ASTM D6786 Test Method for Particle Count in Mineral Insulating Oil
ASTM D6786 Standard Test Method for Particle Count in Mineral Insulating Oil Using Automatic Optical Particle Counters
4. Summary of Test Method
4.1 Samples are taken in particle-clean bottles that are suitable for particle analysis. The sample bottle is agitated to redistribute particles in the oil, then the oil is placed in an automatic particle counter, where the number of particles and their size distribution are determined by the light extinction principle.
4.2 As particles pass through the sensing zone of the instrument, the quantity of light reaching the detector is obscured. This signal is translated to an equivalent projected area diameter based on calibration with a NIST-traceable fluid (ISO Medium Test Dust suspension).
5. Significance and Use
5.1 Particles in insulating oil can have a detrimental effect on the dielectric properties of the fluid, depending on the size, concentration, and nature of the particles. The source of these particles can be external contaminants, oil degradation byproducts, or internal materials such as metals, carbon, or cellulose fibers.
5.2 Particle counts provide a general degree of contamination level and may be useful in assessing the condition of specific types of electrical equipment. Particle counts can also be used to determine filtering effectiveness when processing oil.
5.3 If more specific knowledge of the nature of the particles is needed, other tests such as metals analysis or fiber identification and counting must be performed.
6. Interferences
6.1 Dirty environmental conditions and poor handling techniques can easily contaminate the sample and/or test specimen. Care must be taken to ensure test results are not biased by introduced particles.
6.2 Air bubbles in the oil may be counted as particles giving false positive readings. Mixing or agitating the sample introduces bubbles into the oil, but these readily dissipate and generally do not interfere with insulating oil particle counts.
6.3 Suspended or free water in the oil will generally be counted as particles. Normally, free water concentrations below 10 ppm will not interfere.
6.4 Excessive concentrations of particles in the oil will cause coincidence and/or electronic saturation errors. Limits are determined by ISO 11171 and are generally supplied by the instrument manufacturer. These errors may be avoided by diluting the sample with particle-clean dilution oil or particle-clean solvent.
6.5 Odd-shaped particles and fibers may be improperly analyzed, depending on their orientation as they pass through the sensing zone of the instrument.
