corresponding signal indicative of a detected leak. For example, when the first and the second leak-detection wires  10 ,  20  shown in  FIG.  4    are electrically coupled to each other, the second leak-detection wire  20  can be pulled high (e.g., can have a voltage potential corresponding to the voltage of the power plane, V 1 ), and can activate a relay  35 . When the illustrated relay  35  is activated, the latch  40  electrically coupling the pump and the fan header to each other can be switched to open (e.g., disconnect) the coupling between the pump and the fan header. Such a disconnection of at least one coupling between the pump and the header can serve as a signal to a monitoring system that a leak has been detected. The monitoring system can in response initiate an alert or a system command. 
     In  FIG.  5   , a leak detection sensor is schematically illustrated as extending from an integrated pump and heat exchanger assembly (sometimes referred to in the art as a “Head Module”). U.S. patent application Ser. No. 12/189,476 and related patent applications describe examples of such Head Modules. The leak detection sensor  125 ,  125   a  shown in  FIG.  5    has first and second leak-detection wires  110 ,  120  (referred to in  FIG.  6    as “Cable Conductor  1 ” and “Cable Conductor  2 ”, respectively) spaced apart from each other to form a gap  121 . Such a leak detection sensor is sometimes referred to in the art as a “Leak Detect Cable.” One or both of the leak-detection wires  110 ,  120  can be partially or fully embedded (or otherwise surrounded by) a semi-conducting carrier. The first and/or the second leak-detection wires  110 ,  120  can be formed from an alloy of copper. 
     A conductive fluid spanning the gap between the first and second leak-detection wires  110 ,  120  can provide a “non-trivial” resistance between the first and the second leak-detection wires. As used herein, a “non-trivial resistance” means a finite resistance sufficient to electrically couple the first and the second leak-detection wires to each other. With a circuit configured as shown in  FIG.  6   , a non-trivial resistance between the first and the second leak-detection wires can supply the analog Leak Sense line  122  with a non-zero voltage. 
     As indicated in  FIG.  5   , some leak detectors have a functional module  130  (sometimes referred to in the art as a “Glue Module”) configured to respond to a leak detected by a leak detection sensor  125 . The Glue Module shown in  FIG.  5    can be configured to deliver a logic high signal to the FG line (labeled as “Output Tach” in  FIG.  5   ) responsive to a signal indicative of a leak received over the Leak Sense line  122 . 
     In some embodiments, the Glue Logic module is configured to monitor the Leak Sense line  122  continuously. In other embodiments, the Glue Logic module is configured to sample the Leak Sense line  122  at defined times (e.g., at selected intervals, or at selected intermittent times). The Glue Logic can also be configured to transmit a signal over an Enable Detect line  123 , and, as shown by way of example in  FIG.  6   , the Leak Detection Circuit  125   a  can be configured to become operative in response to a signal received over the Enable Detect line  123 . 
     A Glue Logic module can be configured to interrupt operation of a pump motor responsive to a signal received over the Leak Sense line  122  indicative of the existence of a leak (e.g., an electrical coupling between the first and the second leak-detection wires). For example, a Motor Cutoff line  126  can carry a signal emitted by the Glue Logic, and a Motor Control Circuit  127  can respond to a signal received over the Motor Cutoff line  126  by interrupting power to the motor  128 . Alternatively (or additionally), the Glue Logic can force an output tachometer signal  129  (e.g., an FG signal) from the Head Module to a logic 0 (e.g., low logic) to signify to a monitoring system that there has been a failure associated with the Head Module. 
     Many other leak-detection sensor and leak detector circuit configurations are possible. As but several examples, such sensors can include a capacitive moisture sensor, an optical sensor, an infrared sensor, a pressure sensor configured to observe a pressure within the heat-transfer system, a sensor configured to detect a low fluid level in the heat-transfer system, and other sensors now known and hereafter developed. 
     Some leak detectors can have an electrical circuit operatively coupled to an FG signal pin of a header and be configured, in the absence of a detected leak, to emit a simulated tachometer signal  129  having a waveform similar to a waveform emitted by a properly operating fan.  FIG.  7    shows a header operatively coupled to such an electrical circuit. The electrical circuit (not shown) can be further configured to emit a simulated tachometer signal  129  having a waveform similar to a failed or failing fan in response to a detected leak of a liquid from a liquid-base heat-transfer system (e.g., when a circuit between first and second leak-detection wires is closed). Alternatively, the electrical circuit can be configured to emit no tachometer signal, similar to a fan having a locked rotor (see  FIG.  2   ) in response to a detected leak of a liquid from a liquid-based heat-transfer system. 
