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The Ninth International Conference on Electronic Measurement Non metallic wear particles which result from combustion and failure of the seal and filter device Environmental pollutant particles including various particles in the air especially seen in open systems where the contamination level is higher and the contaminant usually exists as oxide The sizes of these particles are between 1 40 m and those between 20 30 m have the greatest impact on the equipment while those below 1 m have no effect on the equipment 5 As a result the sensor should be most sensitive to particles of size between 1 40 m The suspended particles in the oil have a significant impact on the oil s optical properties Turbidity can be used to describe the impact of the particles on the light transmittance character of the oil The connection between the turbidity and contamination level allows this level to be semi quantitatively determined by measuring the turbidity Figure 1 shows the schematic of the optical fiber sensor The light emitted from the semiconductor laser passes through the optical fiber and fiber optic collimator and then enters the oil pool of interest The photodetector collects the photons transmitted and reflected and transforms them into voltage which conveys information of the solid particle contamination in the oil Fig 1 The schematic diagram of the optical fiber sensing 4 808 978 1 4244 3864 8 09 25 00 2009 IEEE Authorized licensed use limited to CHINA UNIVERSITY OF MINING AND TECHNOLOGY Downloaded on March 31 2010 at 21 17 48 EDT from IEEE Xplore Restrictions apply The Ninth International Conference on Electronic Measurement N is the concentration of particles D is the particle diameter and g305 is the cross sectional area of the particle facing the light Summing formula 3 for particles of different size in a multiparticle system one can get m 20 ii i i 1 I g652 Ln D N K g540 m D L I4 g166 5 Where m is the refractive index of the particle relative to the surrounding medium N i is the particle number with the diameter D i Letting g545 denote the proportion of particles the relation between gravimetric frequency W and the particle size distribution is expressed as follows 3 iii g652 W D g545N 6 6 Plugging 6 into 5 one can get the formula in the case of a single wavelength m 0 i i i 1 i I W Ln C K g540 m D ID g166 7 From 3 4 we can see for the monodisperse particle system that when penetration depth is fixed the value of 0 I Ln I is proportional to the particle s number concentration and thus also the particle s weight concentration i e the particle s weight concentration can be indirectly obtained by measuring 0 I Ln I In this way the oil contamination level can be determined SELECTION OF WAVELENGTH Light is made up of photons of certain energy dependent on wavelength The molecules and atoms constituting the substance are at different movement modes corresponding to different energy levels When the light travels in the oil the absorption by oil molecules is quantized by those energy levels such that the absorbed photon can excite the molecule s rotational or vibrational modes or electron energy levels The wavelength of the light applied by the sensor should meet the following conditions 1 sensitive to the solid particles in the oil 2 The absorption rate of light by the oil remains essentially unchanged at the vicinity of the selected wavelength 3 The decay rate should be low when the light passes through the oil 1 20 engine oil 2 vacuum pump oil 3 MD 1130 4 HL 110 5 32 hydraulic oil Fig 2 Transmittance characteristics curve of several oil Generally the solid particles of sizes between 5 30 m are the most dangerous contamination According to light absorption and scattering theory we can see that the absorption and scattering of the light by oil is strongest when the ratio of particle size to the wavelength of light is about 10 In order to enable the selected wavelengths of light sensitive to these pollutants wavelength of 0 5 5g541m shall be selected With light of such wavelengths transmittance curves were obtained by collocating different types of oil samples and using Hatachi340 Recording Spectrophotometer as shown in Figure 2 The left panel is the transmittance curve of the five oil samples between the wavelength 0 4 1 0 m while the right one corresponds to wavelengths of 1 0 2 2 4 809 Authorized licensed use limited to CHINA UNIVERSITY OF MINING AND TECHNOLOGY Downloaded on March 31 2010 at 21 17 48 EDT from IEEE Xplore Restrictions apply The Ninth International Conference on Electronic Measurement this is because of the commonness of the oil structure made up