How to Monitor Water Concentration in Oil

How Does Water Enter the Oil?

Ask the Expert: Monitoring Water Concentration in Oil

A crucial part of any oil condition monitoring program is routinely monitoring the water concentration in oil. Water can enter the oil due to condensation from cooler areas of the equipment, environmental conditions, and/or compromised seal integrity.

Three Phases of Water

Preferably, water stays in its dissolved state, dispersed molecule-by-molecule throughout the oil. When the oil arrives at its saturation point, it then disperses as microscopic droplets throughout the oil make an emulsion where the oil appears cloudy or foggy. Eventually, in a water/oil emulsion, the water forms as free water in the oil as the water concentration grows.

Usually, mineral oils and PAO synthetics have a certain gravity of under 1.0, which means the water is heavier than the oil; so, the free water starts to gather at the bottom of the tank or sump.

What are the Effects?

High levels of moisture in the oil can result in alterations to the fluid (hydrodynamic) film, accelerated wear, oxidation, and even possible bearing damage from cavitation and/or hydrogen embrittlement.

Measuring Moisture

There are multiple choices available for calculating water in oil which include online sensors, the FluidScan device, Karl Fischer Titration, and the crackle test, to name a few. Due to cost, online sensor technology may not make sense in every application. Karl Fischer reagents can be harsh and so are impractical to utilize on site.

The FluidScan device is an IR technique which can supply a parts per million water measurement in seconds and correlates to the ASTM Standard for Karl Fischer Titration. The total water measurement feature on the FluidScan measures both free and dissolved water down to 300 ppm on turbine oils and over 1000 ppm on other oils. This fast test makes the FluidScan the perfect choice for on-site monitoring of water concentration.

Setting Alarm Limits

When creating an oil condition monitoring program, setting up the correct alarm limits for water concentration is vital. Make sure to consult with the OEM to establish the proper alarm limits, and review the ASTM Standards if possible:

  • ASTM D7720 (Standard Guide for Statistically Evaluating Measurand Alarm Limits when Using Oil Analysis to Monitor Equipment and Oil for Fitness and Contamination) for guidance on overall oil condition monitoring strategies and guidance for analysis
  • ASTM D6224 (Standard Practice for In-Service Monitoring of Lubricating Oil for Auxiliary Power Plant Equipment)
  • ASTM D4378 (Standard Practice for In-Service Monitoring of Mineral Turbine Oils for Steam, Gas, and Combined Cycle Turbines)

Depending on the temperature and length of time in service, turbine oils, hydraulic fluids, and other industrial fluids can hold as much as 200 to 600 ppm of water (0.02 to 0.06%) in the dissolved state. Older oils are actually able to hold three to four times more water in the dissolved state than new oil.

This is because of the polar by-products forming from the oil oxidizing, holding on to the water molecules and keeping them in solution. Crankcase oils usually have this tendency too because they are highly additized with polar additives that tend to hold water in solution.

Turbine oils, which are not as highly additized, usually possess a lower saturation point so free water will form faster. This is why turbine oils typically require lower alarm limits and should be monitored more closely.


When there is a routine monitoring of the water concentration and the proper alarm limits are set, there are a number of choices available to remove the water from the system if an elevated moisture level is detected. The choices are diverse and range from simple to fairly complex alterations in maintenance practices to return the water concentration back to normal levels.

When deciding which option should be used, time, availability, budget restrictions, and criticality of the equipment are all crucial factors. Gravity separation, coalescing filters, vacuum dehydration, centrifugal separation, and the utilization of heating elements are all common ways to drive off moisture.

It is vital to monitor the moisture concentration throughout the process when initiating these methods in order to understand the effectiveness of the technique. For example, the FluidScan would be a useful tool to employ on-site to monitor moisture ppm values to ensure the effectiveness of the moisture removal method.

Furthermore, it may also be wise to carry out more oil condition monitoring tests to assess the integrity of the oil. For example, a demulsibility study (ASTM D1401) is advised if free and emulsified water were present in the oil for an extended period of time, as these two phases tend to be the most harmful to equipment.








This information has been sourced, reviewed and adapted from materials provided by AMETEK Spectro Scientific.

For more information on this source, please visit AMETEK Spectro Scientific.


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