     As an example, a leak detector circuit  225  can be operatively coupled to an available fan header. In response to a detected leak, the simulated signal can be interpreted as by switching a relay as described above in relation to  FIG.  4   . 
     Alternatively, a leak-detection sensor  225  can be operatively coupled to an electrical circuit associated with one or more pumps  210  of a liquid-based heat-transfer system. For example, such a pump  210  can be electrically coupled to a header  231  having a power pin, a ground pin, a PWM pin and an FG pin. The power pin can be operatively coupled to the pump motor to convey an electrical current to the pump to operate the pump. The PWM pin be operatively coupled to a pump controller and convey a pump-control signal to the pump controller, e.g., to control a speed of the pump. The FG pin can convey monitor a tachometer signal emitted by the pump to a sensing circuit configured to monitor the pump (or fan) speed. 
     In one example (e.g., shown in  FIG.  8   ), a leak detector circuit  225  can be operatively coupled between the power pin of the header  231  and the pump motor  210 . In such an embodiment, the leak detector circuit  225  can interrupt a supply of electrical current to the pump (or increase a supply of electrical current to the pump) in response to a detected leak, causing a corresponding reduction (or increase) in pump speed. A corresponding FG signal emitted by the pump can reflect the diminished (or increased) pump speed. A system configured to monitor the FG signal emitted by the pump can, in response to a reflected change in pump speed, transmit an alert signal (e.g., to a system administrator), a system command (e.g., a command to increase a pump speed of another pump in an attempt to compensate for a diminished performance of a stalled pump, a system-shut-down command, etc.), or both. Some implementers might elect not to interrupt power to a pump if stopping a pump might be considered a catastrophic failure. 
     In an alternative embodiment, a leak detector circuit  225  can be operatively coupled between the PWM pin of the fan header  231  and the pump  210 . In such an embodiment, the leak detector circuit  225  can interrupt a PWM signal conveyed to the pump  210  by the PWM pin of the fan header and convey an alternative PWM signal (or no PWM signal) to the pump in response to a detected leak. The alternative PWM signal can cause the pump to speed up, to slow down, or to stop. An FG signal emitted by the pump can reflect the change in pump speed. A system configured to monitor the FG signal emitted by the pump can, in response to a reflected change in pump speed, transmit an alert signal (e.g., to a system administrator), a system command (e.g., a command to increase a pump speed of another pump in an attempt to compensate for a diminished performance of a stalled pump, a system-shut-down command, etc.), or both. 
     In still another alternative embodiment, a leak detector circuit  225  can be operatively coupled between the FG pin of the fan header  231  and the pump  210 . In such an embodiment, the leak detector circuit  225  can interrupt an FG signal emitted by the pump and convey an alternative FG signal (or no FG signal) to the FG signal pin in response to a detected leak. The alternative FG signal can simulate a diminished pump speed, a selected increased pump speed, or no pump speed. A system configured to monitor the simulated FG signal can, in response to a selected change in the simulated FG signal corresponding to a change in pump speed, transmit an alert signal (e.g., to a system administrator), a system command (e.g., a command to increase a pump speed of another pump in an attempt to compensate for a diminished performance of a stalled pump, a system-shut-down command, etc.), or both. 
     A leak sensor  225  can be positioned adjacent to (e.g., routed around) a pump  210  or other component of a liquid-based heat-transfer system, as indicated by way of example in  FIGS.  8  and  9   . For example, a sensor  225  can be positioned on, embedded in, affixed to, positioned adjacent to, or otherwise operatively coupled to a printed circuit board  205  such that the sensor defines a sensor region  226 . The sensor region can be selected to correspond to a region that might be susceptible to wetting by a working fluid in the event of a leak. 
