Moisture contamination in in-service lube oils is perhaps one of the most destructive forms of engine corrosion second only to particle contamination. Water contamination in oil can be categorized into three forms of water; Free, emulsified, and dissolved. All forms of water have direct consequences to the oil and gears of the engine, but indirect consequences as well. Direct consequences include changes in pH and viscosity which can be problematic in themselves. Other factors that affect engine corrosion are contamination of glycol, soot, and particle corrosion. The presence of water exacerbates each of these factors and can cause irreparable damage to the engine. It is therefore imperative for routine moisture testing of in-service lube oils. Traditional methods for water analysis are fourier transform infrared (FTIR) spectroscopy and Karl Fischer titration. The problem with these two methods are their sensitivity and the hassle of sampling (respectively). In place of a Karl Fischer titrator (KFT), a relative humidity sensor moisture analyzer is capable of the same precision as the KFT (10 ppm) but without the use of hazardous chemicals or breakable glassware. Using the relative humidity sensor instrument does not require a strong background in science to operate and is easy to optimize for each sample.
Moisture Contamination: The Three-Headed Beast
No matter the type of engine, gearbox, turbine or bearing, in-service lubricant oils are designed to accommodate heavy compressibility while remaining chemically inert and unreactive. It is important that the viscosity of the oil remains unchanged and that the oil remains free of contaminants to prevent costly wear on the engine. Moisture contamination in in-service lube oils is perhaps one of the most destructive and expensive forms of engine corrosion, second only to particle contamination. Water contamination in oil can be categorized into three forms of water; Dissolved, Emulsified and Free Moisture each of which causes its own direct and indirect problems to the engine. 
Dissolved moisture is the lowest level of moisture contamination in lubricant oils. This moisture is derived from ambient air humidity slowly interacting with the lubricant oil overtime. Usually, the more additives the oil contains, the more hygroscopic (water attracting) the oil will become. Acceptable levels of dissolved moisture typically range from 50-300 ppm (or 0.0050% – 0.0300%).  This level of moisture does not greatly affect the compressibility or viscosity of the oil but is the most chemically reactive water species since it is dispersed throughout the oil.
The bottom line:
Dissolved water can degrade metal and deplete additives
- Dissolved water molecules will coat any polar metal surface it finds within the engine. Under routine engine pressure and heat, this small amount of water can be stripped of its oxygen constituent and release hydrogen ions, which will chemically degrade the face of the metal (ball bearings and gears). This weakens the metal and microscopic flakes will begin to shed from the metal leading to particulates in the oil leading to gear corrosion.
- Dissolved water molecules will actively seek other polar molecules which may include additives such as detergents, antioxidants (amines or phenols), friction modifiers, and anti-wear additives (such zinc phosphates). Because water molecules are sequestering these agents, these additives lose their functionality which may cause problems to the engine.
If left unchecked, the dissolved moisture will continue to increase within the oil sample until it reaches a saturation point. At this point, any added water will precipitate out as cloudy emulsified micro droplets. Like the mayonnaise you would put in your sandwich, this form of moisture is created by the continuous churning, heating and high pressurization of water into oil. The saturation point varies for different lubricants. Mineral oil has a saturation level of 100 ppm (0.0100%), while some hydraulic fluids have a saturation level as high as 5000 ppm (0.5000%). As some oils age, the saturation point may increase depending on the type of oil and the additives used in the lubricant.
The bottom line:
Emulsified water can cause oil to behave unexpectedly, and can cause rust, clog filters, and increase oil acidity
- Emulsified moisture has a direct effect in the compressibility of oil. The bulk modulus (109 Pa, N/m2) of a liquid speaks to the compressibility of the liquid. For example, standard 30wt. engine oil has a bulk modulus of 1.5 while pure water has bulk modulus of 2.15. The higher the bulk modulus, the harder it is to compress. If water is emulsified in oil, the bulk modulus of the blend is now greater than water at 3! This can lead to worn out parts and costly repairs.
- Like dissolved water, emulsified moisture is ubiquitous within the oil sample and can also cause rusting along metal parts leading to particulates in the oil. Once these particulates combine with the emulsified water, a sludge will be created that can grind gears producing even more particulates. This positive feedback loop will continue unless the oil is changed or dried, but the damage is permanent.
- Because of their size, emulsified water droplets will also clog many oil filters. This restricts the normal flow of oil and prevents true particulates from being filtered efficiently.
- Although there are different definitions use to describe the acidification of oil over time (pH or Total Acid Number), there is one variable that exacerbates the issue and that is emulsified water.  With so much water in the oil, the number of hydrogen ions will increase and react with additives. As acids increase in concentration, so does the corrosion of the engine.
Free moisture is water that is neither dissolved nor emulsified within an oil sample but remains in a distinct and separate aqueous liquid phase. This form of moisture will never incorporate into the oil and usually originates from condensation or leaks. The free moisture is denser than the oil and will settle to the bottom of most engines or create a thin film covering all available polar metal components.
The bottom line:
Free moisture causes significant rusting.
- Having any amount of free moisture a lubricant is detrimental to the engine. Gears will substantially rust, grind and fracture. The engine will suffer irreparable damage and the cost of replacing the engine and labor will be substantial.
