MANY times, we take our vehicles for servicing and are told to buy this or that kind of oil. Have you ever bothered to find out why you should use this or that oil? Viscous, actually, when you see 10W50 oil, the ‘10’ and the ‘50’ refer to viscosity of the oil at different temperatures.
But what do those numbers actually mean? Well, they’re called ‘Saybolt Seconds’ and are measured using a Viscometer. There are three different types of viscometer. First up we have the Redwood Viscometer, also known as the Standard British viscometer. Redwood Seconds refer to the number of seconds required for 50ml of the oil to flow out of the device at a predefined temperature.
The instrument is available in two sizes: Redwood type-I and type-II. When the flow time exceeds 2,000 sec, the type-II must be used. It is not recommend putting 10W2000 in your car.
In true VHS vs. Betamax fashion, the industry can never settle on one standard, so there’s also the Engler Viscometer. This measures in Engler Degrees, rather than Redwood Seconds, and is preferred by the rest of Europe. On the Engler Viscometer, the reading is the time (in seconds) required for 200ml of the oil to flow through the device at a predefined temperature. The conversion of Engler degrees to absolute units requires an appropriate table, a degree in rocket science and an intricate knowlege of fluid dynamics. Alternatively, an oil page written by a garage hack will do the trick. To wit: For liquids having a viscosity of 100 centistokes or more the Engler degree is roughly equal to 7.6 centistokes.
So for kinematic viscosity, the formula is: kinematic viscosity in centistokes (cSt) = Ct, where C is the calibration constant of the particular viscometer and t is the observed time of flow. The value computed by this formula is reported in centistokes, units of kinematic viscosity.
Finally, in the category of “America” = “The World”, there’s what is affectionately referred to as Saybolt Viscosity seconds - the term I used at the top of the page. For the Saybolt Viscometer, the amount of oil to be measured is 60ml. There are two types of Saybolt Viscometer, as with the Redwood system. Type-I is called the Furol Viscometer, Type-II is called the Universal Viscometer. “Furol” is a made-up word based on “Fuel and Road Oil” – i.e. That’s what’s used to test the oil you put in your car. The Universal Viscometer is used for other industrial lubricants and oils, and has largely been superseded by kinematic viscosity methods - those performed using the type-I system. You’ll notice for all of the above, they measure time for a predetermined temperature. Under these classifications, the winter grades of 5W, 10W and 20W are determined by the oils’ viscosity at 0° Fahrenheit (-18°C), while grades 20,30, 40 and 50 are determined by its viscosity at 212° Fahrenheit (100°C). Those are the predetermined temperatures.
Saybolt viscosities are reported as the number of elapsed seconds indicated by the timer. For Saybolt Universal viscosities, the units are Saybolt Seconds Universal (SSU), and for Saybolt Furol viscosities, the units are Saybolt Seconds Furol (SSF). For a given oil, the Saybolt Universal value will run about 10 times as high as the Saybolt Furol value at the same temperature. Redwood and Engler viscosities are also based on the time of flow and are reported as “Redwood seconds” or “Engler degrees,” as the case may be. In all instances, the test temperature is reported along with the corresponding viscosity. In case you’re at all inclined, here’s some sample values for common oil ratings. 7.4 (To be continued)