Wear 259 (2005) 1502–1506 Case study Low-cost condition monitoring sensor for used oil analysis Surapol Raadnui * , Srawut Kleesuwan Faculty of Engineering, King Mongkut’s Institute of Technology North Bangkok (KMITNB), 1518 Pibulsongkram Road, Bang-Sue, 10800 Bangkok, Thailand Received 7 September 2004; accepted 22 November 2004 Available online 23 May 2005 Abstract The development of a low cost condition monitoring sensor for used oil is described. The sensor is designed as a direct measurement of the overall quality of the used oil as compare to that of the un-used oil. The system detects the relative variation of lubricant degradation i.e. degradation of physical/chemical properties, suspended wear particles and ingested contaminants, by using the grid capacitance sensor configuration. The system consists of grid sensing unit and a multitude of small holes (1 mm in diameter) between each parallel sensing grids. The system works on a principle of measuring for the relative variation of the dielectric constant of lubricant caused by contaminants such as water, fuel dilution, water, wear debris, etc. In this particular paper, the performance of this specific sensor is assessed systematically through the utilization of statistical design of experiments (DOE). © 2005 Published by Elsevier B.V. Keywords: Dielectric constant; Lubricant degradation; Oil analysis; Sensor 1. Introduction The purpose of most methods of early failure detection in oil-lubricated machinery is to discover the degradation of lubricant within the oil circulation system. These methods can be divided into continuously measuring ones and those requiring the taking of an oil sample for laboratory analy- sis. A different method is offered by the lubricant itself. The condition of the oil is subject to many influences that may degrade its two primary functions, namely cooling and lubri- cation, to a point where severe damage occurs. Accordingly, the lubricant itself is an important source of information in the strategy of defect avoidance comparable to the role of the human blood in the detection and prevention of diseases. [1,2]. Main parameters that have an influence on the condition of the lubricating oil are: * Corresponding author. Tel.: +66 2 9132500–9132524x8217–8218, 8221; fax: +66 2 5870029/5856149. E-mail addresses: s raadnui@yahoo.co.uk, srr@kmitnb.ac.th (S. Raadnui). • Elevated temperature • Presence of air • Water • Fuel or other lubricants • Solid matter: wear debris, dust, dirt, etc. The lubricant quality detecting is one of the main fac- tors in condition monitoring of the oil-lubricated machinery. Contaminants such as dust, wear debris, etc., chemical by products and physical changes of the used oil normally alter the performance of lubricant. These changes always reflect a change of dielectric constant of the lubricant, therefore any abnormal condition, such as contaminant ingression, gener- ated wear debris, severely chemical and/or physical oil degra- dation, etc. can be detected in timely manner by monitoring the dielectric constant of the used lubricant [3]. 2. Structure of the sensor The sensor is a grid capacitance sensor. The construction of which is shown in Figs. 1–3, which consists of two series of parallel poles. While the oil cavity shell of the sensor is 0043-1648/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.wear.2004.11.009