Abstract: Apparatus for monitoring particles in fluid systems in which a detector has electrical contacts that are bridged by particles captured from suspension in the fluid. Upon detection of a particle, a control microprocessor stores a predetermined quantity of electrical energy on a capacitor, which is then discharged through an inductor to burn off the particle at the detector. The burn-off charge and return lines from the charge storage capacitor are un-grounded, and are connected by respective electronic switches through relay contacts to the conductors connected to the chip detector. The relay contacts are also connected to the inputs of a differential amplifier for detecting presence of a chip bridging the detector contacts and monitoring continuity of the conductors to and from the detector. The relay is powered by system voltage, and in the event of system power failure connects the chip detectors directly to warning lamps on an operator panel.

Abstract: Apparatus is disclosed for detecting the presence of the type of debris in a fluid system. The apparatus detects the presence of debris, measures the accumulation and identifies the specific types of the debris. A plurality of sensors, connected in a bridge detecting circuit, are assembled in the apparatus in the fluid flow, each of said sensors are tuned to a specific preselected frequency which is characteristic of one material type of the debris. A variable frequency power supply is connected to the detecting bridge for exciting the bridge. Switching means are provided external to the detecting apparatus for selecting one of said sensors as the detecting leg of the bridge. When the sensor has been selected, the bridge is excited at the resonant frequency of the selected sensor for identifying a specific material type of the debris.

Abstract: A method and apparatus for detecting the degree of deterioration in lubricating oil including a grid-like capacitive sensor, that uses the lubricating oil as a dielectric medium, and a magnetic field imposed upon the oil to attract ferromagnetic wear particles into the vicinity of the sensor. Preferably, the magnetic field is generated by a permanent magnet and an electromagnet aligned such that the magnetic field produced by each magnet acts upon the oil along the same axis. A plurality of capacitance measurements are taken at periodic intervals for one type of classification and analysis. Magnetically induced particle concentrations are analyzed visually under the aid of periodic magnetic flux reorientation and optical magnification.

Abstract: In one aspect of the present invention, an apparatus for detecting particles within a fluid is disclosed. A bobbin defines a debris collection chamber. Spaced apart first and second coils are wound about the bobbin in the proximity of the debris collection chamber, wherein the inductance of the coils are responsive to the particle accumulation within the collection chamber. An electromagnet is wound about the second coil to attract ferrous particles into the proximity of the second coil. A reference coil is wound about the bobbin to compensate for the changing temperature of the fluid. An oscillator is coupled to the first, second and reference coils for selectively producing an oscillating waveform at a frequency that induces eddy currents in the particles. The frequency of the oscillating waveform is indicative of the amount of accumulation of both the ferrous and non-ferrous particles.

Abstract: In one aspect of the present invention, an apparatus for detecting particles within a fluid is disclosed. A bobbin defines a debris collection chamber. A first coil is wound in a helix about the bobbin in the proximity of the debris collection chamber, wherein the induction of the first coil is responsive to the particle accumulation within the collection chamber. A reference coil is wound in a helix about the bobbin to compensate for changing fluid temperature. An oscillator is coupled to the first and reference coils for selectively energizing the coils with an oscillating waveform at a frequency that induces eddy currents in the particles. The frequency of the oscillating waveform is indicative of the amount of accumulation of both the ferrous and nonferrous particles.

Abstract: A metal particle detecting sensor (10), having a simple structure, can perform in-line analysis for metal particles in fluid and also can precisely estimate or detect the number of metal particles for each particle size. Metal films (1) and insulating films (2) are alternately laminated one upon another, and every other metal film (1) in the laminate is connected in parallel with one another to one of two lead wires (3a, 3b) so as to form comb electrodes in the laminate of the sensor, and one end face of the laminate is immersed in a fluid (4) containing metal particles so as to detect the metal particles.

Abstract: A chip detector is an open circuit device that attract metal chips circulating throughout a piece of equipment such as a transmission. If a chip closes the circuit in the chip detector, then it is subjected to a first pulse of energy in an attempt to burn or displace it from the chip detector contacts. The energy content of the first pulse is selected to burn fuzz, which represents chips of an acceptable size. If the chip remains in the chip detector, then it is subjected to one or more additional pulses each having a greater energy content than the first pulse. The number of pulses and their energy contents are recorded to determine the size of the chips captured by the chip detector and to provide a history of the equipment being monitored. If the chip in the chip detector exceeds a predetermined threshold, then an indication is given on a user interface.

