Proper Oil Sampling Methods—The Steps you Should Follow for Best Results
by Michael Holloway, CLS, OMA I
National Sales Manager

Ideally, oil samples should be taken in a manner that is easily repeatable and effectively represents the actual condition of the oil in the equipment. Good sampling procedures ensure consistency and reliability of data. Oil samples must be taken on a regular preventive maintenance schedule. Do not take samples soon after a filter change, or after makeup oil has been added. Adding new oil dilutes the levels of contaminants and wear metals found, which may result in conditions appearing better than they actually are.

Each sample drawn must be taken regularly from a single location in a system. Take samples during normal operating conditions, downstream of pumps, cylinders, bearings, and gearboxes and upstream from the filter. When obtaining a sample from a lubricated system, always have the oil hot and thoroughly mixed before sampling. Make sure that the sample bottle is clean and free of any moisture before obtaining sample. When utilizing the vacuum pump method, make sure that sample is not obtained from the bottom of the oil compartment where sludge accumulates. Aim for the midpoint of the reservoir. Obtain samples during normal equipment operation or at least within 30 minutes after equipment is shut down. This is the best way to obtain a truly representative sample of conditions within a lubricated compartment or a machine compartment. Make sure that sample bottle and container are properly sealed before shipping.

Sampling Methods

Valve Method
Install valves upstream of any filter in order to capture wear particles. Make sure the valve is clean and adequately flushed. Using a sample valve, such as the 1/8” NPT Push Button Valve, helps in producing reliable test results. Install valve properly on a pressurized oil line or oil galley. Avoid areas where oil does not circulate as freely, such as the bottom of a sump.
Taking an Oil Sample Using the Valve Method:

  1. Unscrew dust cap from sample valve.
  2. Depress the button on the sample valve.
  3. Flush the oil line allowing several ounces to drain before taking the sample.
  4. Place the empty sample bottle under the sample valve discharge opening.
  5. Fill the sample bottle 3/4 full and release the sample valve.
  6. Tighten the cap on the sample bottle to secure a tight seal.
  7. Screw the dust cap back on the valve. Prepare for shipment.

Pump Method
It is important that vacuum pumps are used with appropriate tubing. Make sure that new tubing is used for each sample in order to avoid cross contamination. Cut the tubing to the same length each time you sample. Avoid scraping the tubing along the sides or bottom of the tank or reservoir.

  1. Measure and cut new tubing to the length of the dipstick. Tubing must reach half way into the depth of the oil (or long enough to reach the midpoint of the reservoir).
  2. Insert the tubing through the head of the vacuum pump and tighten the retaining nut. The tubing should extend about 1/8 inch beyond the base of the vacuum pump head.
  3. Install a new sampling bottle onto the vacuum pump and insert the end of the tubing into the oil – do not allow the tubing to touch the bottom of the compartment.
  4. Pump the vacuum pump handle to create a vacuum. Hold the pump upright to avoid oil from contaminating the pump. If oil enters the pump, disassemble and clean it before taking the sample. Fill the oil sample bottle at least 3/4 full.

Drain Line Method
The drain line method is considered the least preferred method of sampling. Make sure that an ample amount of oil is drained before collecting a sample. The sludge, particles and water that settle to the bottom of a tank or reservoir provide poor and sometimes unreliable results. Taking an Oil Sample Using the Drain Method:

  1. Clean area around the drain plug to avoid sample contamination.
  2. Allow ample amount of oil to flush through the oil pan drain hole.
  3. Fill sample bottle 3/4 full.
  4. Screw bottle cap on tightly. Wipe bottle clean and prepare for shipment.
  5. Remove the tubing from the compartment and dispose of it correctly.
  6. Do not reuse tubing. Remove the bottle from the vacuum pump and secure the cap on the bottle. Prepare for shipment.

ALS Tribology has developed a series of technical bulletins to provide clarification and guidance on matters regarding oil, fuel and coolant testing and analysis. For a complete list of Technical Bulletins, visit the Downloads section of our website. Technical Bulletins are available in PDF format for easy downloading and printing.