In this paper, a second order polynomial model for predicting the pressure-temperature behaviour of the density of any hydraulic fluid is presented. The model is an extension of the previously published model by the same authors for more moderate operating temperatures. Nevertheless, for a user the extension will not add any more complexity to the model. Even at a wider operating range, the density model can still always be parameterized without any unknown variables, once the standard fluid characteristics are available.

It is shown that compared to the measured values the maximum modelling error is well within 1% at the studied pressure range of up to 1500 bar, and at the studied temperature ranges overall covering from +20 to +130°C, with all the studied fluids.

This study includes 10 highly different hydraulic fluids used in various fluid power applications as power transmission fluids or fuel oils. The studied fluids have a density range from 827 to 997.2 kg/m3, and an ISO VG range from 2.6 to 1187. Also the studied base fluids cover a wide range. Moreover, the studied fluids contain different additives or not even additives at all (crude oils). Neither the base fluid nor the additives will be discovered to affect the received modelling accuracy.

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