Abstract: In an inductive debris monitor (IDM) an inductive probe is disposed at a fluid flow passageway to be monitored, and an RF bridge containing a variable resistance network is used to detect changes in the impedance of the probe due to the passage of metallic debris. An indirectly heated thermistor is utilized in the resistance network for balancing the bridge. Thus the operating temperature range is significantly extended without compromising reliability or experiencing degradation.

Abstract: An apparatus for detecting the concentration of particles in a liquid includes a light-emitting portion for emitting examination light which is totally reflected-by a boundary surface contiguous to the liquid to be examined, a photosensor for receiving the totally-reflected light, a reference sensor for directly receiving the examination light, and a judgment portion for finding the reflectance from outputs of the two sensors to compute the particle concentration. In case that the liquid to be examined is lubricating oil of an engine, the concentration of carbon particles is calculated, thereby judging the deterioration of the lubricating oil.

Abstract: The on-site analyzer (10) includes a housing (14). The housing (14) contains an infrared spectrometer (18) and an optical emission spectrometer (24) for testing a sample (12) of oil. A conduit (32) by operation of a pump (40) draws the sample (12) into the conduit (32), and controllably moves the sample (12) to an infrared spectrometer (18) for testing, and subsequently to an optical emission spectrometer (24) for testing. The pump (40) and spectrometers (18, 24) are controlled by a computer (50) for complete automation of testing. The computer (50) performs analysis and diagnosis of the test results from the spectrometers (18, 24) based on sets of known and standard information.

Abstract: A debris detecting apparatus is provided which collects debris from a fluid system in a screen member and measures the accumulation of debris by measuring the change of inductance in a coil surrounding a portion of the screen member. A bridge detection means is used which includes the inductive coil as one leg of the bridge. A second sensor in the fluid connected as a second leg of the bridge provides temperature compensation for the bridge. The bridge is excited with a high frequency voltage at or near the resonant frequency of the sensing coil.

Abstract: The on-site analyzer (10) includes a housing (14). The housing (14) contains an infrared spectrometer (18) and an optical emission spectrometer (24) for testing a sample (12) of oil. A conduit (32) by operation of a pump (40) draws the sample (12) into the conduit (32), and controllably moves the sample (12) to an infrared spectrometer (18) for testing, and subsequently to an optical emission spectrometer (24) for testing. The pump (40) and spectrometers (18, 24) are controlled by a computer (50) for complete automation of testing. The computer (50) performs analysis and diagnosis of the test results from the spectrometers (18, 24) based on sets of known and standard information. The computer (50) may communicate with a central, remote computer (70). Remote computer (70) accumulates results from a plurality of on-site analyzers (10) to obtain trend and wear evaluations, and to update the standard information in each of the on-site analyzers (10).

Abstract: In one aspect of the present invention, a sensor detects particles within a fluid. The sensor includes a housing that defines a cavity. A magnet is disposed adjacent the cavity bottom to attract particles into the cavity. A first coil is wound about the outer surface of the cavity. The induction of the first coil is responsive to the particle accumulation within the cavity. A second coil is wound about the magnet. The induction of the second coil is responsive to the temperature of the fluid and independent to the particle accumulation within the cavity.

Abstract: A system for sensing metallic debris in fluid flow, including an inductive probe utilizing one or more capacitors with a single turn resonant coil disposed about the flow passageway wherein a second, co-linear, multiple turn coil is provided about the passageway and loosely coupled via mutual inductance to the resonant coil such that the two windings produce a step-up transformer with increased impedance to closely match the impedance of the electrical output cable. This impedance matching reduces output signal loss permitting the sensitive components of the system to be remotely located from the usually hostile environment of the probe. Additionally, a toroid element of low loss, low reluctance material is disposed between the probe and its conductive housing to provide a guiding path for the magnetic "lines of force" of the sensing field produced by the probe, to avoid losses in the housing.

Abstract: An electrode structure of a metallic particle detecting sensor capable of detecting with high efficiency metal powders floating in a wide range in an oil tank. The electrode structure is formed by opposing, on a substrate, a pair of electrodes (1, 2), formed of thin film metals of such as Ta, W, Pt, Cr, Au or the like, having combed (toothed) structures with their respective teeth being meshed or interdigitated with each other.