       FIGS.  8  and  9    show examples of a sensitive region defined by a leak sensor  225 . The illustrated sensitive region  226  extends along the leak sensor (e.g., between points “A” and “B”) routed on a surface of a printed circuit board  205 . With circuits configured as shown in  FIG.  8   , the leak detector can be configured to interrupt a tachometer signal emitted by each pump  210  in response to a detected leak. Alternatively, each of the illustrated pumps  210  and the leak detector circuit  225  can be configured to emit one or more simulated fan-tachometer signals corresponding to one or more respective observed operational states. The one or more simulated fan-tachometer signals can be transmitted over the illustrated fan headers  231 , for example, to an IPMI bus. A controller can receive and interpret the one or more signals as a proxy for the observed operational state, and responsively issue one or more corresponding system commands. 
     In  FIG.  9   , the leak detector circuit  225 ′ is configured to interrupt a simulated tachometer signal in response to a detected leak. Such interruptions can simulate a tachometer signal emitted by a fan having a “locked rotor.” A corresponding control system configured to monitor a tachometer signal emitted from a fan can respond to a simulated “locked rotor” signal by initiating an alert or other system command 
     Overview of Flow Sensors 
       FIG.  10    illustrates a fluid circuit  310  having a pump  320 , an optical flow-rate sensor  330 , a heat exchanger  340  configured to transfer heat  341  from a heat dissipating component (e.g., a microprocessor), and a radiator  350  configured to dissipate heat  351  from the working fluid to an environment  352 . In some fluid circuits, the pump  320  and the heat exchanger  340  are combined into an operative subassembly, as described by way of example in U.S. patent application Ser. No. 12/189,476, among other patent applications. 
     A flow-rate sensor  330  can include a rotational member  332  positioned within a segment of conduit  331  and a tachometer  334  configured to detect a rotational speed of the rotational member. As shown schematically in  FIG.  11   , the rotational member  332  can be configured to receive momentum from a flow of a working fluid passing over the rotational member, in a manner similar as a turbine of a windmill receiving momentum from a flow of air passing over the turbine. A rotational member  332  of the type disclosed herein can include a generally axisymmetric arrangement of wings, foils, blades, faces, or screws positioned within a conduit suitable for conveying a flow of a working fluid such that a flow of a selected fluid passing over the arrangement of wings, foils, blades, faces, or screws applies a torsional force to the rotational member to urge the rotational member in rotation about the axis of rotation. 
     Some body portions comprise a thin shell member having opposed first and second sides  337   a ,  337   b . As shown in  FIG.  13   , a thin shell member can define a primary axis  338  extending longitudinally of the shell member and a secondary axis  338   a  extending transversely relative to the primary axis. The thin shell member can be twisted about the primary axis  338  so as to define a foil-shaped member configured to convert momentum from a passing fluid to a torsional force applied to the thin shell member.  FIG.  13    shows but one possible example of such a foil-shaped member. 
     A torsional force applied to the rotational member (e.g., member  332 ) can correspond to a rate of flow of a working fluid past the rotational member, with higher flow rates corresponding to relatively higher torsional forces. For example, a lift force on a wing in a stream of an incompressible fluid can increase in proportion to the square of the speed of the approaching fluid. A lift force applied to the rotational member  332  at a position spaced apart from a central, longitudinal axis  338  applies a turning moment (e.g., a torque, or a torsional force) to the rotational member about the axis  338 . The turning moment can urge the rotational member  32  in rotation. In some axisymmetric embodiments of rotational members  332 , the turning moment can urge the rotational member  332  in rotation about the central, longitudinal axis  338 . The turning moment can correspond to the lift force (torsional force) generated by the flow of the working fluid past the rotational member. Moreover, the speed of rotation of the rotational member  332  can correspond to the torsional force applied to the rotational member. Accordingly, the rotational speed of the rotational member can correspond to the speed of an approaching flow of the fluid. And, a volumetric flow rate (or a mass flow rate) of the fluid through a closed conduit corresponds to the speed of the fluid through the conduit. 
     Thus, a rotational speed of the rotational member  332  positioned within a fluid conduit can correspond to a volumetric flow rate (or mass flow rate) of a fluid through the conduit  331 . Although determining an algebraic expression for a relationship between rotational speed of a given rotational member in a flow of a selected fluid might be possible, such a relationship or correlation can be determined experimentally for each combination of rotational member configuration, conduit configuration, and working fluid. 
