How is viscosity measured?

[PaJohn]

I've seen numerous posts about oil being a "low 40 weight", etc and presume there is some chart that tells the ranges for each oil weight. Can someone point me to it and tell me how to read/measure oil with it. Thanks

 

[J]

quote:

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Originally posted by PaJohn: I've seen numerous posts about oil being a "low 40 weight", etc and presume there is some chart that tells the ranges for each oil weight. Can someone point me to it and tell me how to read/measure oil with it. Thanks

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Hi, You asked two questions. For relating cSt, cP, mPa-s, SAE and SUS viscosity measurements, refer to Bobistheoilguy home page. Viscosity is a measurement of resistance to flow. So, one natural way to measure viscosity is to make it flow through an opening. Measurement device used in this type of viscosity measurement is a timer such as a stop watch. Jae

 

[Patman]

Here is the guideline I go by in order to determine viscosity of an oil: 20wt oil is 5.6cst to 9.3 cst (at 100c) 30wt oil is 9.3 to 12.5 40wt oil is 12.5 to 16.3 I can't recall the range for 50 and 60wt oil though, someone else jump in here and help me out.

 

[Quattro Pete]

Go to http://www.bobistheoilguy.com/ and click on "Viscosity Chart", although I've seen different sources use slightly different thresholds.

 

[userfriendly]

Patman' SAE 50...16.3-21.9 SAE 60...21.9-26.1 A low side 50 would be around 17.0-18. Trivia 101: Airplane oils use the old SUS numbers which are about double the SAE grades. (SAE60=120 aero)

 

[labman]

Patman gave you at least part of what you want. Expanding on that, the Cst he give is centistokes. Most methods of measuring viscosity are based on gravity. A less dense material will have a higher viscosity measured in stokes or centistokes. Multiplying by the specific gravity gives poise or centipoise. Often viscosity is measured using a cup with a hole in the bottom, and timing how long it takes for the material to run out and reporting it in seconds. Naturally there are several different cups, Saybolt, Ford, Redwood, Engler, etc. Another common method is Gardener Holt. These use small glass tubes with lines etched on them. You time how long a bubble takes to rise from one line to another or compare them to standards. The time in seconds is the centistokes. That is too simple. The standards have letters of the alphabet. They ran out and had to go to Z, Z1, Z2, .... That is where the famous Z7 came from. Another method is the Brookfield viscometer. It measures the torque it takes to spin a disk in the fluid. It gives readings in poise. I am not sure where you would find a standard cup if you wanted to measure the viscosity yourself. A paint supply house might be the best place to start. I think I may have seen them as part of a spray gun outfit. Any viscosity is affected by temperature. Oil isn't too bad, but many materials are affected by shear, thinning out under shear. Oil is permanently degraded somewhat by very high shear, but has almost the same viscosity at different shear rates below what it takes to break it down. Another possible source of viscosity measuring equipment may be http://www.sciencelab.com/.

 

[PaJohn]

Ahhh, now I get it. I just find the Kinematic Viscosity @ 100°C for the specific oil and depending on where it falls on the chart makes it low or high in that grade. For example, Amsoil S2K 0W30 is 11.3@100c so it's about in the middle of the 30 grade. Do I have that right?

 

[Patman]

quote:

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Originally posted by PaJohn: Ahhh, now I get it. I just find the Kinematic Viscosity @ 100°C for the specific oil and depending on where it falls on the chart makes it low or high in that grade. For example, Amsoil S2K 0W30 is 11.3@100c so it's about in the middle of the 30 grade. Do I have that right?

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You got it!

 

[PaJohn]

And Mobil 1 0W30 is 10.3 so it's a low 30 weight. I get it! Thanks.

 

[Dr. T]

Guys...just to continue this thread....what do Gear oil grades mean and how do theycompare?? Why are they different from motor oil grades?