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Proper Sampling Techniques

How to Take Representative Oil or Fuel Samples

By analyzing a sample of fuel or oil, you can determine the amount of contamination, the wear rates and overall condition of your fluids. The real benefit of fluid analysis is that it acts as an early warning system, identifying potential problems before they become an equipment failure. The frequency of sample collection and the procedures followed will determine how informative the samples will be. Here are some of the most important factors to consider.


To measure filter performance, samples must be taken at the filter head or manifold immediately before and after filtration. Samples taken farther downstream may have picked up additional contamination passing through pipes or hose reels, especially when they are older. Sampling at the dispenser is critical for determining fluid cleanliness as delivered into equipment, but may not accurately reflect filter efficiency.


Appropriate sample ports and equipment are critical to taking a representative sample. Use of a properly located mini-mess sample valve is recommended. Similar to a check valve, it is closed and capped while not in use, minimizing contamination.

Sampling port adapters for bulk filtration filter heads are available for both upstream and downstream sampling. They are easily installed with hand tools, even in existing systems. Simply remove the plug in the indicator port and replace with the adapter. Select a sampling valve that fits directly onto the adapter’s threads. See Donaldson’s Bulk Filtration Product Guide for a variety of sampling accessories. Be sure to take into account system pressure to select appropriate sampling accessories.


The rule of thumb is to pre-flush six to ten times the total volume of static fluid in a sample tube, port, port adapter and any dead legs of pipe in the systems upstream of the sample port location. For best results install the sample connection as close as possible to the main line flow. All testing equipment must be thoroughly cleaned prior to use. Mineral spirits are recommended for offline cleaning of tools and supplies.

Bottle Cleanliness

Most bottles are not certified to any particular cleanliness level. “Clean” bottles generally have less than 10,000 particles greater than 10µ per 100ml and may be unsuitable for critical applications. Be sure to thoroughly pre-rinse the sample bottle by filling with fluid and shaking two to three times before taking the final sample. Note: Just removing the cap from the sample bottle in a dusty or windy environment may make the bottle unsuitable for use.

Document Procedures

To achieve consistent results that can be meaningfully compared over time, test procedures must be documented and enforced without exception.

Downstream Sampling From Clean Solutions Single or Dual Heads


  1. Power off pump or isolate head with valve 
  2. Relieve pressure by pressing nozzle 
  3. Remove electronic indicator plug from head with 11/16”wrench or socket set 
  4. Install port adapter, then sample port, then hose 
  5. Turn on pump or turn off isolation valve 
  6. Begin flow by depressing nozzle (filling vehicle or bucket) 
  7. Adjust hose connection to start or stop sampling 
  8. For best results, take sample while fuel is flowing 

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