     Though not to scale, the plot in  FIG.  15    generally illustrates one example of a correlation between an observed rotational speed of the rotational member  332  and a (volumetric or mass) flow-rate past the rotational member. An apparatus of the type shown in  FIG.  14    can be used to assess such a correlation and to generate such a plot. 
     For example, a conventional flow-rate sensor (e.g., a Venturi-type sensor) can be used to determine each of several selected (mass or volumetric) flow rates of a working fluid, and the tachometer can emit a signal indicative of the rotational speed of the rotational member at each respective flow rate. Each flow-rate/rotational speed pair of readings can be plotted as indicated by the plot shown in  FIG.  15   , to reveal an experimentally determined correlation between rotational speed of the rotational member  332  and fluid flow rate through the conduit, for a particular combination of rotational member configuration, conduit configuration, and working fluid. In general, a unique correlation between rotational speed of the rotational member and flow rate of working fluid exists for each combination of working fluid, rotational member configuration, and conduit configuration. However, once a correlation between (volumetric or mass) flow rate and rotational speed is determined for a selected combination of conduit configuration, rotational member configuration, and working fluid, the rotational speed of the rotational member can be observed, and, based on the correlation of flow-rate through the conduit segment  332  and rotational speed of the rotational member, the corresponding flow rate (e.g., volumetric or mass flow rate) can be determined. 
     The tachometer can include any of a variety of known and hereafter developed sensor arrangements suitable to detect a rotational speed of the rotational member  331 . As but one example, a suitable tachometer can include an optical sensor having an emitter, a detector, and a counter. 
     For example, the rotational member  332  can be positioned in a conduit  331  having a transparent outer wall  331   a , or other suitable port configured to permit a selected frequency range (or band) of electromagnetic radiation (e.g., radiofrequency, X-rays, or light in the infrared, visible, or ultraviolet spectra) to pass therethrough. Although many suitable emitter and detector configurations are possible, the following discussion will refer to the emitter as a light emitter and the detector as a light detector by way of example, and not limitation, for ease of description. 
     In some embodiments of flow sensors, a light emitter  333  can emit light (e.g., for a duration substantially longer than a period of rotation for the rotational member  332 ) in a direction toward the rotational member  332 , and one or more portions  335  of the rotational member can reflect incident light (or other radiation band) from the emitter  333  toward the light detector  336 . The counter  337  can increment a count each time the light detector  336  detects light reflected by the reflective portion  335  of the rotational member  332 . Such detection can be responsive to a detected presence of light compared to a detected absence of light, or to a detected absence of light compared to a detected presence of light. With such an arrangement, a rate at which the count increases (e.g., a time-rate-of-change of the count) can correspond to a rate at which the light detector  336  detects a reflection of light from the rotational member  332 . In turn, the rate at which the light detector  336  detects a reflection of light from the rotational member  332  can correspond to a rotational speed (i.e., a frequency of rotation, or an angular speed) of the rotational member  332 , and thus, as noted above, a rate of flow of a working fluid through a selected conduit. 
     For example, the rotational member  332  can be configured to reflect incident light toward the detector  336  once per revolution of the rotational member about the axis of rotation, as with the member  332  shown in  FIGS.  13 ,  16 A and  16 B . As but one example, the rotational member  332  can have a relatively less reflective body portion  337   a ,  337   b  that rotates about an axis of rotation  338  and a relatively more reflective reflector portion  335  affixed to or on, or integral with, the relatively less reflective body portion  337   a ,  337   b . The reflector portion  335  can be so arranged as to reflect light toward the detector  336  once during each revolution of the body portion  332 . 
     The reflector portion  335  can comprise a reflector member positioned on one of the opposed sides  337   a ,  337   b  of the thin shell member shown in  FIGS.  13 ,  16 A and  16 B . With such an arrangement, a rate (or a frequency) at which the count increments in response to detected reflections from the reflector portion  335  can approximate the angular speed of the rotational member, which in turn can correspond to a rate of flow of a working fluid through the conduit  331 . 
