Predictive Maintenance Services, Inc.

Oil Sampling Techniques


The oil report you receive back is only as good as the oil sample sent to the laboratory.  The oil sample must be representative of the system from which they are taken in order for the results obtained from the analysis to be realistic and useful.   To insure that samples are representative and repeatable, the following guidelines should be considered:


1.     Take samples while the equipment is running and at normal operating temperature and load if possible.  If this is not possible, samples should be taken as soon after shutdown as possible.  Samples taken from equipment that has been inactive for long periods are not representative.

2.     Take samples from the same point and using the same procedure each time.  This will help insure repeatability.

3.     Don’t sample directly after an oil change or after a large quantity of make-up oil has been added.

4.     Take samples from flowing lines between the lubricated device and before the oil filter wherever possible.  This sample will be the most representative of the actual wear condition.

5.     Clean around the sample point and flush an adequate quantity of oil through the sample piping to insure a representative sample.

6.     Use the sampling materials supplied.  They are clean and hold the proper quantity for the required analysis.  Don’t use water or soft drink bottles, jelly or baby food jars. 

7.     If samples are taken using a vacuum pump, use new hose for every sample to avoid cross contamination between sample points.  It is also advisable to flush the hose by filling the sample bottle two times and discarding the oil then use the third filling as your actual sample.  This process will flush any debris that may have entered the sample hose as it traveled down the dipstick tube and also rinses the sample bottle. 


Sample Points


The sampling technique that gives the most representative sample is one in which the sample is taken from a line carrying oil scavenged from the wearing parts and before filtering.  If the line is large and the flow velocity is low, sampling from the bottom of the line should be avoided. 


Reservoir, crankcase or sump samples should be taken using a vacuum pump not dipped from the tank.  When used correctly and in the same manner each time, the vacuum pump allows a representative sample to be taken quickly and with a minimum amount of effort.  The sample tube used with the vacuum pump should be inserted into the reservoir at a point several inches above the bottom – preferably in the middle of the reservoir and where there is some turbulence.  Care should be exercised while inserting the tube to avoid touching the sidewalls or the bottom of the reservoir.  This might dislodge rust, sludge, or large particles that might be picked up along with the sample.  The important considerations when sampling from reservoirs using this technique are to sample at the same depth each time and to use new, clean sample tubing for each sample.  A dowel rod or other similar device with the sample hose attached using wire ties works very well.  Sample depth is easily controlled and repeated and the old hose can be removed and new hose attached quickly between each piece of equipment.


If samples must be taken from drains (least preferred), flush the drain by allowing oil to flow for a short period of time before taking the sample.  If the drain line is fairly lengthy, make sure warm oil is flowing to insure that the oil that was dormant in the line has been flushed.  Do not adjust the valve during sampling to avoid introducing wear from the valve into the sample.  If the sample is taken from an engine drain plug during an oil change, allow approximately half the oil to drain then dip the sample bottle into the flow. 


Vacuum Pump Basics


Remember to use a new piece of sample tubing for each sample to avoid cross contamination between samples.  Insert the sample tubing through the top of the vacuum pump until it extends approximately ½” below the body of the pump and tightened the compression nut sufficiently to secure the tubing in the pump.  Screw the sample bottle into the pump.  Do not stick the cap in your pocket or allow it to become contaminated!  Placing the lid in a plastic bag or our mailing container will protect it from becoming contaminated.  Insert the sample tubing into the sump as described above.  If inserting the tubing through a dipstick, only insert the tubing to the same depth as the dipstick.  KEEPING THE GUN AND BOTTLE IN AN UPRIGHT POSITION pull the plunger back a few times until oil begins to flow into the bottle.  When the oil reaches the shoulder of the sample bottle (just below the threads), break the vacuum by pressing the button on the front of the gun (or unscrew the bottle to break the vacuum).  Replace the cap on the bottle.  Remember that it is a good idea to throw away the first two samples when sampling through dipsticks to insure any contamination that might have entered the tubing as it traveled down the dipstick does not become part of the sample.  Use a new piece of tubing for each sample.


Sample Valve method


Sample valves and petcocks offer quick, easy and representative samples if placed in the proper location and used correctly.  The primary location of sample valves is typically before the return line filter.  Insure the sample valve you are using is appropriate for the pressure at the sample location.  Wipe the exterior of the sample valve and surrounding area clean prior to sampling.  Flush the sample valve by allowing four to eight ounces of oil to flow before taking the sample.   Insert the sample bottle in the flow while depressing (or opening) the valve to avoid adding wear from the valve into the sample (especially petcocks and ball valves).


Oil Drain Method


Clean the drain plug and surrounding area to avoid contaminating the sample.  Allow a sufficient amount of oil to flow from the drain to flush the inside of the reservoir near the drain plug. A quart is usually sufficient or about one half the capacity of the sump.  While the oil is flowing, tip the sample bottle into the flow of oil and pull it out when the oil is within ½ inch from the top. 


Sample Identification


Historical data is extremely useful as a trending tool but can only be accomplished when the oil samples are labeled the same way each time so the data can be associated with the equipment being sampled.  Identify the sample or sample point the same way each time.  A label or tag at the sample point is sometimes useful when there are many sample points in a plant or several different individuals will take the samples. 


Don’t hold samples.


Send samples to the laboratory as soon as possible.  Any corrective action that may be indicated will be delayed by the amount of time the sample remains unsent.