Dissolved, Emulsified, and Free Moisture Ranges in Oils
|Oil||Dissolved (ppm)||Emulsified (ppm)||Free (ppm)|
|New Hydraulic Fluid||0-200||200-1000||>1000|
|Aged Hydraulic Fluid||0-600||600-5000||>5000|
|New R&O Oil||0-150||150-500||>500|
|Aged R&O Oil||0-500||500-1000||>1000|
|New crankcase oil||0-2000||2000-5000||>5000|
It is evident that any of the three forms of water are destructive to both the lubricant and to the engine. Engineers and machinists go through great lengths to keep their oil/lubricants clean and dry, but over time moisture will eventually absorb into the engine system. Monitoring for moisture content therefore important in in-service lube oils, but what is the appropriate method of moisture analysis?
Moisture Analysis for In-service Lubricants
There are many forms of moisture detection assays out there but they all come with advantages and disadvantages. For the scope of moisture detection in oils, this paper will survey four: Crackle Test, Fourier transform Infrared Spectrum (FTIR), Karl Fischer Titration (KFT) and Relative Humidity Sensor Technology. There are other forms, but these four are considered to be the most accurate and reliable in the industry.
The Crackle Test
Range: 0 – 2000+ ppm
Resolution: ~ 500 ppm
The Crackle Test utilizes the differences in vapor pressures between lubricant oils and water vapor. Synthetic or petroleum based lubricants can tolerate high temperatures before vaporizing. The crackle test utilizes a drop of oil on a hot plate set to 160C, to quickly vaporize the water within the oil (with a boiling point of 100 C). The outcome is an audible ‘crackle’ when tiny pockets of water vapor escaping the oil drop. Depending on the number of ‘pops’ heard by the analyst, an estimation of water content can be made.
This method is not a quantitative measurement of moisture, but does give gross estimates of the moisture content within an oil sample (see diagram below).  This method is quite variable and is technique driven and dangerous. Dissolved gasses or other aqueous liquids may be confused for water vapor. This form of analysis is also potentially dangerous if the technician does not take the proper precautions.
Fourier Transform Infrared Spectroscopy
Range: 1000+ ppm
Resolution: ~ 50 ppm
Fourier Transform Infrared Spectroscopy (FTIR) is a form of absorptions spectroscopy that involves illuminating an infrared wavelength of light onto a sample of oil. The wavelength of light is either reflected, scattered or absorbed by the sample. After passing through the sample, the remnants of this wavelength are ‘collected’ by a detector as a signal and using a mathematic algorithm, water content can be deduced.
This method can give the technician better resolution than the crackle test but relies on a mathematical algorithm to predict the water content in an oil sample. Water may not be the only other contaminant within the oil sample and small particulates may reflect or absorb the wavelength giving the technician a false positive.
Karl Fischer Titration
Range: 10 ppm – 100%
Resolution: ~ 10 ppm
Karl Fischer Titration (KFT) is a widely accepted form of moisture specific analysis in lubricant oils. This technology requires a sample of oil to be heated or dissolved into a titration cell where water molecules undergo a chemical reaction with an iodine species to produce an electric current. This electrical current can either be measured volumetrically or coulometrically based on the total moisture anticipated from the sample; free & emulsified water to be analyzed by volumetric KFT, dissolved water to be analyzed by coulometric KFT.
Volumetric Karl Fischer Titration
Coulometric Karl Fischer Titration
The drawback to Karl Fischer Titration is the use of expensive, hazardous chemical reagents and delicate glass pieces. The reagents must be replenished continuously throughout the life of the instrument. Some oils contain additives that also undergo the chemical reaction intended for water, leasing to erroneous readings. Routine cleaning of the parts are also labor intensive and consume a substantial amount of time. Trouble shooting also takes a trained analyst should problems arise.
Computrac Vapor Pro Moisture Analyzer
Relative Humidity Moisture Analyzer
Range: 10 ppm – 100%
Resolution: ~ 10 ppm
Accuracy and precision are important when monitoring the moisture content within in-service lubricant oils. Like the Karl Fischer Titrator, a relative humidity (RH) moisture analyzer is moisture specific, but is not plagued with as many interferences as the KFT. The RH moisture analyzer does not consume chemical reagents and is easy to operate at the touch of a button. An oil sample is heated inside a capped vial, the moisture is released and the vapor is carried passed the relative humidity sensor where the water molecules are quantified. Using this method both accuracy and precision can be equivalent or better than existing KFT methods (see below). 
For testing lubricants, relative humidity moisture analyzers prove to provide significant advantages when compared to Karl Fisher titration. The solvent free testing reduces the amount of hazardous chemicals that must be kept on hand and eliminates the expensive glassware required for testing.
There are many reasons to test for moisture contamination in lubricant oils, but it is sometimes difficult to decide on which moisture testing method to use. Although the crackle test may be easy to perform, it lacks precision and accuracy. If lower levels of moisture contamination are to be examined, FTIR may be a reasonable method to use. However, this method is prone to interferences and cannot detect ultra low moisture levels. If accuracy, precision and ultra level moisture contamination is required for routine maintenance, then the moisture specific Karl Fischer titration and relative humidity sensor methods are best. When comparing KFT to RH moisture analysis it is important to note that there are many different forms of KFT some with different co-solvents or different sample preparations. These methods are all prone to interferences and unless you have a PhD at the end of your name it may not be intuitive to which method you should use to get the ‘right’ result. That is why RH sensor technology is so important to the industry. It allows a technician at a push of a button to detect moisture contamination in minutes without the expensive glassware, hazardous chemicals and upkeep of the KFT.
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