METHOD AND APPARATUS FOR ASSESSING PARTICLE DEPOSITS

Abstract: A method of assessing a property of the particle burden in a liquid system which comprises, subjecting a sample of the particles from the system, with or without the liquid that is/has been entraining it, to an electric or magnetic force field and monitoring the change caused to the force field when the sample is and is not placed therein.

Description: Method and apparatus for assessing particle deposits This invention relates to a method and apparatus for obtaining a quantitative measurement of some property (typically the amount) of particulate material present in a liquid sample. The particulate material can be in contact with the liquid or it can be deposited, for example on a solid substrate. The invention has particu¬ lar, but not exclusive, application to the quantification of wear debris present in a used lubricating oil or other fluid circulating in a machine subjected to wear. It is known to draw down solid particles from their entrainment in a liquid onto a suitable substrate for the purpose of examining the particles and so determining such information as their composition, shape, number or other useful characteristics. Examples are the use of a magnetic plug in a lubricating oil system or the use of a filter to separate particles above a specific size from a liquid. In a further example, magnetic or electric fields may be used to separate polarised or polarisable particles from suspension in a liquid medium, as in U.K. Patent 1,415,311 (Trans-Sonics Incorporated), U.K. Patent Spec¬ ification 2103358 (Jones & Kwon) and U.K. Patent Applica¬ tion No. 8329151 (The University College of Swansea). In a system of moving parts which is operating nor- mally, the loss of particles to a fluid circulating in the system will be roughly at a constant rate so that if a sample of lubricating oil (for example) used in the system for a given period is taken at periodic inter¬ vals, it will be carrying similar loads of wear debris. Thus one early indication of a change in the performance or operating mode of a system can be a change in the debris load carried by the circulating fluid. An engine or other machine having oil lubrication may continue to operate in a steady manner for hundreds or thousands of hours. Any change in the quality of lubrication or in the condition of some lubricated surface in that machine may be first detected by a change in the wear debris separated out of the oil system onto a suitable substrate. It can therefore be important to monitor such separated debris routinely in order to detect any change in the condition of the engine or other machine as soon as possible. The traditional method by which deposited particulate material is examined involves subjecting the deposit to a detailed examination by means of optical or electron microscopy, typically coupled with compositional analysis and particle counting techniques. Such detailed examin¬ ation is relatively time-consuming and requires the ser¬ vices of highly-trained personnel, and is therefore rela- tively expensive. For this reason such detailed examina¬ tion may not be appropriate for frequent routine monitor¬ ing of large numbers of liquid systems. This invention makes use of a second method of exam¬ ination which seeks merely to obtain a numerical quantifi- cation or an index of the quantity of the debris present in the liquid or deposited therefrom onto a suitable sub¬ strate. Such a quantification or quantity index will remain substantially unchanged so long as no significant change occurs in the liquid system, but a change in the quantification or quantity index will indicate a change in the liquid system. Provided therefore that a technique can be provided for obtaining the quantification or quan¬ tity index that does not involve too much expense, it can be applied routinely to samples of liquid or particu- late material separated from a liquid system in order to detect changes in that system. Once a significant change in the particle burden in a liquid system has been detected, a deposit of par¬ ticulate material that heralded the change can be examined in detail as hereinbefore described in order to provide more extensive information about the nature of the changes which have occurred in particle burden and by inference in the machine in which the liquid system was circulating. ' In its broadest aspect the invention relates to a method of assessing a property of the particle burden in a liquid system which comprises, subjecting a sample of the particles from the system, with or without the liquid that is/has been entraining it,to an electric or magnetic force field and monitoring the change caused to the force field when the sample is and is not placed therein. The sample of particles can be a small volume of the liquid from the system placed in a shallow container (e.g. of glass). It is -desirable that the material of the container does not affect the force field to any sig¬ nificant extent since this permits a quantification of the sample to be effected simply by noting the change produced in the force field when the container with its sample is, or is not, within the force field. As an alternative, the sample of particles can be removed from the liquid system, e.g. by removing the de¬ posit captured by a magnetic plug in the liquid system from the