     As another example, the rotational member  332  can be configured to reflect incident light toward the detector  336  twice per revolution of the rotational member about the axis of rotation. For example, the rotational member  332  can comprise opposed first and second reflective portions (not shown) so arranged relative to the opposed faces  337   a ,  337   b  that each of the first and the second reflective portions reflects light toward the detector  36  once during each revolution of the rotational member (i.e., such that light is reflected toward the detector  36  twice per revolution of the rotational member). With such an arrangement, one-half of a rate (or a frequency) at which the count increments in response to detected reflections from the first and the second reflector portions can approximate the angular speed of the rotational member, which in turn can correspond to a rate of flow through the conduit. 
     In general, the rotational member  332  can be configured to reflect incident light toward the detector N times per revolution of the rotational member about the axis of rotation. With such an arrangement, 1/N of a rate (or a frequency) at which the count increments in response to detected reflections from the rotational member can approximate the angular speed of the rotational member, which in turn can correspond to a rate of flow through the conduit. 
       FIG.  12    shows a particular exemplary embodiment of a flow-rate sensor  330  of the type described above. The illustrated sensor  330  has a transparent (in relation to a selected spectrum of incident electromagnetic radiation) segment  336  of conduit with a rotational member  332  positioned therein. A tachometer  334  is positioned externally of the conduit and is arranged to emit light (or other band of radiation) through the transparent segment  36  and toward the rotational member  332 . The tachometer  334  is further arranged to detect light (or other radiation) reflected by the rotational member  332  through the transparent segment  336 . 
     The illustrated sensor  330  also has a retainer  360  configured to suspend the rotational member  332  within the conduit  31  in spaced relation from an interior wall  331   b  of the transparent segment of conduit. Such a suspended arrangement can permit the rotational member  332  to rotate about a selected axis of rotation  338  within the conduit  331  and without being carried away by a flow of a working fluid passing through the conduit. 
     The retainer  360  can include an upstream retainer member  361  and a downstream retainer member  362 . One or both of the retainer members  361 ,  362  can be configured to urge outwardly against an inner wall  331   b  of the segment  331   a  of conduit. In some embodiments, one or both retainer members  361 ,  362  comprise an elongate member that resiliently urges against the wall  331   b , as shown in  FIG.  12   . 
     In some embodiments, one or both retainer members  361 ,  362  include a segment  363  configured to matingly engage with a correspondingly configured region of the inner wall  331   b  of the conduit  331   a . As  FIG.  12    shows, the segment  363  configured to matingly engage with the inner wall  331   b  can include a bent segment  364  of wire configured to rest within a corresponding détente  365 , or other recessed region of the inner wall  331   b.    
     The rotational member  332  can be rotatably coupled to the retainer  360 . For example, a first swivel member  366   a  can rotatably couple the rotational member  35  to an upstream retainer member  361  and a second swivel member  366   b  can rotatably couple the rotational member  335  to a downstream retainer member  362 , as shown in  FIG.  11   . 
     The conduit  331   a  having a rotational member  32  positioned therein can be fluidly coupled in series (or “in-line”) with one or more other components of a fluid circuit  310 . Such a placement of the conduit  331   a  can facilitate measurement of a rate of flow of a working fluid through the one or more components, once a correlation (e.g.,  FIG.  15   ) between observed rotational speed of the rotational member  332  and a volumetric (or mass) flow rate of the working fluid through the conduit  331   a  has been determined. 
     A flow sensor  330  of the type described herein can be incorporated in a cooling system, such as, for example, a cooling system configured to cool an electronic component or other device that dissipates waste heat during operation. As noted above, a tachometer output of such a flow sensor can indicate a rate of fluid flow through the sensor. 
     In some embodiments, the output of the tachometer  334  can be multiplexed so as to be compatible with a known or installed communication bus, e.g., over an IPMI bus. As noted above, a computer system incorporating a cooling system can include a controller configured to transmit an alert or other command in response to an observed signal exceeding a selected upper threshold or falling below a selected lower threshold. The observed signal can be emitted by a flow sensor. In some instances, the emitted signal can be emitted by a tachometer  334  configured to observe a rotational speed of a rotational member  332 , and the control system can transmit an alert or other command in response to an observed tachometer signal exceeding a selected upper threshold or falling below a selected lower threshold. The upper or lower threshold can correspond to an upper or a lower fluid flow-rate threshold. 