of two elements namely carbon and hydrogen The above analysis shows that when wavelength is between 0 7 0 9g541m transmittance is relatively stable and large combining the typical transferring window of the optical fiber eventually the wavelength of the testing optical is determined as 850nm STRUCTURE DESIGN OF THE SENSOR The sensor is mainly used for online monitoring for the oil contamination of the reducer of the coal mine scraper conveyor To that end the sensor is embedded in the reducer s tank In order to prevent the particles in the oil from accumulating on the glass baffle with time which may lead to inaccurate measurement the internal glass baffle must be cleaned regularly The mechanical structure of sensor is as follows Fig 3 The mechanical structure of sensor 1 fiber 2 thread of photodetector 3 lazer 4 flange 5 inner core 1 6 upper chamber of exterior sheath 7 sealing sheath 8 fastening nut 9 Seal circle 10 fastening bolt 11 quartz glass baffle 1 12 Seal circle on the quartz glass 13 lead cavity 14 quartz glass baffle 2 15 photodetector 16 lower chamber of exterior sheath 17 orientation sheath 18 inner core 4 19 inner core 2 20 inner core 3 Explanation of the sensor s structure 1 The main structure of the sensor is made up of inner core exterior sheath and sealing sheath 2 The inner core is made up of four parts which are connected by fastening bolts The quartz glass baffle which constitute the left and right baffle of the oil pool is installed in the inner core 3 The exterior sheath is made up of upper and lower chambers the external oil can flow through the holes in the exterior sheath into the oil pool measured Through the cooperation of lower chamber of exterior sheath and sealing sheath the inner core can be separated from the external oil so that the inner core can be taken out and the quartz glass baffle be cleaned TEST CALIBRATION Use the particle counter to calibrate the oil of different contamination degrees which are collocated in accordance with the law of gravimetric method Put the oil into the sensor and measure the output signals corresponding to oil of different degrees and the signal of the point for changing oil Because N320 lubricant is widely used in the reducer of heavy duty machines in the coal mine it can be used as based lubricant for the collocation of different contamination degrees of oil Currently NAS1638 contamination standard is widely used First of all it can be seen from Table 1 that the upper limit of the solid particle contamination in the oil is 0 5 percent According to table 2 we can also see that the weight percentage of 0 5 percent corresponds to NAS18 contamination degree Totally 13 kinds of oil with the oil contamination degrees from NAS8 to NAS20 are collocated NAS1638 contamination standard totally has 14 contamination degrees from NAS00 to NAS12 the ratio of particle concentration of adjacent two levels is 2 Therefore when the contamination level is above 12 extrapolation can be used to decide it The contamination level of the oil used in mining machinery and equipment is far greater than NAS12 so we can just use the method to solve this problem 8 Table 1 L CKC exchanging standard of industrial closed gear oil SH T0586 item Exchanging oil standard appearance abnormity Movement viscosity 40 rate of change 15 or 20 moisture 0 5 Mechanical impurities g149 0 5 Copper corrosion 100 3h degree g149 3b Timken OK value N g148 133 4 4 810 Authorized licensed use limited to CHINA UNIVERSITY OF MINING AND TECHNOLOGY Downloaded on March 31 2010 at 21 17 48 EDT from IEEE Xplore Restrictions apply The Ninth International Conference on Electronic Measurement Instruments ICEMI 2009 Table 2 Weight contamination standard OIL CALIBRATION Calibrate the oil sample collocated by gravimetric method with the particle counter and get the accurate contamination level of them But the calibration will have the following questions 1 The oil viscosity used for Particle Counter should be between 10 and 15 while N320 does not meet this requirement of viscosity 2 The upper limit contamination degree of the oil tested by Particle Counter is NAS12 This problem can be solved by oil diluting but this can cause large error for high contamination degree oil For problem 1 collocate the mixture of oil and Petroleum ether according to different proportion measure the viscosity of the mixture with the viscometer and get the conclusion that when the oil and Petroleum ether meet the proportion of 3 1 the viscosity of mixture meets the requirement of particle counter For problem 2 choose four out of the 13 oil samples with the contamination level NAS12 NAS14 NAS16 NAS18 to measure using Particle