Oil sampling programs can provide incredible insight into machine conditions, but if you don’t get that sample right then time and cost invested can be wasted. If you’re not sure how to take an oil sample, read on, and make sure you don’t make any of these five oil sampling procedural problems we see.
Is your machine at operating temperature?
If your machine has been out-of-service for more than a hour or so, the compartments have started cooling, and any debris that was circulating may be settling out of the oil.
If you take a sample from a cold compartment, the oil is not likely to be representative of the debris circulating during operating conditions.
If the oil is at operating temperature, it’s safe to assume the compartment is well-mixed.
Consider how this could be achieved in normal operations.
If the oil sample will be drawn during a scheduled service, then sequence the service activities so the oil sample is drawn immediately after the machine has stopped.
Perhaps your operation draws samples during a pre-service inspection. In this case, you’re probably conducting the inspection during a downtime event (like a meal break). The machine is likely to be warm from normal operations. This is the perfect time to draw your samples.
Tracked machines (dozers, shovels and excavators) will need to be walked a short distance to circulate oil in their final drive compartments. If a digging machine has walked off the active face for a scheduled service this is probably sufficient. If a dozer has tracked to the workshop and is still warm this is also adequate.
Mistake #1: Are you drawing samples at compartment operating temperature?
Is your equipment clean?
Typically, on earthmoving equipment and heavy vehicles, oil is sampled from the compartment in one of two methods:
- Vacuum pump
- Oil probe method
Vacuum pumps are used to extract a sample from a non-pressurised compartment. Oil probes can be used to extract a sample from a pressurised compartment such as engine lubricating oil.
The oil probe method – or live oil sampling - does require the machine to be operational when taking the sample, which may be difficult or prohibited by site operating procedures, especially for large mining machines. In this case, the vacuum pump method is perfectly acceptable when the machine has been isolated and made safe for maintenance procedures.
Vacuum pumps are the workhorse of any oil sample collection program.
Firstly, you should absolutely have a separate vacuum pump for coolant. While most people are very careful and methodical with cleaning their vacuum pumps, it only takes a minor lapse in attention or a distraction during sampling to overlook cleaning the pump.
No matter how careful you are, at some point you’re likely to trigger “oil in coolant” or “coolant in oil” reports from the laboratory. And this could send the team on an investigation for a non-existent problem when it was simply due to a contaminated vacuum pump.
Vacuum pumps can also be contaminated if they’re tipped over when a full sample bottle is attached. The sample oil then contaminates the internal components of the pump. Now you have a chance the oil that has contaminated the pump then subsequently contaminates the next oil sample. The lab will likely return a “wrong oil” or “contamination” result.
Finally, drawing the tube back through the vacuum pump can also contaminate the pump internals. A better method is to remove the sample bottle, then push more of the clean tube though the pump. This allows you to cut off the “wet” end of the tube and then pull the “clean” tube back through the pump.
Even cleaning the vacuum pump with dirty rags is a source of contamination. While solvents are effective at dissolving oil and washing debris from the pump, wiping them with anything other than pristine cloth or shop towel can potentially introduce more contamination.
Live sampling ports
Our equipment operates in a harsh environment. Oil sampling valves are exposed to dust, and if they are at all “wet” from the last sample taken, they become a magnet for dust.
Washing that dust into your sample bottle will be obvious to the lab and trigger a “possible bottle contamination” result.
Live sampling ports should be cleaned before use. If the machine has been washed prior to a scheduled service, they might need a quick wipe with a shop rag or paper towel. If not, they’ll definitely need some attention before the sample is drawn.
It's impossible to know whether the live sampling port has been installed in a “dead leg” or stagnant flow area. To flush the sampling port and any lube that might not be representative of the compartment, drain an amount of oil into a waste container prior to drawing the sample. A volume of three times the sample quantity would be a good target to flush the line and the valve itself.
