Remaining Useful Life or RULer was introduced now going on 20 years ago few as an alternative to RPVOT (Rotating Pressure Vessel Oxidation Test – formerly known as RBOT) to establish remaining life of lubricants. Up until this point, the traditional method of measuring how much antioxidant was left in the oil was done indirectly by artificially ageing the oil in a rotating pressure vessel which could take up to a day per sample to perform on expensive instruments that required huge investment to provide capacity for clients samples. From a lab perspective if you only had 1 instrument and you received a batch of 20 samples all in good condition this could take 20 days to perform which in condition monitoring terms is a lifetime.
In came the inventors of the RULer test to shake everything up with its direct measurement of anti-oxidants rather than indirectly that could measure all those 20 samples on the same instrument in a few hours and with a very small volume of sample. The manufacturers did an excellent job of marketing the RULER product and had all the studies to back up their claims. So much so the test has become synonymous with their brand name with the RULER brand being used instead of its technical name of linear sweep voltammetry of antioxidants (yes it is a mouth full and I see why RULER caught on).
The price point was also affordable that this was a product not only for high tech labs, but could be used by field engineers too. The product worked by directly measuring the anti-oxidants of the used and new oils with different solutions to highlight the different anti-oxidants. Then you divide one by the other and get a RUL% value of how long the oil had left.
This sounded like the end for RPVOT and I’m sure some labs even started to mothball them as they started to use RULer.
All references are not equal…
However, RULer is very dependent on reference oils in that the value without a reference was pretty meaningless. In contrast understanding a RPVOT value was relatively simple, measured in minutes and the lube oil suppliers often quoted a typical new value on their turbine oils to allow comparison. Equally since the RPVOT were all measured in minutes then general rules could be applied such as a value less than 100 minutes was time to consider an oil change. Whereas for RULer a value of 700 could be fantastic or poor and without a reference it was difficult to compare.
So you may say well just get a reference and indeed this is what you ask the customer for and you hopefully can get one. However, because each batch and blend and formulations change from country to country for the same product this means the oil is not the same even for the exact same product name in terms of the antioxidant mix. This means you not only need to get a reference for each oil you are testing but for each oil for each customer on each delivery. Although time-consuming, this is doable, but the problem with the original instruments was they were designed for field use and with that price, which means expensive things like instrument memory and programming to hold tens of thousands of reference points were never considered in the design. My very first RULer actually came with a floppy disks to show you the type of memory that these things came with. Even the more modern versions you were stuck with a couple hundred data points. Again for field use, this is perfect as that would be about 50 years of data for quarterly measurements for a single turbine on one solution, but I used to find that it was like watching an episode of the squid games trying to work out which reference data point had to be deleted next before people able to test the next sample. For a lab-quality auditing purpose, you can imagine this was a nightmare.
Red light – Green light (and also blue and yellow lights too) – too many confusing options
To add more confusion depending on the application you used a different set of solutions for the test (red, green, yellow and blue), which meant for oils that e.g. may be used in hydraulics, turbines and compressors with complex anti-oxidant additive packs you had to run at least 3 of the 4 solutions options to cover all the bases. I did try a policy at an old job of the blanket only running one solution for everything, but then you had the scenario of a customer moving from one lab to another they always seemed to be on a different solution. So you ended up going back to running multiple solutions for everything instead and asking the customer to send a reference oil with each sample to compare against. This not only spent lots of extra data points in the instrument software, meant the man time on the test took 8 times longer, cost a fortune in these exceptionally expensive chemicals where the only supplier was the manufacturer of the instrument, but also meant the report could have 85% of one additive and 25% of another, so the customer didn’t know which one was the actual remaining life to use for decision purposes.
Is the test a waste of time?
Having read all the above you may think I think the test is a waste of time, but I think it’s very useful. My criticism is not of the principle, but in how it is been implemented. The original systems are not designed for labs to use and for field use they are ideal. However, I come at this from a lab perspective, where getting not just a rough guide, but a precise right answer is very much key to what we do. From a commercial lab perspective, it has also got to be something that can be scaled up and doesn’t use proprietary chemicals where the prices are 100x what they would normally cost. The original technology of the instruments are based on 20-year-old principles now, so I might actually be just being critical of something that is good but just needs a refresh and a different approach to its use. In fact, additional players in terms of instrument manufacturers are likely to enter this market as there is a clear gap in the coming years to address the limitations of the original technology for the lab market.
My prediction was indeed correct, other manufacturers have indeed entered this market 4 years on from my original article and the international methods are being reviewed to allow for improved voltammetry electrode accuracy, laboratory levels of precision, all in one solution options, huge reference database sizes and automation. In fact, our laboratory has provided samples for some of the first blinded international round-robin comparisons and will be one of the first in the world to offer these new updated linear sweep voltammetry methods. Combined with our own additive testing methods developed exclusively at oil analysis laboratories, we now are able to offer the most in-depth lubricant degradation methods available. If you would like to find out more then contact us using the contact us blue button on the bottom right of this page.