Abstract: A device to detect the presence of particles, principally metallic particles, in fluid lines. The fluid to be sampled flows through a former on which is wound a coil assembly comprising a sensor coil flanked upstream and downstream by two field coils. The former is constructed with a double wall and air gap such that pressure fluctuations in the fluid are not transmitted to the coil assembly to adversely affect the positions and readings of the coils. The field coils are driven by a high frequency signal derived from a crystal oscillator and the upstream and downstream sections are wound in opposite directions such that the magnetic field from each section is cancelled out in the plane of the sense coil.

Abstract: Wear particle monitoring is carried out on samples of a lubricating oil extracted from lubricated equipment together with the naturally appearing burden of ferromagnetic particles. The extracted sample is collected in a bottle to a depth sufficient to exceed the flux field of a sensing coil to which the bottle is subsequently applied whereby the numerical value (PQ value) assigned to the particle burden is substantially insensitive of the volume of liquid in the bottle. From time dependent measurements information as to the size of the particles in the burden can be obtained.

Abstract: A metal particle detector apparatus for use with flowing non-conductive fluid such as lubrication oil from an engine or other rotary device wherein the lubricated components include bearings or the like susceptible to failure, thereby placing harmful metal particles in the non-conductive fluid. A circuit grid comprising two electrodes affixed on a perforated electrically non-conductive plate which is disposed in the fluid flow in a manner which will catch the particles being carried by the fluid and when one, or an accumulation of these particles bridges the gap between electrodes completes an electrical circuit passing into the device from outside. In one embodiment, a primary and secondary seal are provided for sealing fluid against leaking into the electrical chamber leading to the electrical circuit or alarm system while still permitting the electrical circuit system to be electrically connected to the detector unit which is disposed in the flowing fluid. Compressible O-ring seals are used.

Abstract: An electrical magnetic detector disposed in the passage of a flow of fluid to detect the magnetic particles transported by that flow and to trigger an alarm after detection of a predetermined quantity of magnetic particles. It includes a magnet surrounded by two polar masses separated by an axial air gap. The external surface of the detector has, at least adjacent the air gap (4), peripheral surfaces (5, 6) converging toward the air gap. The detector is useful in the surveillance of wear members to detect the degree of wear thereof.

Abstract: A small sample of combustion engine lubrication oil is analyzed, simultaneously, for its viscosity and contamination level in a compact, self-contained, computer-controlled unit. If the combination of viscosity and contamination level falls outside of the acceptable range, which is determined by comparison to a digitally stored matrix of the various states of used motor oil, a warning is displayed recommending that the oil be changed.

Abstract: An inductive debris monitor for detecting chip-particles in tubing of a fluid wetted system. The inductive debris monitor includes a multi-turn detector and an electronic circuit having a resistor bridge configuration. The multi-turn probe detector wraps around the tubing of a fluid wetted system a plurality of times to detect the presence of the chip-particles while the electronic circuit analyzes and deciphers the detected chip-particle information.

Abstract: A simpler, multiple element chip detector having two parallel, electrical conductors shrouding a plurality of discrete magnets which are aligned along the longitudinal axis of the chip detector is provided. The poles of each magnet abut opposite conductors defining a chip gap. A plurality of electrical isolators are mounted about the conductors and positioned between each of the magnets to define a chip element therebetween. The number of chip elements can be varied by adding or subtracting a magnet and isolator. The conductors are mounted in a housing to receive an electrical potential. Thus, regardless of the number of chip elements no additional circuitry is required. In an alternative embodiment the discrete magnets are replaced with a single magnet. In this embodiment the number of chip elements can be increased or decreased by only adding or removing an isolator.

Abstract: A debris monitor apparatus and method and optical sensor apparatus and method that use the Faraday Effect to detect accumulation of debris on a magnetic probe. The probe contains a magnet and the optical sensor and is immersible in the fluid. As debris accumulates on the probe, the magnetic field changes. The optical sensor includes a magneto-optic material located within a portion of the changeable magnetic field, preferably near a node of the magnet. Means are provided to bi-directionally address the sensor magneto-optic material with linearly polarized light. Means are also provided to convert rotation of the light polarization angle into a modulation of the light's intensity. An electro-optic transducer converts the light intensity into electrical signals that indicate the quantity of debris accumulation on the probe, with a differentiated value of these signals with respect to time yielding rate of debris accumulation on the probe.

