Although lubricants are usually miscible (with the exception of PAGs) – especially if they share the same nominal viscosity and are designed for the same purpose – they are not always compatible with each other. In particular, when zinc-free and zinc-containing HLP mineral oils or PAO or ester synthetic oils are mixed with mineral oils or with synthetic oils from another producer, important characteristics such as air release time, water separation, foaming tendency, filterability, and seal compatibility can be negatively affected. Mixing of different hydraulic fluids, as happens for example in the case of construction machinery through the exchanging of different attachments on a daily basis, cannot always be avoided. In the event of difficulties following dismantling or an oil change, questions concerning the compatibility of two hydraulic fluids often arise. OELCHECK tribologists are able to determine the cause of the Chemical structure of mineral oil Chemical structure of synthetic oil 6 difficulties more easily if they have access to the following information:
- exact oil type, with information on base oil types and additives
- working temperatures and pressures
- machine designation and components in use
- usage times of the fluids
A standard analysis, which is conducted using an analysis kit 2 or 4, is usually sufficient to provide an indication of the problem‘s cause. Sometimes, however, it is necessary to carry out an explicit compatibility analysis. For this, the lab requires five samples (each of 1 litre): a fresh oil sample of the two hydraulic fluids used, as well as three mixtures in the ratios 50:50, 95:5, and 5:95. The Sample Information Form must be marked „compatibility test“. The OELCHECK lab will then begin by carrying out the same tests conducted using analysis kit 2. Air release time, foaming tendency, water separation, and water content are also determined. In addition, experienced OELCHECK tribologists also assess whether the oils are compatible with one another, and/or what problems may arise from mixing them.