Counter if the contamination level measured by Particle Counter is the same with their Nominal level it illustrates that the oil contamination collocated by gravimetric method is consistent with that measured using Particle Counter Or else if certain rule can be found from the measurement results the actual contamination levels of other oil sample can be speculated using this rule From the measurement result of the four oil samples we can see that the contamination level measured by Particle Counter is 2 degree higher than that of the oil sample collocated with the gravimetric method In accordance with the law the actual contamination level of the 13 kinds of oil is from NAS10 to NAS22 The corresponding contamination level of the point of changing oil is NAS20 Measure the oil contamination of the collocated oil using the sensor respectively During the measurement the light from the semiconductor laser travels through the optical fiber the optical fiber collimator the oil and finally reaches the surface of the photodetector By measuring the output voltage of the photodetector we can obtain information of the contamination level from NAS10 to NAS22 as shown in Table 3 Table 3 Data of light penetration test NAS contamination level 10 11 12 13 14 15 16 Output voltage mv 402 401 401 401 401 401 400 NAS contamination level 17 18 19 20 21 22 Output voltage mv 398 392 390 368 357 338 338 Data analysis as can be seen from figure 4 1 The output voltage value of the photodetector is basically unchanged and is the same compared with the clean oil This shows that the blocking of the light by the particles in the oil when the oil contamination level is below NAS16 may be negligible 2 The output voltage of the photodetector falls apparently when measuring the oil of the contamination level above NAS17 This shows that the blocking of the light become obvious from this contamination level The contamination level of the point of changing oil is NAS20 so this point is measurable Fig 4 The output voltage of the photodetector corresponding to the NAS contamination level CONCLUSION The oil contamination sensor is described in this paper The measuring principle selection of wavelength mechanical design features collocation of testing oil calibration of sensor and the analysis of the calibrating data are analyzed It can be seen from the data that the point of changing oil is measurable which proves that the sensor can meet the requirements for in line oil contamination monitoring of the scraper conveyor gear reducer However the sensor also has some shortcomings for example the color of the oil will change after long term use which will affect the transmittance and leading to measuring errors The test is carried out at room temperature and is inconsistent with the actual status of oil temperature which affects the rate of transmission So the next step would be to study how to eliminate the impact of the oil temperature and color to improve the accuracy of the sensor Contamination degree NAS1638 12 13 14 15 16 mg 1000ml 50 100 200 400 800 Contamination degree NAS1638 17 18 19 20 mg 1000ml 1600 3200 6400 12800 12800 4 811 Authorized licensed use limited to CHINA UNIVERSITY OF MINING AND TECHNOLOGY Downloaded on March 31 2010 at 21 17 48 EDT from IEEE Xplore Restrictions apply The Ninth International Conference on Electronic Measurement Instruments ICEMI 2009 REFERENCES 1 Jones M H Tribology a key element in condition monitoring Conf On Condition Monitoring Oxford Proc of Inter 2001 2 Qingmin Meng Study on the On line Oil Monitor Based on Optical Fiber Sensor Chinese Hydraulics Pneumatics Vol 2006 5 pp 34 37 May 2006 3 Hanliang Xiao Ferrography technology and its application in mechanical diagnosis China Communications press Beijing 1944 4 Yonghui Yin Xinping Yan Hangliang Xiao Development on Optic f iber Transducer in Oil Contamination Monitoring Instrument Technique and Sensor Vol 2006 11 pp 3 4 Dec 2006 5 Ball P G Machine wear analysis a rational approach to methods integration for maximum benefits Lubrication Engineering Vol 54 No 3 pp 18 22 Mar 1998 6 Yutian Wang Optoelectronics and optical fiber sensing technology National Defence Industry Press Beijing 2003 7 Fuquan Zhang Rongshan Sun Guowei Tang Optical Beijing Normal University Press Beijing 1985 8 ZhengduoPang and Guoying Meng collocating and experimental demarcating of the N320 lubricating oil servicing the design of the contamination sensor Chinese Journal of Sensors and Actuators Vol 20 No 1 pp 247 250 Jan 2007 4 812 Authorized licensed use limited to CHINA UNIVERSITY OF MINING AND TECHNOLOGY Downloaded on March 31 2010 at 21 17 48 EDT from IEEE Xplore Restrictions apply