     As well, or alternatively, a flow-rate sensor, e.g., an optical flow-rate sensor  330  of the type described herein, can emit a simulated fan-tachometer signal in correspondence with an observed flow rate (or indicia thereof). For example, until a lower threshold flow rate (or indicia thereof) is observed by the flow-rate sensor, the sensor can emit a simulated fan-tachometer signal indicative of a given condition of a fan, for example, a stalled fan rotor. Between the lower threshold flow rate (or indicia thereof) and a selected upper threshold observed flow rate (or indicia thereof), the sensor can emit a corresponding simulated fan-tachometer signal indicative of a selected fan speed. As but one example, a correlation can be defined between simulated fan-tachometer speed and observed flow rate (or indicia thereof) between selected upper and lower threshold flow rates (or indicia thereof), as shown in  FIG.  17   . With such a pre-defined correlation, the sensor  330  can emit a simulated fan-tachometer signal correlated to (or encoding) a flow rate (or indicia thereof) observed by the sensor. The simulated fan-tachometer signal can be conveyed over a known bus using known protocols (e.g., an IPMI bus) and observed by a control system. The control system, in turn, can decode the simulated fan-tachometer signal using the known correlation ( FIG.  17   ) between observed flow rate (or indicia thereof) and simulated fan speed. 
     Some flow-rate sensors can have an electrical circuit operatively coupled to an FG signal pin of a header and be configured to emit a simulated tachometer signal having a waveform similar to a waveform emitted by an operating (or stalled) fan. The electrical circuit (not shown) can be further configured to emit a simulated tachometer signal having a waveform similar to a failed or failing fan in response to an observed flow rate (or indicia thereof) below a selected lower threshold. 
     A flow-rate sensor can be operatively coupled to a control system associated with one or more pumps of the liquid-based heat-transfer system. The control system can emit a control signal for adjusting operation of one or more pumps in the fluid circuit (e.g., a cooling system) responsively to an observed proxy (or other) signal emitted by a flow-rate sensor. 
     For example, if the signal emitted by the flow-rate sensor indicates a lower-than-desired flow rate (e.g., based on an observed system workload, such as a microprocessor workload, or read/write traffic across a memory bus), the control system can emit a control signal. Such a control signal can cause a given one or more pump to increase speed, can cause a supplemental pump to become operational, and/or can cause a valve to open (or close), to increase flow rate through a desired portion of a cooling system. Such a control signal can alter an operational state of a computer system. For example, if additional system cooling is unavailable by increasing pump speed, opening a valve, or operating a supplemental pump, the computer system can reduce or limit workload of a subsystem at risk of overheating (e.g., microprocessor workload can be limited or reduced, read/write traffic across a memory bus can be limited or reduced) absent increased cooling. 
     As another example, if the signal emitted by the flow-rate sensor indicates a higher-than-necessary flow rate (e.g., based on an observed system workload, such as a microprocessor workload, or read/write traffic across a memory bus), the control system can emit a control signal. Such a control signal can cause a given one or more pumps to decrease speed (e.g., to save power and/or lower acoustic emissions by the pump), cause a supplemental pump to slow down or to stop operating, and/or cause a valve to open (or close), decreasing flow rate through a portion of a cooling system to a suitable level. 
     Other Method Proxies 
     As one generalized example, a sensor circuit can be configured to emit a proxy signal corresponding to an observed operational state. Each in a plurality of discrete proxy signals can correspond to each respective observed operational state in a plurality of observable operational states. 
     In some instances, such a proxy signal can be a simulated fan-tachometer signal. Each discrete simulated fan-tachometer signal can correspond to a respective observed operational state. For example, a simulated fan-tachometer signal corresponding to a fan speed of 200 RPM can constitute a proxy for a selected observed flow rate (or indicia thereof, such as, for example, a rate of increasing count of detected reflections from a reflective portion  35  of a rotational member  32 ). With such an example, a simulated fan-tachometer signal corresponding to a different fan speed (e.g., 250 RPM) can constitute a proxy for another (e.g., higher) observed flow rate (or indicia thereof) within the system  10 . 
     As another example, a simulated fan-tachometer signal corresponding to a fan speed of 200 RPM can constitute a proxy for an observed first flow rate at a location within the system and a simulated fan-tachometer signal corresponding to a fan speed of 250 RPM can constitute a proxy for an observed second (e.g., different) flow rate.  FIG.  17    shows an example, pre-defined correlation between simulated fan-tachometer signal and flow rate. 
