Why choose RULER?

Lubricants degrade throughout their lifetime. As part of condition monitoring, we typically employ several types of oil analysis tests to help us better understand the rate of degradation or guide us towards finding the root cause of degradation. However, with so many oil analysis tests on the market, how do we choose the suite of tests which can help us to determine the health of the lubricants in our machinery? This is where a comprehensive understanding of the criticality of the machine, the particular functions of the lubricant and its environment all aid in determining which tests should be used.

Choosing Oil Analysis Tests

As stated earlier, when developing an Oil analysis program, particular attention should be paid to the equipment being monitored, its criticality and environment. For most equipment, the purpose of monitoring the health of the lubricant is to allow the users to determine whether there are any challenges which may lead to unplanned downtime. Typically, these users are interested in getting these results quickly and accurately to allow them to make informed decisions regarding the operation of the equipment.

Typically, most basic oil analysis programs include tests for Viscosity, TAN, wear metals, contaminants and additive concentrations. While the results of these can all contribute to understanding health, the one area we have not touched on is the rate of depletion of antioxidants. When a lubricant is exposed to oxidation, its antioxidants are readily depleted which then affect its Viscosity, TAN, presence of wear metals, contaminants and additives. Why then is the test for the Remaining Useful Life seen as a secondary test rather than a primary test?

What does RULER represent?

RULER represents the Remaining Useful Life Evaluation Routine. It is a test which is performed to quantitatively determine the remaining amount of antioxidants in an oil. When oxidation occurs, the antioxidants become depleted as they are used to neutralize the free radicals produced during the process. This neutralization process will continue until the antioxidants have been depleted which leaves the base oil open to becoming oxidized by the free radicals. It is at this stage that the tests for Viscosity and Acid Number show significant increases. At this time, the base oil has already begun oxidation, hence using the Viscosity and Acid Number tests do not adequately warn the users.

On the other hand, through the use of voltammetric techniques, the oil can be analysed to determine the quantity of antioxidants which remain. This gives us a better estimate of whether oxidation is occurring in the oil, the rate at which it is occurring (if we do comparison samples at different intervals) and can even let us know if there are any “strange” antioxidants in the oil. By including the RULER test in our primary suite of tests for critical equipment, we can make better informed decisions about the health of the oil and steps to protect the machine or plan for downtime.

Comparing techniques

When evaluating which oil analysis technique should be added to your program, one of the methods used is to compare these techniques and use this to guide your selection.

The TAN (Total Acid Number), RPVOT (Rotating Pressure Vessel Oxidation Test) & RULER (Remaining Useful Life Evaluation Routine) tests were all compared based on their characteristics and summarized in this table below:

Ease of Operation

Sample size

5ml 50ml 0.4ml
Sample Preparation Extensive Moderate Minimal
Analytical Procedure Extensive Minimal Minimal
Safety Requirements Extensive Moderate Minimal
Analysis Time

Sample preparation

<10 min <10min <2min
Analytical Procedure >10min >1 hr <1min
Duplicate Analysis (without resampling) No No Yes
Instrument for RUL Evaluations No Yes Yes
Standardized test Yes ASTM D 664 Yes ASTM D2272 No
On-site condition monitoring capability No No Yes
Disposal cost Very high High Low


From this table, it is clear that there are both benefits and disadvantages of all three techniques. However, when we examine this table from the perspective of the addition of a particular test to the oil analysis program, there are a couple of items which stand out:

*For equipment which have a higher frequency of oil analysis such as monthly or bi-monthly, it is more effective to use smaller sample sizes with shorter sample preparation and analytical procedure times. This saves time for the analysts and gets faster results without having to top up the sumps due to the amount of oil being removed for sampling. When new oil is added, it skews the trend of degradation and can produce misleading results.

*The ability to duplicate analyses with the RULER test really allows this test to shine. In the oil analysis world, it should be ensured that tests have the ability to be repeatable. This ensures that should anything be questioned or in need of verification, the test can easily be repeated and the results would be the same.

*On-site condition monitoring using the RULER test can greatly expedite the rate at which results are returned. This is crucial for critical pieces of equipment where unplanned downtime can turn into catastrophic failures to the plant.


Why should RULER be incorporated into your oil analysis program?

There are 4 reasons you should incorporate RULER into your oil analysis program:

1 Low cost of RULER Analysis – for the value that is obtained from this test, the cost is relatively lower than other tests which may not produce repeatable results. The overall cost savings to your equipment far exceeds the cost of having this instrument at your site, at your disposal.

2 Trending Capabilities – when the antioxidant depletion trends are combined with particle counting, water analysis and viscosity analysis, users can detect abnormal operating conditions. If these RULER samples are done frequently, then the rate of antioxidant depletion can be easily trended and addressed accordingly.

3 Proactive information – From past case studies, we have found that the levels of TAN only increase when the Remaining Useful Life of the oil reaches 30% of remaining antioxidants. It is at this point in time that the oxidation levels tend to rapidly increase which do not give the operators sufficient time to address the ongoing issue. However, through the implementation of the RULER test in critical pieces of equipment from the onset, operators are able to proactively determine the rates of oxidation and plan accordingly. However, if these maintenance plans were done according to the TAN levels, then the oil would have already been far too oxidized and not much could be done at that time.

4 Portable technique – Fluitec has developed the RULER instrument which is easily portable into the field and can produce results quickly. This allows operators to easily and frequently test their oils and receive the results.

Overall, there is a high favourability for the integration of the RULER test into a regular oil analysis program especially for critical pieces of equipment. Its ability to quantify the presence of antioxidants and help operators trend the rate of oxidation greatly assists in planning potential downtime for these equipment. The RULER test significantly escalates the ability of operators to increase equipment availability and essentially reduce costs associated with unplanned downtime.