# Viscosity

### Viscosity

More Details on on Viscosity are to be found in the links below the table

Viscosity

A plate of area "A" moving under the action of a force "F" with a resulting velocity "V" over a fixed plate separated by a film of fluid "h" thick.   The shear stress on the fluid is "F / A".    For a Newtonian fluid the shear stress is directly proportional to the velocity gradient in the fluid- "dV /dh" .

P / A = m . dV /dy

The proportionality constant " m " is the viscosity and has units of ( N/m 2) .( m ). ( m / s ) = N . s / m 2...

Dynamic Viscosity

The coherent SI unit of dynamic viscosity is the pascal second (Pa s).   This equates to a Newton second per square metre (N.s / m 2 ).  In the cgs unit the poise is the corresponding unit which has dimensions of dyne seconds per cm 2.   The centipoise (cP = 10 -2 poise) is the most common unit used in tables and handbooks.... 1 Pas = 1000. cP.   The viscosity of water at room temperature is approximately 1 cP

Kinematic Viscosity

This is the ratio of the dynamic viscosity and the fluid density r ( kg /cubic m )  The SI unit of kinematic viscosity is the m 2 / s.   The equivalent cgs unit is the stoke (St) which has the unit of cm 2 / s.  The unit commonly used is the centistoke(cSt)....... 1 m 2 / s = 10 6 cSt

Absolute Kinematic Viscosity (m 2 / s ) = m ( Pa s ) / r (kg / m3 )

Kinematic viscosity (centiStokes) = m (centiPoise) / r ( grammes /cubic cm )
For water with a density of about 1000kg/m3 the conversion is simply 1 cST = 1 cP

Viscosity conversion table Viscosity conversion table

Saybolt Universal Viscosity

A method of determining a lubrication fluids viscosity is to measure the rate of flow of the fluid through a test device.  The rate is measured in seconds the greater the number of seconds the more viscous the fluid.  This viscosity is seconds t = Saybolt universal Viscosity - seconds. The dynamic viscosity relates to the Saybolt universal viscosity (t) by the following formula

Viscosity m (Poise) = (0.00022 .t — 0.18 / t )

SAE (Society of automobile engineers ) has allocated numbers for specifying the viscosity of Engine Oils..

The SAE grades 0W through 25W, where W stands for Winter, have a maximum viscosity specified at low temperatures (—5 through —35C), to ensure easy starting under low temperature conditions, and a minimum viscosity requirement at 100C to ensure satisfactory lubrication at the final operating temperature.   The SAE grades 20 through 60 only have limits set at 100C as these grades are not intended for use under low temperature conditions.

For marine applications, monograde oils of SAE 30 or SAE 40 are used because of the steady operating conditions in a ship's engine room.

On the other hand, automotive oils are normally formulated with Viscosity Index Improvers (VI Improvers) to provide multigrade performance.    VI Improvers are very large molecules, which are chemically made by linking together smaller molecules in a process called polymerization.   The resulting products, called polymers, may have molecular weights 1000 times or more greater than the base stock molecules.    The use of these special polymers makes it possible to meet both the low temperature viscosity requirements of the W grades as well as the high temperature requirements of the non-W grades.  Thus a SAE 20W 40 has the low temperature viscosity value of a SAE 20W oil at low temperatures and the high temperature viscosity of a SAE 40 oil at the higher temperature.

Different tests are completed to establish the viscosities for different operating regimes.   Tests are completed for low temperature

• Pumping in engines starting at low temperature;
• Lubricating the crankshaft bearings at low temperature
• Lubricating the engine at normal operating temperatures
 SAE Viscosity Grade Viscosity (cP)* at Temperature (oC), Max Viscosity mm2/s (cSt) at 100o C Cranking Pumpability Min Max 0W 3250 at—30 30,000 at —35 3.8 — 5W 3500 at —25 30,000 at— 30 3.8 — 10W 3500 at —20 30,000 at —25 4.1 — 15W 3500 at —15 30,000 at —20 5.6 -- 20W 4500 at —10 30,000 at —15 5.6 -- 25W 6000 at —5 30,000 at —10 9.3 -- 20 — — 5.6 9.3 30 — — 9.3 12.5 40 — — 12.5 16.3 50 — — 16.3 21.9 60 — — 21.9 26.1

The ISO 3448 grading system for Industrial Engine Oils simply relates to the midpoint viscosity of oil in cStokes ( at 40odeg ) as shown in the table below.