magnetic plug. Thus, after removing a magnetic plug from the liquid system, the deposit on the plug can be removed therefrom (with or without prior washing to remove residual system liquid) by contact with a substrate having a tacky surface. As a further alternative the sample can be produced on apparatus described in Jones and Kwon's UK Patent Application 2103358. Th€* present invention will now be further described by way of example with reference to the accompanying drawings,in which Figure 1 shows, in section, a suitable container for holding a liquid sample, Figure 2 shows the container in place on a magnet to ' create a particle deposit from a liquid sample in the container. Figure 3 shows how a particulate sample can be taken from a magnetic plug for subsequent use in the method of the invention. Figure 4 is a schematic view of apparatus for quanti¬ fying a deposit, and Figure 5 is a further deposit for quantifying. In a typical embodiment of the invention a sample of the system liquid containing the particulate material is delivered to a cylindrical container 10 of the shape shown in Figure 1. The material of the container is such that it does not prevent the use of a measurement tech¬ nique which depends on monitoring the change caused by the particulate material in a magnetic or electric force field. The dimensions of the container are not restricted to those given in the drawing, but must be suitable for the dimensions of the magnetic or electric force .field used in quantification of the particulate material. The volume of the system liquid delivered to the container 10 is chosen to contain sufficient particulate material to generate a change in the magnetic or electric force field which is large enough for reliable detection and measurement. A solvent may be added to the system liquid at any time prior to the measurement in order to improve ho ogenisation, fluid flow or particle movement. The sample of system liquid may be delivered to the con- tainer 10 by any suitable means, such as, by way of example, a fixed or variable volume pipette, a measuring cylinder, or a direct flow line from the system in which the liquid circulates. The container 10 is then transferred to a measuring system generally as shown in Figure 4 hereof and an index derived of the quantity of particulate material present in -the container 10. In one embodiment of carrying out the method of the invention, the container 10 is placed within the field of a permanent magnet or an electromagnet 11, or within an electric field, either during or after the delivery of the fluid sample in order to assist in the stabilisation or the particulate material within the container 10. Such a magnet 11 can also be rotated and may be as shown in Figure 2. In a further embodiment of carrying out the method of the invention the container 10 may contain a pad 12 of absorbent material to absorb the carrier liquid and stabilise the particulate material. In a still further embodiment of carrying out the method of the invention, a suitable material may be added to the liquid in the container 10 in order to thicken or gelate the liquid and again stabilise the particulate material. In any of the previously described embodiments or at any stage in the method of the invention, a suitable chemical material may be added to the liquid or to the container 10 in order to generate or modify the electric or magnetic property of the particulate material by which the quantification will be made. In any of these embodiments or at any stage in the method of the invention, a suitable chemical material may be added to the liquid or to the container 10 in order to stabilise the chemical nature of the particulate mater¬ ial. Such a chemical material might include, but is not ' restricted to, an anti-oxidant or corrosion inhibitor. Figure 3 shows an end 13 of a magnetic plug from an -oil sump of a machine with a particulate deposit 14 held thereon. 15 is a fibrous filter pad coated on one face with an adhesive layer 16. By pressing the pad 15 onto the deposit 14 s the latter can be removed from the plug and the pad with its deposit can then be quantified on the apparatus as shown in Figure 4 hereof. The apparatus shown in Figure 4 does not constitute any part of this invention being merely a convenient way of quantifying a deposit. Referring to Figure 4 a magnetic core rod 21 is surrounded by an excitation coil 22, the coil 22 being connected to an oscillator 23 operating at between 1 and 50 kiiohertz (e.g. 5 kilohertz) and incorporating a push-pull circuit to reduce pick-up of stray signals. The core rod 21 serves to concentrate the AC magnetic field produced by the coil 22. Two torroidal pick-up coils 24, 25 are disposed in axial alignment with the core rod 21 and are slightly spaced by equal amounts from the ends of the rod 1 so that each coil 24, 25 is in a similar diverging magnetic field. The coils 24, 25 are electrically linked so that their outputs subtract one from the other, the resultant output being fed to a simple amplifier 26 of limited gain. The amplifier 26 feeds its output to a series of amplification stages 27a, 27b, etc., each of which provides a measure of high and/or low pass filtering so that the band pass of the amplifying system is centred on the AC drive frequency of the coil. The output from the final amplification stage is fed to an AC/DC