Field tests are some of the most overlooked, yet valuable tools that lubrication professionals can have in their arsenal. Most field tests are quick, inexpensive, simple to conduct and yield great information. One of my favorite field tests is odor.

Many characteristics and properties of an oil can be detected with our senses. We use our eyes to check level gauges, color, clarity, opacity, etc. We use our ears to determine conditions like cavitation, overloading, misalignments, etc. Why shouldn’t we utilize our noses more often?

Smell is a very direct sense. In order for you to smell something, molecules from that something must find their way to your nose. Therefore, everything you smell is releasing molecules. These molecules are mostly small, light, volatile chemicals that find their way into your nasal passages. Once in the nasal passages, these molecules come in contact with a special patch of neurons. These neurons have very small, hair-like projections called cilia that increase the surface area to capture more of the molecules. The molecules attach to the cilia and trigger the neurons to send a signal to your brain, which causes you to perceive a particular smell.

So which oil odors should you be trying to distinguish? Here are a few you should be able to recognize.



Oxidation has a sour or pungent odor, similar to rotten eggs. It occurs when the hydrocarbon constituents of lube oil combine chemically with oxygen. As with most chemical reactions, oil oxidation is accelerated by heat and pressure. It is no different than other commonly encountered oxidation reactions, such as rusting. Just like the effects that rusting and other corrosive processes have on metal substrates, oil oxidation results in a catastrophic and permanent chemical change to the base oil molecules. The net effect of prolonged oxidation is that the oil becomes acidic (chemically), causing corrosion, while an increase in viscosity occurs (physically).

Thermal Failure

Thermal failure has the smell of burned food. It typically occurs when the base oil comes in contact with hot surfaces within the oil-wetted path or due to a sudden and rapid increase in temperature associated with the adiabatic compression of entrained air bubbles in pumps, bearings and other pressurized lubrication environments. When this takes place, the layer of oil that comes in contact with the hot machine surface or compressed air bubble can change chemically.


Bacteria can produce a road-kill smell or stench. Once established, bacterial colonies will clog control systems, quickly degrade oil quality and performance, and generate corrosive byproducts. If not detected early, the problem will manifest itself into expensive repairs, extended downtime and a significant expenditure of scarce resources.


Contaminants such as solvents, refrigerants, degreasers, hydrogen sulfide, gasoline, diesel, kerosene and process chemicals all have a distinct smell of their own.

Sulfur Compounds

Sulfur compounds have a skunk-like odor. The various oxides of sulfur and water, both of which are combustion byproducts, react together to form sulphuric acid. This acid is neutralized by the basic reserve in the oil’s additive package (overbase detergent) and normally results in the formation of metallic sulfates.

Nitrogen Compounds

Nitrogen compounds have an almond-like scent. Nitration is another form of oxidation. It results from the reaction of oil components with nitrogen oxides (NO, NO2 and N2O4), which are produced from the oxidation of atmospheric nitrogen during the combustion process. In addition to causing oil thickening, nitration products are major contributors to the buildup of varnish.

Esters and Ketones

Esters and ketones have a perfume (fruity) odor. Esters are produced when carboxylic acids are heated with alcohols in the presence of an acid catalyst. Their odor is due to their volatile nature, which is caused by their chemical composition and conformations.

84% of visitors use smell as an oil analysis tool.

Although it’s not a cutting-edge science and there aren’t any cool handheld devices (yet), smell should be an essential part of your oil analysis program. It’s fast, cheap and easy. Very few things in the machinery reliability world offer all three of these attributes.

One of my favorite stories about using smell as a field test involved a client who sent me an oil analysis report that was very basic. It had the normal range of tests for an “economy” report. It showed an increasing viscosity, a darkening, the formation of some sludge and varnish, etc., all the telltale signs of an oxidative failure, yet the oil had not been in service very long.

Over the phone, I told the client to open the bottle and take a big whiff of it. I could tell by the awkward silence that he was making a face - you know the one you make when you think you’ve heard what someone has said but know it couldn’t be right. He repeated my request with a bit of sarcasm in his voice, to which I replied, “Just do it.” A few seconds later, I heard, “Wow! It smells like burnt oil!” That was our clue that the problem was thermal failure.


More Than a Sniff Test

Even in laboratories these days, there are headspace instruments that are not actually analyzing what’s in the oil but what is coming out of the oil. Gas chromatography (GC) is one of the most widely used techniques in modern analytical chemistry. In its basic form, GC is used to separate complex mixtures of different molecules based on their physical properties, such as polarity and boiling point. It is an ideal tool to analyze gas and liquid samples containing many hundreds or even thousands of different molecules, allowing the analyst to identify both the types of molecular species present and their concentrations. So if your oil has a unique odor, you may want to run a GC test to see what these odors might be.