The relation with viscosity and pressure of lubricant has been considered in detail from the past. Barus equation that was presented in 1893 is known as the basic equation. This paper describes the new linear viscosity equation, that included dimensionless density and temperature other than pressure as functions in comparison with Barus equation. Author measured high pressure viscosity by using reciprocating-piston type test apparatus and measured high pressure density by using capacity type test apparatus in each temperature and pressure. After that, author attempted to derive the density into Barus equation and it was found that dimensionless density was preferable as density. Furthermore, author attempted to introduce temperature into Barus equation. It became able to in this way deal with the new viscosity equation as a linear equation. It found out that the logarithm of dimensionless viscosity was proportional to pressure and it was inversely proportional to temperature and dimensionless density cubed. And it was found that these slope δ of lubricants by this linear equation are intrinsic constant.

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