During the operation of a hydraulic system the particles of dirt that occur in the pressure fluid. They are either
- Added from the outside, e.g. due to leaks, a poor breather filter or are due to inadequate scraping at the piston rods or
- Due to abrasion in the system. These are metal chips as well as abraded material from the seals or
- Ageing products from the hydraulic fluid (sludge among others).
Sometimes there can also be water (that has entered the system or condensation).