Abstract: An apparatus is adapted to detect particles within a fluid. The apparatus includes first and second electrodes within a housing. The first and second electrodes form a "V" shaped recess. A magnet is positioned within the housing to attract ferrous particles into the recess. The resistance between the first and second electrodes is measured as an indication of particles within the fluid.

Abstract: For use in conjunction with a fluid retentive housing defining a fluid volume and containing a lubricant, wherein there is provided a fluid port for the lubricant, an integral filler cap and metal debris detector assembly for removable placement within the fluid port comprising a filler cap defining an outer surface for insertion into the fluid port and further defining a bore and magnetic apparatus arranged within the bore and immersed in the fluid volume for attracting metal debris suspended in a lubricant fluid flow so as to remove metal debris from circulation.

Abstract: An inductive coil shaped screen traps debris in a fluid and senses the accumulation of the debris. The shaped screen coil comprises non-conductive wires parallel to one another and at least one conductive wire coil perpendicular and woven with the non-conductive wires. The conductive wire is a coil of an inductor. As debris accumulates within the coil, inductance immediately changes and indicates the accumulation of debris in the screen. Another coil connects with the first coil in a bridge circuit to compensate for changes in temperature of the fluid.

Abstract: A particle sensor having a magnet for attracting and retaining metallic particles in a lubricating fluid is disclosed having a sensor to determine when the particles attracted and retained on the magnet have reached a predetermined level and to transmit this information to a machine operator. The sensor, which includes a plug removably attached to the machine casing, has a magnet which extends into the path of the lubricating fluid to attract and retain metallic particles. A pair of coils, one attached to the moveable plug and one attached to the casing adjacent to the plug are inductively coupled such that the accumulation of particles on the magnet changes the inductance of the coils. An electronic circuit converts the changes in inductance to voltage changes to activate an alarm or other signal to the operator when the threshold value has been reached.

Abstract: An apparatus for detecting magnetic particles suspended in a liquid, such as particles suspended in lubricating oil flowing through an engine. The oil is caused to pass through the area coupling two or more coils while the mutual inductance of the coils is monitored. When a magnetic particle enters the area coupling the coils, the mutual inductance of the coils will increase, thereby indicating the presence of the particle.

Abstract: A lubricating film thickness measuring system and method includes a rolling element bearing having a relatively rotatable electrically insulated inner and outer races forming a raceway therebetween. Rolling elements are movably mounted in the raceway. Apparatus is provided for measuring capacitance across the bearing. The measuring apparatus includes an oscillator circuit and a phase detection circuit and is responsive to a phase angle shift between a first signal going from an oscillator circuit to the bearing and from the bearing to the phase detection circuit, and a second signal received by the phase detection circuit directly from the oscillator circuit. The method involves applying a predetermined electrical signal across the rolling elements and measuring the capacitance across the rolling elements thereby providing a measure of the lubricating film thickness.

Abstract: A monitoring system for detecting contamination of a liquid entrained in a circulating fluid, including a housing having an inlet for receiving the fluid, a reservoir for containing extracted liquid, and an outlet portion disposed in predetermined relationship with the reservoir for venting the fluid from the housing. There is also a separator, which may be formed as part of the housing, for extracting liquid from the fluid and directing the extracted liquid to the reservoir. A probe, disposed in the reservoir, measures an electrical parameter representative of the electrical resistance of the liquid to detect a change in resistivity indicative of contamination of the liquid.

Abstract: Centralized greasing installation, each stage of which comprises a metering iston (12) with two rods (19, 20), the ends of each of the two rods (19, 20) sliding respectively inside a lubrication chamber (21) and a resetting chamber (22), said chambers being supplied with air and connected to the atmosphere by a bore (23, 24) of an adjusting stopper (17, 18) which is closed by the end of the corresponding rod at the end of corresponding lubrication and resetting travel of the piston (12), with means for detecting the establishment of pressure, wherein the various lubrication air chambers (21) are connected together and to a common air source having a limited flow rate as well as to a common pressure switch, while the various resetting air chambers (22) are placed in communication with each other and with another compressed-air source having a limited flow rate and to another pressure control switch, in order to verify, with limited means, that between the resetting phase and the lubrication phase, all the mete

Abstract: An arrangement for detecting metallic particles carried by a fluid includes a metallic probe member which surrounds an elongated passage through which the fluid flows and which is constituted by a split tube having two marginal portions delimiting a gap which extends along said passage and completely separates the marginal portions from one another. Capacitors are arranged at the gap and alternating electric current is caused to flow in the probe member around the passage so that the probe member and the capacitors form a tank circuit having resonance characteristics that are influenced by any inclusion then present in the passage in a manner dependent on the electromagnetic properties of the inclusion. The character of any metallic particle then present in the passage is determined from variations in the alternating electric current that reflect the influence of such metallic particle on the resonance characteristics.

