Type a line (SUS 200 at 100 °F to cSt) or fill in the fields: the line and the fields stay in step.
Enter a viscosity as an older fuel or lube specification or an instrument plate gives it, in Saybolt Universal seconds (SUS), cSt (mm2/s) or cP with a density, and the converter gives it in the other units at the same temperature. It shows the working with your numbers, draws the value on two scales, and gives a line you can copy or add to a request.
SUS follow the ASTM D2161 equation with its temperature factor. Redwood No. 1 and No. 2 seconds, Engler degrees and Saybolt Furol seconds are read and not converted, because the published relations for them disagree, and the converter says so instead of giving a figure. No density is assumed for cP, and below about 10 cSt the second scales are approximate.
ASTM D2161 gives SUS at 100 °F from the kinematic viscosity v in cSt: SUS = 4.6324 v + (1 + 0.03264 v) / (3930.2 + 262.7 v + 23.97 v^2 + 1.646 v^3) x 100000. 24.85 cSt gives 118.7 SUS. At another temperature the figure is multiplied by 1 + 0.000061 (t - 100), t in °F. From SUS to cSt the same equation is solved for v: 200 SUS at 100 °F is 42.90 cSt.
Redwood No. 1 seconds: two relations are in use, v = 0.247 t - 50 / t and v = 0.26 t - 179 / t, and from 200 s up they differ by 4 to 5 %: 1000 s is about 247 cSt by one and about 260 cSt by the other. The published relations and tables for Redwood No. 2 seconds, Engler degrees and Saybolt Furol seconds disagree in the same way. The converter names these units and asks for the cSt figure instead of picking one of two numbers.
The dynamic viscosity is the kinematic viscosity times the density, both at the same temperature: cP = cSt x density in g/cm3. 1000 cP at 0.98 g/cm3 is 1020.4 cSt. The converter never assumes a density.
| Quantity | Relation | Note |
|---|---|---|
| SUS at 100 °F | SUS = 4.6324 v + (1 + 0.03264 v) / (3930.2 + 262.7 v + 23.97 v^2 + 1.646 v^3) x 100000 | ASTM D2161; v in cSt; from 1.81 cSt (32 SUS) |
| SUS at t °F | SUS(t) = SUS(100 °F) x (1 + 0.000061 (t - 100)) | the same kinematic viscosity at t |
| SUS to cSt | the same equation solved for v | by bisection |
| Dynamic viscosity | cP = cSt x density | density in g/cm3 at the same temperature; 1 cP = 1 mPa·s |
| Not converted | Redwood No. 1 and No. 2 seconds, Engler degrees, Saybolt Furol seconds | the published relations disagree |
| Units | 1 cSt = 1 mm2/s; 1 St = 100 cSt; 1 Pa·s = 1000 cP | kg/m3 / 1000 = g/cm3 |
The conversion is at one temperature. SUS at 100 °F unless a temperature is given. Below about 10 cSt the second scales are approximate.
The SUS equation, its lower limit of 1.81 cSt (32 SUS), its temperature factor and the worked example 24.85 cSt = 118.7 SUS are those of ASTM D2161, as independent engineering references give them, and they agree with published cSt to SUS tables at 100 °F and 210 °F. For Redwood No. 1 seconds two relations are in use from 100 s, one in an engineering knowledge-base article and the other in an online unit converter; one published conversion table follows the first and another the second, 4 to 5 % apart, so Redwood No. 1 is not converted. For Engler degrees the Ubbelohde relation and the published conversion tables differ by about 4 %, for Saybolt Furol seconds the ASTM D2161 equation and a published table by about 3 to 4 %, and for Redwood No. 2 seconds a relation and a table by about 2.4 %, so these units are not converted either. That the second scales are approximate at low viscosity, and that cP = cSt x density at the same temperature, is stated in engineering references on viscosity and pumping.
Because two relations are in use and they disagree. From 100 s, v = 0.247 t - 50 / t and v = 0.26 t - 179 / t (v in cSt, t in Redwood No. 1 seconds) both give 24.2 cSt for 100 s, but 1000 s is about 247 cSt by the first and about 260 cSt by the second: 4 to 5 % apart from 200 s up, and the published conversion tables follow one or the other. Below 100 s the relations differ further. The converter reads Redwood No. 1 and No. 2 seconds, names them and asks for the kinematic viscosity in cSt with its temperature instead.
By the ASTM D2161 equation at 100 °F: SUS = 4.6324 v + (1 + 0.03264 v) / (3930.2 + 262.7 v + 23.97 v^2 + 1.646 v^3) x 100000, with v in cSt. 24.85 cSt gives 118.7 SUS, and 10 cSt gives 58.8 SUS. The equation holds from 1.81 cSt (32 SUS) up. From SUS to cSt the converter solves the same equation for v: 200 SUS at 100 °F is 42.90 cSt.
Because the Saybolt equivalent of one kinematic viscosity changes with the temperature of the test. ASTM D2161 gives SUS at 100 °F and multiplies it by 1 + 0.000061 (t - 100) at another temperature t in °F: at 210 °F the factor is 1.0067, so 10 cSt is 58.8 SUS at 100 °F and 59.2 SUS at 210 °F. Without a temperature the converter gives SUS at 100 °F and says so. It changes the units at one temperature, not the temperature.
For the same reason: the published relations disagree. The Ubbelohde relation, v = 7.31 E - 6.31 / E, gives figures about 4 % away from the published Engler conversion tables, and the ASTM D2161 Furol equation at 122 °F and a published Furol table differ by about 3 to 4 % above 100 cSt; for Redwood No. 2 (Admiralty) seconds a relation and a table differ by about 2.4 %. One figure would not be honest, so the converter names these units and asks for the kinematic viscosity in cSt instead.
The dynamic viscosity in cP (mPa·s) is the kinematic viscosity in cSt (mm2/s) times the density in g/cm3, both at the same temperature: cP = cSt x density. 1000 cP at a density of 0.98 g/cm3 is 1000 / 0.98 = 1020.4 cSt. The converter never assumes a density: without one it asks for it, in g/cm3 or kg/m3.
Less certain: below about 10 cSt the conversion of the empirical second scales is approximate, and the converter says so with the result. The calculator takes viscosities up to 100 000 cSt, temperatures from -20 to 150 °C and densities from 0.5 to 2 g/cm3; SUS from 1.81 cSt (32 SUS) up.
The converter gives the viscosity in the units the documents use. When parts for a fuel or lube oil system are needed, send the line from the converter with the equipment details and a photograph of the nameplate, and Vessel Core checks the details before an offer goes back.
