Chemical Contamination Sensor for Phosphate Ester Hydraulic Fluids

Abstract

The paper deals with chemical contamination monitoring in phosphate-ester-based hydraulic fluids using nondispersive infrared (NDIR) optical absorption. Our results show that NDIR monitoring allows detecting the take-up of water into such fluids and their hydrolytic disintegration as these become additionally stressed by Joule heating. Observations on the O-H stretching vibration band (3200-3800 cm −1 ) are used for determining the free water content (0-1.5%) and the Total Acid Number (0-1 mgKOH/g). Both quantities can be assessed by monitoring the strength and the asymmetry of the O-H vibration band with regard to the free water absorption band centred around 3500 cm −1 . As such optical parameters can be assessed without taking fluid samples from a pressurised hydraulic system, fluid degradation trends can be established based on regular measurements, before irreversible damage to the fluid has occurred. Therefore maintenance actions can be planned accordingly, which is very important for the airline, as unscheduled maintenance disturbs the flights organisation and often generates money loss.

Multi-channel IR sensor system for determination of oil degradation

“…In addition, these gases in combination with water contamination can lead to formation of acids, further intensifying the polycondensation process. Ester-based fluids, e.g., native or phosphate esters, which are often used as hydraulic fluids, can break up into alcohols and acid components (hydrolysis) when exposed to increasing water content, especially at high mechanical load or elevated temperatures (Totten et al, 2003). Comprehensive online condition monitoring would therefore require a sensor capable of analyzing the degree of degradation and also the relevant degradation processes to allow an estimation of the remaining lifetime of the oil in the specific application.…”

“…Thus, these solutions while achieving promising results have not yet entered the market on a larger scale. Sumit et al (2010) presented a system with an infrared source, a fluid cell consisting of two sapphire windows and a quadruple infrared detector equipped with different filter windows for analysis of phosphate ester hydraulic fluids, especially for aerospace applications.…”

Practical online condition monitoring of gearbox oil using non-dispersive infra-red sensors

“…Phosphate ester hydraulic oils degrade through water absorption, which can react with the oil. This shows up in the region around 3500cm -1 [9]. For gearbox oils, oxidation may be much more subtle, especially with high-performance synthetic oils, and condition monitoring is based on analysing the depletion of the oil additives [12], which can vary depending on the manufacturer and oil brand.…”

“…Nondispersive infra-red (NDIR) analysis concentrates on measuring the absorption at specific wavelengths relating to the chemical components of interest. Such equipment is considerably cheaper, and has been demonstrated with a variety of fluids [1,3,9,11]. Such a system could be incorporated in each turbine as part of an online oil condition monitoring package consisting of multiple low-cost sensors, for improved accuracy and diagnosis ability [h].…”

“…This can be achieved by either using a second test cell with a different narrowband filter, or by using a mechanical system to swap between two filters. Another option is to use a cluster of detectors with different narrowband filters, and a single emitter [9], but this requires a custom-made detector, which is more applicable for a production system. NDIR-based oil condition monitoring has been demonstrated, using both fixed samples of oil [1] and online systems in which oil is pumped through a test cell [9].…”