Take a whiff, why if your oil doesn’t smell right you should get it tested.

After years maintaining machinery you start to develop a 6th sense to know when something is not right. There is not really a 6th sense, but the other 5 play a crucial role of on sight inspections. Well maybe only 4 as I wouldn’t advocate tasting your machine fluids. You do however, listen for abnormal noises, use your eyes to inspect damage, deposits or contamination, and touch to feel vibration etc. The final one is smell. Smell is an extremely sensitive sense and is a fundamental diagnostic tool.

Before I begin this article I will start with a disclaimer as smelling samples although normally not a problem and odour is in many ASTM and OEM requirements as a test to perform on samples, can be dangerous if not done correctly. I recall early in my career having 10 thin lube oils in a row that had a weak EP additive smell I was checking if were gears or not and I picked up the next sample to smell assuming to be another gear oil and it was an ammonia compressor oil (think of smelling salts) and because I breathed too close to the bottle it had me coughing and splutering with watering eyes. That taught be a good lesson and I always listen for a fizzing sound when opening all sample bottles now.

Please note: being able to identify an odour is not the same as advocating breathing in lungs full of fumes. When identifying odour of a sample you should not have your nose directly over the bottle and simply removing the lid should be sufficient to waft sufficient molecules of the material into the air to be detectable by smell. Although most lubricants are on the whole not harmful to be smelled in this way, you should always check material safety data sheets of anything you test the odour of and consult with your health-care provider if you have any breathing or other conditions that smelling a sample may adversely affect. If in doubt don’t smell the sample and talk to your laboratory who can perform the necessary testing to identify failure modes. Smelling samples is at the sole discretion of the individual and learnoilanalysis takes no responsibility of any adverse consequences from doing so.

How sensitive is smell?

Smell is quite a powerful sense, which extreme sensitivity to low concentrations of odourous compounds. It can sometimes have a lower limit of detection (termed odour detection threshold – ODT) than the most advanced lab instruments. In the example of Ammonia I mentioned earlier as little as 50ppm can be detected by an expert nostril. Petrol can be detected as low as 2ppm, which considering Gas Chromatography values of petrol in diesel are very difficult to identify below 1000ppm your nose can often identify the contamination before an instrument. With oxidised oils the common burnt smell comes from short chain carboxylic acids that can also be detected at less than 10ppm by odour.

In the case of mercaptans which is what give gear oil that sulphur smell (and what I was looking for when I smelt that fridge compressor oil), these can be detected as low as 1ppb or 0.001ppm. Hence the smell test for presence of EP additive is so simple when using odour.

Your nose is so powerful, the day they invent the nostril viscometer the analysis lab business should start worrying. I’m obviously joking, but hopefully you get the point that odour is a powerful tool in analysing samples.

So how does it all work?

The fact you smell something means molecules of the substance have made it up your nose to send signals via the olfactory nerves to the brain. For this to work the moelcules need to be reasonably volatile and low molecular weight to be become airborne and hence smelt. That means heavy compounds or weak odour compounds are unlikely to be detected by smell.

What can be detected by smell?

Some common things that can be detected by smell in your lube oil samples include:

  • Partial Oxidation or hydrolytic degradation can have rather pungent odour and is what can sometimes be described as a rancid smell. You may be familiar with with off food or bin smells having an odour similar to this. It it caused by chemical reaction of oil with oxygen leading to production of aldehydes and ketones. Many low molecular weight aldehydes have a rotten fruit smell, and some ketones can range in smell from nail polish (acetone / propanone) to blue cheese (2-heptanone). This is probably the most difficult to identify as there are a variety of smells, but the main thing is to notice a change of smell than be able to identify specifically the smell – the lab can try help with that.
  • Thermal failure oxidation – this is typically a burned smell and is quite obvious to notice, but this may not always be unpleasant. I recall every few months a PHD student sent us his pyrolysis oils for ICP and viscosity testing as part of his research project to produce an alternative biofuel and the whole lab smelled of smokey bacon crisps (my favourite). The process of the burnt odour occurs as oil comes into contact with hot surfaces, or increases in temperature from adiabatic compression of entrained air bubbles such as in hydraulics, pumps or bearings. FYI adiabatic compression is like the process of a diesel engine compression of fuel droplets in air being compressed to burn the fuel, but in reverse. I.e. air bubbles in a fuel (the oil) meaning when the bubbles collapse under pressure very high temperatures are formed and the oil around the bubble is burnt to form carboxylic acids and soot particles.
  • Microbes such as bacteria have a variety of smells and in industries using cutting fluids the main failure mode can be the odour, in which whole rooms in factories need to be vacated until fixed with e.g. increased biocides because the smell is so unpleasant. Bacteria grow anywhere there is water present and in ideal environments quickly multiply, which can clog filters, valves and can also degrade oil quality and performance. The microbial activity leads to acid build ups that can be corrosive to machinery. Prevention by of use of biocides in cutting fluids and in non-water based products minimising water ingress are the main methods of prevention.
  • Sulphur compounds – this is the bad egg type smell and can be used to identify contaminants as well as additives such as EP.
  • Miscellaneous Solvents and other contaminants – many distinctive chemicals such as degreasers, cleaning solutions and fuels have their own odours and can be detected by smell. Equally as mentioned earlier refrigerants can also be smelled too.

Limitations of smell.

There is a problem with smell as a diagnostic tool. It obviously doesn’t detect everything and only works when small odour compounds reach the nose. It is more qualitative than quantitative in that you may be able to describe a smell as slight, moderate or strong, but not be able to say it is 5000ppm for instance. Smell also varies from individual to individual and some people are better at detecting certain odours than others. Equally, in high concentrations for prolonged periods, you desensitise to it. My wife often tells me my clothes smell like ‘oil lab’ when I come home, but I don’t smell it, just like someone who works at a fast food establishment will become less sensitive to the smell of chip (fries) fat or someone working at a swimming pool will become less sensitive to smell of swimming pool Chlorine. If you recall back to the days when pubs were all allowed to be smoking when you were in a pub you rarely noticed the smell of smoke, but if you smelt your clothes the following day they were extremely strong in cigarette smoke smell. This is because your brain when faced with constant signals for an odour tunes out and treats it as a background smell so your attention can focus on other things.

Unless you work in an oil testing laboratory you probably never will intentionally need to smell an oil. However, when changing oils or performing maintenance you cannot help but notice the odour if something is wrong. So if you do notice a strange odour, take a sample and let your lab know what you believe you smelt to help rule in or out that as a cause of potential problems.

If you have a question about an odour coming from your oil or fluid systems then feel free to ask about it by clicking the chat button at the bottom right of the screen.