WHITEPAPER Oil is the lifeblood of many industrial plants and machines. It is used for cooling, lubrication, insulation, and power transmission. To operate a gearbox, a transformer or an industrial plant safely and stable over a long period of time, monitoring the condition of the lubricants with the support of process sensor technology is recommendable. Every oil changes with chemical and physical aging, as additives degrade and impurities accumulate. The oil’s ability to absorb water can suffer and its lubrication ability and insulation strength are reduced. Unchecked, operating machines with unsuitable oil condition with a too high water content may lead to machine downtime and expensive repairs. Continuous monitoring of moisture in oil is key to running preventive maintenance economically and / or to trigger automated counter-measures within the monitored system.

Contents 1.0 Motivation Continuous monitoring is a crucial first step towards extended equipment lifespan. E.g. transformers need to operate safely and reliably under extreme weather conditions. Offshore infrastructure like wind turbines, oil rigs and cargo ships are prominent examples of machines with high installation and maintenance costs. Also, heavy duty vehicles are subject to high reliability standards for hydraulic and power transmission systems. Measurement Results Thermal management of battery packs with oil immersion cooling is another emerging application example. Prime oil condition...

Figure 2: Measuring principle of a capacitive humidity sensor element: A concentration gradient driven diffusion of water molecules in and out of the polymer layer effects the capacitance of the sensor 2.0 Technology Capacitive humidity sensors are common in air monitoring systems. The polymer layer of the sensor is in contact with the surrounding air through the porous top electrode. Water can diffuse through the pores of the electrode from the surrounding air into the polymer layer and vice versa until an equilibrium state is established (see figure 2). The amount of polymerbound water influences...

Figure 3: Control window for process control expressed in %RH (corresponding to water activity aw) Note: The displayed reaction threshold and absolute water content values are randomly selected for illustrative purposes only The results obtained by Karl-Fischer titration disregard any changes in water solubility due to oil aging and additive depletion as it gives the absolute water content in ppm. Laboratory analysis also disregards the operational conditions. Especially temperature has a strong influence on the solubility of water in oil. The interpretation of laboratory results relies on detailed...

3.0 Measurement Results It takes time to reach physical equilibrium with dissolved water evenly spread throughout a certain volume of liquid oil (see figure 4). The dissolving process of water in oil is sped up at higher temperature and with effective high-speed mixing. Sample homogenity remains the limiting factor for a reliable data read-out. The response time of the humidity sensor to its immediate surroundings is significantly faster. Figure 4: The slow dissolving process of water in oil is demonstrated by monitoring moisture in oil after the addition of sufficient water to saturate the oil...

Innovative Sensor Technology IST AG Your partner and manufacturer for physical, chemical and biological sensors With more than 30 years of experience Innovative Sensor Technology IST AG is one of the world’s leading manufacturers of physical, chemical and biological sensors. We specialize in the development and manufacturing of temperature sensors, thermal mass flow sensors and modules, humidity sensors and modules, conductivity sensors and biosensors. In addition to our standard products, we offer sensor adaptions to individual, customerspecific application needs – right up to the joint development...