     In some instances, a simulated fan-tachometer signal corresponding to a fan speed of 200 RPM can constitute a proxy for an observed leak at a first location within the system and a simulated fan-tachometer signal corresponding to a fan speed of 250 RPM can constitute a proxy for an observed leak at a second (e.g., different) location within the system. 
     Such proxy signals can be transmitted over and observed from, for example, the IPMI bus. A controller operatively coupled to the IPMI bus can observe the proxy signal, interpret the observed proxy signal, as by comparison to a lookup table, and, if appropriate, issue one or more selected system commands responsively to the observed or interpreted proxy signal. 
     By way of illustrate of disclosed principles, the following table summarizes specific examples of proxy signals, proxy signal values and corresponding operational states represented by the proxy signal values: 
                                         PROXY SIGNAL   CORRESPONDING       PROXY SIGNAL   VALUE   OPERATIONAL STATE                                                simulated fan-   200   rpm   Leak at position “A” in system       tachometer signal   250   rpm   leak at position “B” in system           300   RPM   Pump 1 failure           350   RPM   Pump 1 operating normally           400   RPM   Pump 2 failure           450   RPM   Pump 2 operating normally           500   RPM   Observed temperature (e.g.,                   temperature of electronic                   component, pump motor, liquid                   coolant or air) over selected                   threshold or within a predefined                   range           550   RPM   Observed temperature (e.g.,                   temperature of electronic                   component, pump motor, liquid                   coolant or air) over another                   selected threshold or within a                   different predefined range                    
Computing Environments
 
       FIG.  18    illustrates a generalized example of a suitable computing environment  1100  in which described methods, embodiments, techniques, and technologies relating, for example, to control systems, may be implemented. The computing environment  1100  is not intended to suggest any limitation as to scope of use or functionality of the technology, as the technology may be implemented in diverse general-purpose or special-purpose computing environments. For example, the disclosed technology may be implemented with other computer system configurations, including hand held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like. The disclosed technology may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. 
     With reference to  FIG.  18   , the computing environment  1100  includes at least one central processing unit  1110  and memory  1120 . In  FIG.  18   , this most basic configuration  1130  is included within a dashed line. The central processing unit  1110  executes computer-executable instructions and may be a real or a virtual processor. In a multi-processing system, multiple processing units execute computer-executable instructions to increase processing power and as such, multiple processors can be running simultaneously. The memory  1120  may be volatile memory (e.g., registers, cache, RAM), non-volatile memory (e.g., ROM, EEPROM, flash memory, etc.), or some combination of the two. The memory  1120  stores software  1180  that can, for example, implement one or more of the innovative technologies described herein. A computing environment may have additional features. For example, the computing environment  1100  includes storage  1140 , one or more input devices  1150 , one or more output devices  1160 , and one or more communication connections  1170 . An interconnection mechanism (not shown) such as a bus, a controller, or a network, interconnects the components of the computing environment  1100 . Typically, operating system software (not shown) provides an operating environment for other software executing in the computing environment  1100 , and coordinates activities of the components of the computing environment  1100 . 
     The storage  1140  may be removable or non-removable, and includes magnetic disks, magnetic tapes or cassettes, CD-ROMs, CD-RWs, DVDs, or any other medium which can be used to store information and which can be accessed within the computing environment  1100 . The storage  1140  stores instructions for the software  1180 , which can implement technologies described herein. 
     The input device(s)  1150  may be a touch input device, such as a keyboard, keypad, mouse, pen, or trackball, a voice input device, a scanning device, or another device, that provides input to the computing environment  1100 . For audio, the input device(s)  1150  may be a sound card or similar device that accepts audio input in analog or digital form, or a CD-ROM reader that provides audio samples to the computing environment  1100 . The output device(s)  1160  may be a display, printer, speaker, CD-writer, or another device that provides output from the computing environment  1100 . 
     The communication connection(s)  1170  enable communication over a communication medium (e.g., a connecting network) to another computing entity. The communication medium conveys information such as computer-executable instructions, compressed graphics information, or other data in a modulated data signal. The data signal can include information pertaining to a physical parameter observed by a sensor or pertaining to a command issued by a controller, e.g., to invoke a change in an operation of a component in the system  10  ( FIG.  1   ). 