analogue converter 28 and then to a digi¬ tal display unit 29. To compensate for possible variations in the driving power of the oscillator 23, a reference signal can be taken from the oscillator 23, on a line 30, and used to modify the output of the display unit 29. Figure 5 shows a glass plate 31 with an annular deposit of the kind produced on the rotary depositor which is described in UK Patent Application 2103358. This is a particularly suitable deposit pattern for use with an annular pick-up coil 24 as shown in Figure 4. However, other deposit patterns (e.g. such as produced from the apparatus described in UK Patent Specification 1415311) can also be quantified on the apparatus shown in Figure 4. To use the apparatus, the output to the unit 29 is zeroed (i.e. the outputs from the coils 24, 25 are balanced) and then a substrate 31 with the deposit 32 to be assessed is placed on a support 33. The ferromag¬ netic particles in the deposit 32 distort the flux through the coil 24 and unbalance the outputs from the two pick¬ up coils causing a digital read out on the unit 29 which gives an immediate quantification of the ferromagnetic "density" of the deposit 32. Where the substrate used affects the field by itself it is possible to locate a substrate without deposit thereon just below the pick-up coil 25 and to balance the coil output with a further clean substrate above the coil 24. In further developments of the invention, separate quantifications or quantity indices are obtained for different areas, groups or categories of particles within the overall deposit of particulate material appearing on the substrate. By way of example, but not exclus¬ ively, such different areas may be differentiated in accordance with their axial, radial, lateral, circumfer¬ ential or vertical position on the substrate, and such groups or categories may be differentiated by size, shape, colour or composition. The obtaining of separate quantifications or quantity indices for such areas, groups or categories of particles may be achieved by several different techniques, including but not restricted to the following: varying the frequency of oscillation of the electric or magnetic field, vary¬ ing the intensity of the electric or magnetic field, or using a multiplicity of . separate detection systems either individually or in an assembly of such separate detection systems.

Claims: CLAIMS 1. A method of assessing a property of the particle burden in a liquid system which comprises, subjecting a sample of the particles from the system, with or without the liquid that is/has been entraining it, to an electric or -magnetic force field and monitoring the change caused to the force field when the sample is and is not placed therein. 2. A method according to claim 1, in which the sample of particles is a small volume of the liquid from the system placed in a shallow container. 3. A method according to claim 2, in which the material of the container does not affect the force field to any significant extent whereby the quantification of the sample is effected simply by noting the change produced in the force field when the container with its sample is, or is not, within the force field. 4. A method according to claim 1, in which the sample of particles is removed from the liquid system prior to quantifying. 5. A method as claimed in claim 4, in which the sample of particles is formed on a surface of a non¬ magnetic substrate by the application of a magnetic field having a component- normal to the substrate surface while the liquid from the system flows over it, the substrate subsequently being quantified in a magnetic field. 6. A method as claimed in claim 5, in which the sample of deposited particles is prepared on an apparatus as described in GB-A-2103358. 7. A method according to claim 4, in which the deposit captured by a magnetic plug in the liquid system is removed from the magnetic plug and after removing the magnetic plug from the liquid system, the deposit on the plug is removed therefrom, with or without prior washing to remove residual system liquid, by contact with a substrate having a tacky surface. 8 . . A method of assessing particle deposits which comprises forming a deposit of particles on a substrate and monitoring the change caused to a magnetic or electric field when a substrate without a deposit and a substrate with a deposit are placed therein. 9. Equipment to monitor the burden of magnetic wear particles entrained in a sample .of lubricant taken from a machine subjected to wear, which equipment com- prises apparatus as claimed ' in UK-A-2103358 j _produce an annular deposit of magnetic wear particles on a non¬ magnetic substrate and means to quantify a magnetic prop¬ erty of the annular deposit. 10. Apparatus to quantify a deposit of particles on a substrate comprises means to support a substrate, means to direct a magnetic or electric field through the substrate, means to sense the distortion of the field caused by a particulate deposit on the substrate, means to quantify the field distortion and means to display the quantification.

Inventors: JONES DAVID BRIAN, LANSDOWN ANTHONY RICHARD, PRICE ALWYN LEWIS, JONES, DAVID, BRIAN, LANSDOWN, ANTHONY, RICHARD, PRICE, ALWYN, LEWIS
Applicants: UNIV SWANSEA, THE UNIVERSITY COLLEGE OF SWANSEA
Application year: 1985
Main classification: G01N15
IPC codes: G01N15, G01N27