What is the value of 40°C cSt?

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May 25, 2005
CT, YL of USSA aka Oceania
I do not see the value of the 40°C (104°F) cSt viscosity data on a product/technical data sheet. The two cSt viscosities that are of any value are at a cold start between 0°C (32°F) and 5°C (41°F), and the extreme at water boiling 100°C (212°F); a third data value at operating temp around 85°C (185°F) could be useful.

An engine is at 40°C for a short time while warming up unless you are around the 100°+F hot areas in the summer such as Phoenix, the Middle East, or the desert. The next cold temperature viscosity data that some oil manufacturers provide are CCS or MRV at -35°C/-30°C depending on the oil grade that is ~130°F colder than 40°C cSt.

How can we [BITOG] lobby for a change to the standard tests used for a data sheet that will provide more valuable data?
Modern engines are at running temperature of 195-200 I see little valve in a measurement at 185. Often times I see the lowest pumping temperature. Not sure the 40 degree measurement is of much value either Draw a straight line between the two viscosity values to determine the vis at what ever temp you want between the dots. As you approach the pumping temp, the linearity fails I think.
Mo Data = Mo Betta
What's wrong with more info about a product?

BTW, 100C is NOT extreme by any means.
I would like to see specs for oils at more elevated temps.
A straight line extrapolated graph may not be representative of the actual product.
GMorg wrote: "no point along the viscosity curve"

To keep Mori at bay, I will have to correct myself to say that there is "no set of points along" or "no portion of" the viscosity curve that is linear.
Okay, but one would assume that the various visc calculators factor this in the less than simple background formulas that they use. That is, the two point method should be valid in figuring things at colder and hotter temps ..at least to some point where some other set of physics gets involved.
GMorg: If we are looking for laboratory level viscosity values then you are of course correct. However; if we look at the two data points generally given (0 & 100 C)the it appears to be linear for estimating the viscosity in a useful and real world application.
Yes, I do agree that 185°F is on the low end of coolant operating temps, but how many cars have oil temp gauges? That's where I referenced 212°F as an extreme temp where drivers become aware of high operating temps of their engines. Oil temps do exceed it since oil is being sprayed on the underside of pistons along with being at the frictions points through an engine.

If the spread between cSt values were greater than 108°F (60°C), then the linear approach to speculating cSt at other temps could be more accurate. Beyond the known endpoints of the line we are guessing with no accuracy of an unpredictable liquid.
The 40C is the ISO number for all industrial oils. It gives a good reference point (or common denominator) to compare a motor oil with a gear oil, hydraulic oil, compressor oil, etc.
SAE 10W = ISO 32 (because it is 32 cSt (+/- 10%) at 40C. It is the only common factor of relativity.

Also, with 40 and 100, you determine the VI and the actual viscosity curve. Any given application has it's optimum oil viscosity for the temperature it is running. Using the cSt at 40 and 100, plus the temperature of the gear box, hydraulic sistem, etc. you can see what the oil is at that temp, as in this calculator.

There are lots of reasons to run motor oils in other equipment, so the ISO number is valuable.

And for engines, with the exception of pour point depressors that can modify the very low end of the curve, the 2 points let you compare actual performance of the oils, as in this one:
Originally Posted By: GMorg
Eddie wrote: "Draw a straight line between the two viscosity values to determine the vis at what ever temp you want between the dots."

There is no point along the viscosity curve that is linear. The link below shows a curve for several ATMs from an older thread.

If you plot kinematic (low shear) viscosity at various temperatures on a linear/linear scale it will give a curve. If, however, you plot the viscosities on a log/linear scale you will get a straight line, the slope of which is the viscosity index. The straight line will begin to curve as you approach the pour point, but is straight between 40C and 100C and at much higher temperatures as well.

The standard VI plotting paper puts viscosity in a log scale on the Y axis and temperature in a linear scale on the X axis.

Tom NJ
I agree that it is log/linear, but the earlier implication was that a simple plot (without a log scale) would linear.
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