Sample Containers Affect Oil Analyses
Ken Bannister | September 21, 2018
Grabbing the nearest empty container to hold your used-oil sample will produce unreliable test results.
Used-oil analysis is a respected, effective, inexpensive predictive-maintenance technique. It can help determine when to change oil based on its condition, predict incipient bearing failure so that corrective action can be taken in a timely manner, and diagnose bearing failure should it occur.
Setting up and implementing an effective industrial oil-analysis program is relatively easy to do, compared with other predictive technologies. It should be an included strategy in any industrial plant that purchases, stores, dispenses, changes, uses, or recycles lubricants as part of its manufacturing or maintenance process.
Most maintenance departments understand that the first step to a successful used-oil-analysis program is establishing and fostering a continuing relationship with a reputable laboratory. Although such laboratories offer and perform similar testing capability on the same equipment, they will differ slightly in their processes, methods, and reporting capabilities. Success is, therefore, primarily driven by methodologies that use consistent procedures to collect, test, and report on every sample.
Having an established relationship with your laboratory allows them to understand and compensate for working and ambient conditions for each machine being sampled. This will help to develop reliable trending for each sample point.
Arguably, the most important and misunderstood element of the used-oil-analysis process is the consistent collection of a quality representative oil sample for testing purposes. To achieve this, the maintainer must ensure that the sample is collected on an established frequency and in an appropriate manner to maximize data density and minimize data disturbance. This requires the choice and consistent use of appropriate sample containers of the correct material and size.
Containers and Sample Results
If an oil sample is not exactly representative of the oil system being tested, the result will be compromised. Simply put, the test is only as good as the sample. Once a sample is extracted, it should be placed into a clean, neutral environment that will not adversely affect the sample’s integrity as it is shipped from the machine to the laboratory for testing. In addition, the amount of fluid collected for testing should meet laboratory requirements.
Take a tour of an oil-analysis facility and you will be astounded at the variety of containers used to collect and send used-oil samples to the laboratory. These can include used screw-top soft-drink containers, milk containers, mason jars, plastic pill bottles, and ketchup and condiment containers, to name a few. It should be obvious that any used-oil sample placed in a previously used container will likely have added particulate and/or fluid contamination that is not representative of the original oil system. To achieve a representative sample, it is imperative to use a container specifically designed to hold oil samples. These containers are sold according to their cleanliness level, material, and size.
Sample containers are available with three basic cleanliness levels: clean, super clean, and ultra clean. Cleanliness is defined by the residual particulate greater than 10 micron/ml of fluid that might be expected to contaminate the sample. Refer to the table on p. 32 for particulate range and materials by cleanliness levels.
The appropriate cleanliness-level container will depend on the application and is based on the cleanliness levels required by the machine being lubricated. This is established using a signal-to-noise-ratio (SNR) calculation that divides the oil target cleanliness level (signal) by the bottle contamination level (noise). For example, if the target level of a fluid is 50 particles that are >10 microns/ml and a super-clean bottle is chosen, the SNR calculation would be 50/10 = 5. That signifies a respectable SNR value of 5. The higher the number, the cleaner the sample.
Container Materials, Sizes
The three most common container materials are glass, high-density polypropylene (HDPE), and clear PET plastic. The glass and clear PET plastic containers offer great visual-analysis capability for immediate detection of water and heavy or large contaminants. Glass is the most-expensive option, but offers the cleanest environment for storing samples and is compatible with all oils. PET plastics start to deform and break down with fluids exceeding 200 F, but offer visual clarity for lower-temperature fluids and are less expensive than glass. HDPE is opaque and not as clean as glass or PET, but it’s the least-expensive option for less-critical types of fluid analysis.
Where lubricant contamination is a critical factor, such as in hydraulic-fluid testing, glass is always the best option.
The size of sample containers will generally vary from 2 oz. (50 ml) to 8 oz. (200 ml). Size will depend on whether single or multiple tests are to be conducted on the sample. When filling the sample bottle, the amount of lubricant will vary based on oil viscosity and whether the laboratory needs to agitate the sample in the bottle. A rule of thumb is if the lubricant viscosity VG is lower than ISO VG 32, fill to 75%; for ISO 32 to 100, fill to 66%; for ISO VG greater than 100, fill to 50%.
When an oil-analysis program is in place with a dedicated laboratory, the lab will be able to advise the best type and size of sample bottles to use and the approximate fill-level requirements to achieve maximum value density (the most usable data from your sample), minimum data disturbance (cleanest representative sample), and most accurate reporting capability. In most cases, the laboratory is able to provide the correct sample bottles or put you in touch with a recommended supplier. EP
Contributing editor Ken Bannister is co-author, with Heinz Bloch, of the book Practical Lubrication for Industrial Facilities, 3rd Edition (The Fairmont Press, Lilburn, GA). As managing partner and principal consultant for Engtech Industries Inc., Innerkip, Ontario, he specializes in the implementation of lubrication-effectiveness reviews to ISO 55001 standards, asset-management systems, and training. Contact him at firstname.lastname@example.org, or 519-469-9173.