The Redwood Viscometer
Viscosity measured by how long the oil takes to run out.
Origin
Devised by Sir Boverton Redwood, consulting chemist to the British petroleum industry, and the British standard for oil viscosity from the 1880s onward.
The catch
The unit is a second. Redwood seconds are not a physical property but a time: how long a fixed volume of oil takes to flow through a standard orifice at a stated temperature. Change the orifice and every number changes, which is why the instrument had to be standardised before the scale meant anything.
| Fluid | Redwood seconds at 70°F | Use |
|---|---|---|
| Water | 26 | The practical floor of the scale |
| Paraffin | 30 | Lamp oil and heating |
| Light machine oil | 100 | Sewing machines, small mechanisms |
| Engine oil, light | 250 | Motor lubrication |
| Engine oil, heavy | 1,000 | Industrial machinery |
| Cylinder oil | 5,000 | Steam engine cylinders |
| Bitumen, softened | over 20,000 | Road surfacing and waterproofing |
Redwood was the dominant figure in British petroleum chemistry, advising the Admiralty on fuel oil and helping to frame the legislation governing petroleum storage. His viscometer became the British standard while Saybolt served the United States and Engler served Germany, so an oil cargo crossing borders arrived with a number nobody at the far end could use directly. Conversion tables between the three were a standard fixture of engineering handbooks well into the twentieth century.