Abstract: A magnetic sensor for the collection and measurement of ferrous particles through the use of an electronic tuned circuit is disclosed. The sensor contains an electrical inductance along with an integral magnet to attract ferrous particles suspended in the fluid. The ferrous particles collecting on the magnetic surface of the sensor causes a change in the inductance of the integral coil which is measured by an electronic circuit. The electronic circuit operating in conjunction with the sensor utilizes a parallel tuned resonant circuit and time variant frequency multiplying to eliminate undesired response changes of the tuned circuit due to the temperature change of the sensor. The sensor may be located in environmental conditions which include a large variation in ambient temperature. The disclosed invention provides for compensation of such undesired changes caused by temperature change so that the actual change of the inductance in the sensor can be monitored without interference.

Abstract: A monitoring system for detecting contamination of a liquid entrained in a circulating fluid, including a housing having an inlet for receiving the fluid, a reservoir for containing extracted liquid, and an outlet portion disposed in predetermined relationship with the reservoir for venting the fluid from the housing. There is also a separator, which may be formed as part of the housing, for extracting liquid from the fluid and directing the extracted liquid to the reservoir. A probe, disposed in the reservoir, measures an electrical parameter representative of the electrical resistance of the liquid to detect a change in resistivity indicative of contamination of the liquid.

Abstract: A diagnostic debris sensor operating on the inductive pulse is triggered by ferrous particles floating by or attracted to a permanent magnet within a pulse-sensing coil and the entire sensor at that, the pulse-sensing coil is equipped with a second winding for either continuous monitoring or an occasional push-to-test checking of the pulse-sensing coil.

Abstract: An annular absorbent filter element of a sensor probe is exposed to fluid while held by one end of a coil spring in contact with magnetic and non-magnetic discs non-conductively spaced from a third non-magnetic disc to supply a contamination signal to a two-stage detection circuit through which internal deterioration conditions of machinery are monitored. Electrically conductive and magnetic flux paths are established through the filter element and a flux gap to magnetically attract contaminants and establish parallel connected, variable resistances through which said contamination signal is derived. The signal output of the sensor probe thereby provides an early warning alert to prevent machinery damage.

Abstract: An instrument for detecting the condition of an oil is provided in which an ultrasonic wave having a predetermined waveform is propagated through the oil and assumes a change in waveform in accordance with the particular condition of the oil. The instrument includes an arrangement in which any change in the waveform of the propagated ultrasonic wave is translated into a change in terms of time involving the waveform, which is used for the detection of the particular condition of the oil.

Abstract: An on-board sensor system which compares the dielectric properties of lubricating oil in an internal combustion engine with the dielectric properties of unused oil in a contained volume and providing an output signal indicative of the changes in viscosity correlated to the dielectric constant changes of the lubricating oil.

Abstract: This invention relates to a device for detecting electrically inductive particles in a fluid medium and is of particular use for detecting ferrous particles in the lubricant of an engine or mechanical transmissions thereby giving warning of malfunction of the engine or the transmission. The invention is of particular importance for the operation of an internal combustion engine, especially highly stressed helicopter and aircraft engines wherein prior warning of a malfunction permits corrective action prior to a catastrophic failure.

Abstract: An improved method and device for measuring magnetic particles in a liquid wherein a known amount of liquid is contacted with a bed of ferromagnetic fibers maintained in the magnetic field of a magnet with sufficient strength to substantially completely magnetize said particles for a time long enough for said particles to migrate to and be substantially completely captured by said fibers and having a magnetic flux sensor positioned between a pole face of said magnet and said bed wherein the distance from said sensor to the fibers farthest from said sensor is not greater than about 6 millimeters.

Abstract: A method and device for measuring the amount of magnetic particles in a fluid such as ferrous wear particles in the lubricant of a machine. The particles are entraped by contacting a volume of the fluid with a means to entrap the particles and magnetized by subjecting the entraped particles to a magnetic field. The amount of magnetic particles entraped is measured by measuring the magnetization of the entraped and magnetized particles.

