View Full Version : Oil viscosity

26-01-2013, 02:25 PM
Am currently doing my level 2 diploma in refrigeration and air conditioning and I had a question in my test which was to do with oil viscosity. It was- What is the oil viscosity of oil at -20 in a freezer. The answers were- ISO15,ISO32,ISO68,ISO120. If anyone can help it would be great. Thanks, Liam.

monkey spanners
26-01-2013, 04:10 PM
These test questions are written by idiots, the oil will be whatever viscosity the compressor manufacturer recommends, most common would be ISO32 in hermetic and scroll comps, the bigger semi hermetics sometimes use ISO68 (also would depend on refrigerant type)

The actual viscosity of the oil in the evaporator will be difficult to work out as it will be diluted by the liquid refrigerant.

This is my take on it anyway :D

26-01-2013, 04:54 PM
Hi Liam.
Monkey has a good point.
Basically the oil viscosity is measured at 40c with the ISO being the industry standard for those grades.

What you are asking is not a simple case of saying well therefore a 68 oil would be double or half the viscosity.

There are numerous things to concider, such as operating pressures temperature and what refrigerant is within the system.

The attached may help?


26-01-2013, 05:15 PM
Thanks for the help, much appreciated. I agree, some of the questions am getting are an absolute joke.

monkey spanners
26-01-2013, 06:35 PM
I once put some 3gs oil which is ISO32 in a small jar and left in the freezer overnight and it was like honey by the morning, didn't taste as nice though.

Still got to get answers right to past test so guess doesn't matter if stupid questions so long as you know the answer (and the correct answer!)

26-01-2013, 11:17 PM
Question is probably trying test your in-depth knowledge not just general knowledge. Viscosity at -20 would relate the pour point temp of an oil, which is to be considered in a refrigeration system for oil return, some oils freeze/ go gluggy other stay fluid at -20.