Bottles & tubes
Bottles and extraction tube are generally supplied sealed from the elements. Leaving them open in a dusty environment quickly deposits dirt and artificially inflates the dirt results in your lab report.
Bottles are manufactured to high cleanliness standards and keeping the cap in place as long as possible is a good practice. Removing and reinstalling the cap only to fit the bottle to a vacuum pump is ideal.
Similarly with the tube, it is clean when supplied in its sealed packaging. If you are using sample kits with individual pieces of tube supplied then good habits – like those employed for bottles – help keep the tube clean.
Bulk tube supplied on a roll is likely to be located in the store with all the other bulk tube and hose. Which means its probably dusty.
Wiping the tube clean and cutting off the last exposed 10cm of tube is one method to minimise contamination.
Mistake #2: Is your sampling equipment dirty?
Is the sample representative of the compartment?
You might expect the heaviest contamination to settle at the bottom of the engine sump or base of any oil tank on your machines. I’m sure you’ve seen this when draining and cleaning tanks of any sort.
If using a vacuum pump to draw your sample, you don’t want to vacuum this debris into your bottle. This isn’t representative of the oil in the compartment.
When using a vacuum pump, the usual guidance is to cut the tube to the length of the dipstick – plus some arbitrary amount.
But then how do you know where in the sump the tube finishes?
A more accurate method is to measure the extraction tube against the dipstick and use a marker to identify the location of the dipstick cap on the tube. Then its possible to insert the tube down the dipstick accurately to the mark you have made. Now it doesn’t matter if you have one centimetre or one meter of extra tube, you know you’ve installed the extraction tube into the sump, but not touching the bottom.
Similarly with oil tanks, aim to cut the extraction tube to reach mid-way into the tank.
This is also why drawing a sample from the draining fluid (or the “drain line method”) is less than ideal. If the debris from the bottom of the compartment is being drawn towards the drain you have no way to know if it is contaminating your sample.
Take some care to draw a representative sample from the oil in the compartment.
Mistake #3: Are you drawing a representative sample?
Have you provided accurate data to the lab?
While the oil in the bottle is very important, the oil sample label is a close second.
The lab needs basics like the machine identification and compartment, but there are other important details.
The current machine or compartment hours are critical. The lab will look at the condition of the oil and compare this to current hours. A simplified example might be as follows.
The lab sees 10 parts per million of iron in the oil. But when they cross check against the oil they calculate only 100 hours from the last sample they received. So, is 10 ppm bad or not? If they assume the rate of iron generation is linear, 10ppm multiplies to 50ppm if the compartment is scheduled to receive 500-hour oil changes. Now they must make the assessment whether this is problematic for “hours on the oil”.
Secondly, and admittedly most operations have trouble tracking this, is oil top up.
If your engine has a 50-litre sump, but over the course of the maintenance interval you’ve topped up 10 litres because of leaks or consumption, you’ve diluted the oil in the compartment and consequently any contamination also present.
While this top up might be negligible, the lab has no way of knowing. If you’ve been topping up and the lab doesn’t factor this in, your results are worse than the report indicates.
Mistake #4: Are you providing accurate data to the lab?
Avoiding all these mistakes aims to influence one factor: sample consistency.
Providing the lab a representative sample with accurate data gives them the best chance of providing valuable feedback to you about the health of your machines.
There are some other considerations to improve consistency.
All the factors described above might be logical to an experienced technician or engineer, however, new people to the industry or your organisation need training. Otherwise, how can they know why the sample must be drawn so carefully? On massive 24/7 operations, many people will likely be involved in drawing samples and training becomes critical.
In some smaller operations, you can improve consistency by having dedicated people drawing all the samples. If your site is performing pre-service inspections, you might have dedicated equipment inspectors checking for defects. Often these inspectors have high attention to detail and are perfectly suited to drawing samples.
Mistake #5: Is your sample quality inconsistent?
To ensure you’re not wasting time and money on your sampling program, check your operation is not making these five mistakes.