     Computer-readable media are any available media that can be accessed within a computing environment  1100 . By way of example, and not limitation, with the computing environment  1100 , computer-readable media include memory  1120 , storage  1140 , tangible, non-transitory communication media (not shown), and combinations of any of the above. 
     Other Exemplary Embodiments 
     The examples described herein generally concern control systems, with specific examples of control systems being configured to respond to a detected condition or operational state of a liquid-based heat-transfer system, e.g., to issue an alert or other command responsive to a detected leak of a working fluid or to issue an alert or other command responsive to an observed flow rate of a working fluid. As but one example, a pump speed can be adjusted responsive to a signal emitted by a flow-rate sensor. The signal can be indicative of an observed flow rate of working fluid (or an indicia thereof, by way of example, a rotational speed of a rotational member within a segment of conduit). Other embodiments of leak detectors, flow-rate sensors, methods, circuits and/or control systems than those described above in detail are contemplated based on the principles disclosed herein, together with any attendant changes in configurations of the respective apparatus and/or circuits described herein. Incorporating the principles disclosed herein, it is possible to provide a wide variety of control systems configured to issue an alert or other command, and/or, based on a detected change in state or operation (e.g., a detected leak or change in observed flow rate), to adjust an operation of a wide variety of systems, including by way of example, a heat-transfer system for any of a data center, a laser component, a light-emitting diode, a chemical reactor, photovoltaic cells, solar collectors, and a variety of other industrial, military and consumer devices now known and hereafter developed. Moreover, systems disclosed above can be used in combination with other liquid-based systems including, inter alia, reactor vessels. 
     Directions and references (e.g., up, down, top, bottom, left, right, rearward, forward, etc.) may be used to facilitate discussion of the drawings and principles herein, but are not intended to be limiting. For example, certain terms may be used such as “up,” “down,”, “upper,” “lower,” “horizontal,” “vertical,” “left,” “right,” and the like. Such terms are used, where applicable, to provide some clarity of description when dealing with relative relationships, particularly with respect to the illustrated embodiments. Such terms are not, however, intended to imply absolute relationships, positions, and/or orientations. For example, with respect to an object, an “upper” surface can become a “lower” surface simply by turning the object over. Nevertheless, it is still the same surface and the object remains the same. As used herein, “and/or” means “and” or “or”, as well as “and” and “or.” Moreover, all patent and non-patent literature cited herein is hereby incorporated by references in its entirety for all purposes. 
     The principles described above in connection with any particular example can be combined with the principles described in connection with any one or more of the other examples. Accordingly, this detailed description shall not be construed in a limiting sense, and following a review of this disclosure, those of ordinary skill in the art will appreciate the wide variety of fluid heat exchange systems that can be devised using the various concepts described herein. Moreover, those of ordinary skill in the art will appreciate that the exemplary embodiments disclosed herein can be adapted to various configurations without departing from the disclosed principles. 
     The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the disclosed innovations. Those of ordinary skill in the art will appreciate that the exemplary embodiments disclosed herein can be adapted to various configurations and/or uses without departing from the disclosed principles. For example, the principles described above in connection with any particular example can be combined with the principles described in connection with another example described herein. Various modifications to those embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of this disclosure. Accordingly, this detailed description shall not be construed in a limiting sense, and following a review of this disclosure, those of ordinary skill in the art will appreciate the wide variety of filtering and computational techniques can be devised using the various concepts described herein. 
     Similarly, the presently claimed inventions are not intended to be limited to the embodiments shown herein, but are to be accorded the full scope consistent with the language of the claims, wherein reference to an element in the singular, such as by use of the article “a” or “an” is not intended to mean “one and only one” unless specifically so stated, but rather “one or more”. All structural and functional equivalents to the elements of the various embodiments described throughout the disclosure that are known or later come to be known to those of ordinary skill in the art are intended to be encompassed by the features described and claimed herein. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 USC 112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or “step for”. 
     Thus, in view of the many possible embodiments to which the disclosed principles can be applied, we reserve the right to claim any and all combinations of features described herein, including the right to claim all that comes within the scope and spirit of the foregoing description, as well as within combinations recited in the following claims, literally and under the doctrine of equivalents.