Why do particle counters differ in results?

I had an interesting question the other day regarding a customer who uses both online field particle counters and sends samples for oil analysis that includes particle count. The question posed was:

“Why do you not always get the same result with field particle counting compared to lab particle counting?”


When measuring oil cleanliness particle counts there are many methods that can be used. Assuming you are using a laser particle counter and using the same technology basis as lab particle counters, there are a few reasons for differences between any particle counters in the field or between field and lab counters.

But first it helps to explain how light obscuration laser particle counters work. The principle is quite simple as a beam of laser light is shone through the oil and when anything that can change the angle or block the laser light is detected a particle is measured. This can be a piece of dirt blocking light, but also things that bend it too like water, non-dissolved liquids and air bubbles. Now you understand the principle of the technology I will cover some of the ways the counts can be different These are:

Oil Samples are not Representative

I will start with the obvious because when measuring particle counts the most difficult part is obtaining a representative sample. Good sampling practice dictates that the machinery should at operating conditions with good circulation of the oil. The ideal location for taking a sample is from return lines before the filter with a sampling point fitted at a bend to give extra turbulence and mixing before measuring. However a particle counter you would want less turbulence to reduce the amount of bubbles forming.

You also want to run off some oil (at least twice the dead oil volume)* to clean the sampling point before testing and flush the bottle up to 3 times with the oil going to be measured depending on the cleanliness level you are working to.

*Dead oil volume or dead leg volume is if you have a drain pipe at the bottom of a tank the oil will be trapped in this and collect debris which will not be circulated even with good mixing of the tank. Hence the volume of this piping should be flushed at least twice before sampling. It is also worthwhile cleaning around the sampling point before removing the cap too if sampling in a dusty or dirty environment.

If you think about it if you have a 1000L system and you are taking 100ml, you need to be sure that 100ml is representative of the oil in the system. It’s best to speak to your lab to help you identify the best way to take consistent and representative samples of oil from the machinery.

It also helps when collecting a sample for the lab to eyeball it yourself before sending to check for any debris or contamination that may have occurred at sampling, which is a sign of poor sampling technique. If you do spot a problem then resample again and consider sending both samples to the lab for analysis.


Warming generally increases the ability of a liquid to dissolve suspended particles. In particle counting terms we refer to soft and hard particles. Soft particles are things such as oil additives and oil degradation products, which are not generally abrasive or significant to measure. Equally water solubility at low concentrations changes with temperature and so dissolved water may become free water after cooling and be measurable on a particle count. When at operating temperature these soft particles will be dissolved in the oil, but when exposed to colder temperatures outside of the machinery they will start to only be in suspension rather than dissolved and be detected in a count. This can sometimes be observed as the oil going slightly hazier after cooling depending on the severity. So if you are measuring online at e.g. 45’C and then it’s measured at the laboratory at 20’C then there is a likelihood a higher count may be obtained. Part of this issue is why dilution particle counting has become popular in recent years with laboratories to address this issue and remove soft particles and water from counts (discussed later).

Air Bubbles

Each air bubble in a sample is detected as a particle by laser counters. So it is important to reduce the number of naturally forming bubbles such as leaving compressor samples to degas before testing. Moreover, non naturally forming bubbles introduced during testing can also be a problem.

Air bubbles can be introduced in a number of ways. When using an offline system a common mistake I have seen by well intentioned individuals trying to homogenise the sample is shaking the bottle. This is a bad thing for laser counters and introduces thousands of microscopic bubbles into the sample long after shaking.

If using an online system then it is best to ensure the pump driving fluid into the counter is not causing a vacuum around the particle counter which again introduces bubbles. That is why I have always used automatic rollers to roll samples to gently mix the samples with as little air entry as possible.

Solvent effects

Solvent dilution particle counting has become the big thing in particle counting in recent years and did promise for a time to give 100% soft particle free counts close to manual counting methods. The counts are much improved, but with it come new hurdles to overcome on these methods. These include not all oil soft particles truly dissolving in the solvent mixes used, the solvent itself carries particles and the mixing in the solvent with the oil causes bubbles. Hence why regular baseline samples of the solvents need to be used for comparison to minimise these limitations. Dilution counting is unlikely to be used on most customer sites using field testers, although I have seen it used on sites running test rigs or new machinery production quality control. The important thing is to check the same dilution techniques are being used between the lab and the field counter.

Sample Compatability

This tends to be an issue if you have multiple product types on site as laboratories tend to be better at keeping their equipment clean between samples. With online or portable counters it is tempting to go from machine to machine and not spot the fluid type change. This is commonly seen when you have filter caddy systems with particle counters and the operator has not realised the oils between tests are incompatible. For instance silicone oils, polyglycol and water based products all are incompatible with mineral oil. What does incompatible mean? Well in this case they won’t mix. Not mixing means artificially high counts meaning if you go and test one incompatible oil system the next 10 or 15 counts could all be wrong because you need the right order of solvent washes to switch from one product type to another. In a lab this is time consuming, but with many solvents on site it’s not really a problem to do. However, in the field it is unlikely you will have these solvents to hand or the safety and waste solvent procedures to handle them. Hence in this case I would personally recommend investing in a second particle counter of the same model if you intend to regularly have to switch from one product type to another (no I’m not on commission to say this.)

Cleaning and maintenance

Naturally the cleaner and well maintained you keep your counter the better the results you will obtain. However you need to be clever about how you clean the instrument. I have seen this a few times by well meaning individuals on customer sites where they have cleaned the ports with a fresh paper towel. This is a cardinal sin of particle counting as it produces thousands of tiny paper fibres that will give artificially high counts. Whenever going near any of the inlets of a particle counter to clean you should always use a disposable lint and paper free cloth.

Different Calibration Types

Assuming the obvious that your equipment calibration is within date, there are a few ways particle counters can be calibrated. These include my preferred method of NIST dust but there are alternative calibration media including beads/ spheres type materials. These will give slightly different readings between instruments and so it is good to check what the calibration material is if struggling to find a difference.

Are you causing the problem?

If all else fails, look for if you are doing anything different. I had a customer who swore blind the lab was getting too high particle counts. It eventually involved a visit to site and we brought some new oils we had run in our lab to test on their counter, which came very close to our value. So there was a lot of furstration why in practice we were getting such different results to each other. In the end we discovered the customer had been adding a leak dye after they had measured their particle count but before taking an oil sample.

Does a difference between field and lab counters matter?

No not really. Most particle counters are generally accurate to within a code of the correct value meaning small changes in particle count are not necessarily cause for action as they could be within repeatability of the test. The main take home message is to develop a consistent method, to trend and use a reputable lab to obtain an official rating when a specification is required.

Remember particle counting is a target for the most part. Any improvement you can achieve helps and will help your equipment. So try not to be too focussed on absolute limits and beat yourself up for not achieving them. Instead try to work on a general improving trend in cleanliness across your machinery with whichever method you are using, this goal based approach has been found to have up to 7x improvement in component life as a result.

If you would like help with checking the cleanliness of your fluids in the field or via a lab then press the contact button on the bottom right to ask for help.