Viscosity of a mixture of diesel & hydraulic oil

Asked October 1, 2014, 3:25 PM EDT

What is the approximate viscosity of a mixture of 17 gallons of 10W-30 hydraulic fluid and four gallons of #2 diesel? Consider an operating temperature of 160F.

[Apparently someone added diesel to the wrong reservoir in a skid steer while it was on the job site. It then ran for ~ 15 hours or so. I am just curious as to viscosity of the diluted mixture as some means of potentially forecasting the damage.]

Pulaski County Illinois

2 Responses

This is a tough question for multiple reasons, speaking with some tractor engineers I was able get the information below.

I have no idea if mixing 10W-30 and No. 2 diesel would form any other compounds that might affect the viscosity.

If it is strictly a mixture issue (no other compounds formed), my references indicate the viscosity of 10W-30 to be about 88 mm2/s at 160F. I couldn’t find a viscosity for No. 2 diesel at any temperature other than 100F so I used the low end of the range for the diesel fuel at about 2 mm2/s, as viscosity typically decreases as temperature increases. Since most of the mixture (17/21 by volume ) is 10W-30, the viscosity of the mixture should be close to that of the 10W-30. My calculations show an estimate of 72 mm2/s, so I would estimate the viscosity of that mixture (without any other compounds formed) being approximately 70 mm2/s.

Beyond that, refer them to an oil/fuel supplier to check on other compounds that might form and affect the viscosity, and another estimate for the viscosity of the mixture.

I appreciate your taking the time to respond. I did significant research as well and concluded that #2 diesel does not usually have a viscosity over 100F listed because it is specifically engineered for combustion at relatively low temperatures. The hydraulic shop that discovered the issue advised the owner he was "on borrowed time....had perhaps as long as six months prior to a catastrophic failure of a pump or final drive." I am with you on the parts to the mixture. It is still mostly 10W-30. The Rexroth fixed displacement radial piston drive motor cut sheet of specifications indicates a viscosity range of 10 to 200 mm2/s.