Abstract: An apparatus is provided for detecting metal debris particles in a fluid flowing in machinery. The apparatus is adapted to be installed in machinery and has a grid means of electrically insulating material provided with openings of a first type and a second type respectively to allow fluid passage therethrough. Each opening is conductively connected to electric means adapted to provide an electric signal when a conducting particle of metal debris having a predetermined size is trapped in or across any of the openings.

Abstract: A machine oil deterioration detection apparatus having a copper thin film resistor formed on an electrically insulating holding member and immersed in a machine oil. The copper thin film resistor is corroded by acid produced in accordance with deterioration of the machine oil, thus changing the resistance of the resistor. The change in resistance is detected by a detection circuit, connected to the thin film resistor, for detecting the resistance of the thin film resistor.

Abstract: An apparatus and method is provided for infrared qualitative analysis of a fluid independent of the temperature of the fluid. A first signal is generated in response to detected infrared energy passing through the fluid, and a second datum signal is provided for comparison with the first signal, wherein the second signal indicates a non-radiating state. The energy difference between the two signals is provided to a microprocessor control system for computing the fluid quality as a function of the energy difference independent of the temperature of the fluid.

Abstract: The degree of deterioration of lubricating oil used in an internal combustion engine is detected by measuring the lubricating oil dielectric constant with a pair of spaced sensor capacitor electrodes in the lubricating oil. The sensor capacitor is connected to a fixed capacitor to form a voltage divider connected across a constant frequency AC voltage source so that a developed voltage across the sensor capacitor corresponds to the lubricating oil dielectric constant. The frequency of the AC voltage is set at a value ranging from 50 KHz to 500 KHz.

Abstract: Frequently, particularly in the case of old machines such as engines, traissions, and gear boxes, particles, or "chips", are produced as the internal parts wear out. When these chips are present they indicate equipment damage and must be removed since they can cause additional damage to the equipment. For over a decade such electric chip detectors have been installed in helicopter transmissions and engines, as well as in propulsion and drive systems of many military fixed wing aircraft. Detectors usually have a pair of permanently magnetized electrodes that are insulated from each other and positioned to form a magnetic field in the small gap therebetween. One of the underlying problems of monitoring wear particles is that detectors can be triggered by an accumulation of metallic dust, causing a false chip detection warning.

Abstract: An improved method and apparatus of producing Ferrograms in which centrifugal force is used to facilitate the spread of a deposit of magnetically or electrically polarized or polarizable particles onto a substrate to facilitate subsequent analysis of the particles.

Abstract: Means for shutting down operation of an enclosed motor compressor unit for air conditioning and heat pump service consists in immersing a pair of spaced electrodes having a potential gradient thereacross in the refrigerant-oil mixture in the unit enclosure and in employing the ion and electron flow across the electrodes resulting from contaminants in the mixture to break the motor energizing circuit through relay means, and in the provision of a normally open fusible switch responsive to close in response to a predetermined high motor temperature and through relay means break the motor energizing circuit. In a simplified modification, the immersed electrodes are directly connected across the power supply leads and cause a fusible link connected in the power supply leads to open and break the motor energizing circuit when sufficient conduction across the electrodes occurs.

Abstract: A debris detection system for detecting debris in a fluid flow in which both a primary and a secondary fluid flow from a fluid reservoir are maintained. The fluid reservoir is situated downstream of the system being served and receives the debris from the system being served. Within the reservoir the primary fluid flow is shielded from all but the colloidal debris received. All non-colloidal as well as colloidal debris is diverted to the secondary fluid flow where the amount and rate of debris accumulation is detected.

Abstract: Apparatus is provided for detecting ferrous contamination in a fluid such as engine transmission lubricant. The apparatus has two spaced apart electrodes, a magnetic flux extending between the electrodes the lines of force of which are substantially rectilinear, the electrodes being connectable to a circuit for signalling when an electrically conductive path is formed from one electrode to the other by metal particles attracted to the flux and formed into an electrode spanning bridge. For preference the sensor is a hollow plug, which in use is screwed into a transmission housing port, and has a flat end wall in contact with the lubricant in the housing. The plug contains a magnet having both poles disposed towards the plug end wall, a portion of the end wall acting as one electrode. A ferromagnetic disc mounted externally from the plug end wall, and electrically insulated therefrom, overlies the poles.