Updated on 8 July 2020.
Even though oil and water are traditionally immiscible, certain mechanical environments can cause oil and water to mix. As far as this goes for the automotive and industrial machinery systems, this is a bad scenario, as the presence of water in the lubricant systems can cause oxidation and degradation to the surrounding mechanical parts (as well as degrade the oils themselves). In this article, we look at the different types of methods that can be used to determine if a lubricant contains water, and to what extent it is present, where the results can be used in decision-making processes to see if a lubricant requires changing.
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The density differences between water and oil means that they do not generally mix if they are poured together in a stationary container. However, in systems where there is a lot of mechanical movement and liquid shearing, oil and water mixtures can become one. This is true for the lubricants in mechanical systems, and there are many ways in which the oil and water can be mixed, with the most common being as emulsions, free water globules in an oil matrix and dissolved water in the oil/lubricant system.
Water in a lubricant system can cause degradation to the surrounding metal components. Aside from oxidation (and the formation of rust if any iron is present), it can cause chemical changes within the lubricant itself, and this can lead to additive depletion, and the formation of acids, sludges and varnishes. Oil degradation and additive depletion are also two of the top three most common reasons for why an oil/lubricant will fail (the other being contamination, which technically, water is). So, to avoid having to change lubricant frequently, it is advised to get the water content tested on a regular basis before any serious damage is caused to the machinery. Below, we look at how this can be done.
The Crackle Test
The crackle test is the simplest method to determine whether a lubricant sample contains water. To perform this method, a droplet or two of oil is placed onto a hot plate. If the droplets crackle, then there is water in the oil, and the heat will cause the water to turn into a gas and form visible bubbles in the droplets. If there is no crackling and no formation of bubbles, then there is no water in the sample. A rough approximation can be made to the extent of water in the oil (although no quantitative values can be made) and is relative to the size the bubbles, i.e., the larger the bubble, the more water there is in the oil. However, it can only detect water in a lubricant down to 500 ppm.
Calcium Hydride Test
This is another simple method. A known volume oil is placed into a chamber containing a known amount of calcium hydride. Once added, the chamber is shaken vigorously to mix the oil with the calcium hydride, whereupon hydrogen gas is produced (alongside calcium hydroxide). Because the volumes of the two reactants are known, and the stoichiometry is a 1:1 ratio, the amount of gas given off can be used to calculate the amount of water in the sample.
Infrared (IR) Spectroscopy
Infrared (IR) spectroscopy is one of the most promising analysis methods for measuring water content in a lubricant because it doesn’t require the use of chemicals or other media. Water molecules (specifically the dissociated OH– group) absorbs infrared light at a specific wavelength with a characteristic peak that is easily identifiable. There is some slight variation (by on 50 cm-1) when water is freely suspended in an oil environment, but it can still be easily identified, and the absence of this peak shows that there is no water dispersed in the oil.
For water impurities that cause the lubricant to emulsify, the analysis is not as straightforward. The viscous nature of the emulsion causes the light to scatter, and this makes it harder to identify the characteristic water peak. There are a couple of ways around this if the sample is suspected to be emulsified. The first is through the water stabilization method, which pre-treats the water with an additive (commonly a surfactant) that breaks the bi-phasic mixture back into the discrete water and oil droplets, thus, enabling the water peak to be identified again. The second approach to separate the oil and water droplets is through using a homogenizer to shear the emulsion into discrete (and analyzable) droplets.
Karl Fischer Titration
Karl Fischer titration methods are the most widely used method when it comes to determining whether a lubricant contains water. Karl Fischer titration is a type coulometric titration method that yields results with high accuracies and high repeatability compared to many other methods. It is also possible to analyze water contaminants in all forms, which is another reason why it is so widely used. It is used in the way most titration methods are, where a solution of a known concentration (in this case iodine) is titrated into the oil sample. The amount of iodine absorbed by the water molecules relates to the amount of water in the oil sample, so that a quantitative value can be produced.
There are also other methods used, such as humidity sensors, saturation meters, Fourier-transform infrared (FITR) spectroscopy and Dean and Stark distillation methods, although these are often limited in the number of industries that they can be used in.
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