 Viscosity Grade Midpoint Viscosity at 40 o C Kinematic Viscosity Limits( cStoke ) Min Max IS VG 2 2,2 1,98 2,42 IS VG 3 3,2 2,88 3,52 IS VG 5 4,6 4,14 5,06 IS VG 7 6,8 6,12 7,48 IS VG 10 10 9,00 11,00 IS VG 15 15 13,5 16,5 IS VG 22 22 19,8 24,2 IS VG 32 32 28,8 35,2 IS VG 46 46 41,4 50,6 IS VG 68 68 61,2 74,8 IS VG 100 100 90,0 110 IS VG 150 150 135 165 IS VG 220 220 198 242 IS VG 320 320 288 352 IS VG 460 460 414 506 IS VG 680 680 612 748 IS VG 1000 1000 900 1100 IS VG 1500 1500 1300 1650

Viscosity conversion Chart

Kinematic viscosity (cST) = Dynamic Viscosity (cP)) / [1000.Density (kg/m3) ]

Table below is based on fluid with a sg = 1 (1000kg/m3)

 Centipoise (CPS) Poise Centistokes Stokes Saybolt Seconds Universal Millipascal seconds (mPas) (P) (cSt) (S) (SSU) 1 0.01 1 0.01 31 2 0.02 2 0.02 34 4 0.04 4 0.04 38 7 0.07 7 0.07 47 10 0.1 10 0.1 60 15 0.15 15 0.15 80 20 0.2 20 0.2 100 25 0.24 25 0.24 130 30 0.3 30 0.3 160 40 0.4 40 0.4 210 50 0.5 50 0.5 260 60 0.6 60 0.6 320 70 0.7 70 0.7 370 80 0.8 80 0.8 430 90 0.9 90 0.9 480 100 1 100 1 530 120 1.2 120 1.2 580 140 1.4 140 1.4 690 160 1.6 160 1.6 790 180 1.8 180 1.8 900 200 2 200 2 1000 220 2.2 220 2.2 1100 240 2.4 240 2.4 1200 260 2.6 260 2.6 1280 280 2.8 280 2.8 1380 300 3 300 3 1475 320 3.2 320 3.2 1530 340 3.4 340 3.4 1630 360 3.6 360 3.6 1730 380 3.8 380 3.8 1850 400 4 400 4 1950 420 4.2 420 4.2 2050 440 4.4 440 4.4 2160 460 4.6 460 4.6 2270 480 4.8 480 4.8 2380 500 5 500 5 2480 550 5.5 550 5.5 2660 600 6 600 6 2900 700 7 700 7 3380 800 8 800 8 3880 900 9 900 9 4300 1000 10 1000 10 4600 1100 11 1100 11 5200
 Centipoise (CPS) Poise Centistokes Stokes Saybolt Seconds Universal Millipascal seconds (mPas) (P) (cSt) (S) (SSU) 1200 12 1200 12 5620 1300 13 1300 13 6100 1400 14 1400 14 6480 1500 15 1500 15 7000 1600 16 1600 16 7500 1700 17 1700 17 8000 1800 18 1800 18 8500 1900 19 1900 19 9000 2000 20 2000 20 9400 2100 21 2100 21 9850 2200 22 2200 22 10300 2300 23 2300 23 10750 2400 24 2400 24 11200 2500 25 2500 25 11600 3000 30 3000 30 14500 3500 35 3500 35 16500 4000 40 4000 40 18500 4500 45 4500 45 21000 5000 50 5000 50 23500 5500 55 5500 55 26000 6000 60 6000 60 28000 6500 65 6500 65 30000 7000 70 7000 70 32500 7500 75 7500 75 35000 8000 80 8000 80 37000 8500 85 8500 85 39500 9000 90 9000 90 41080 9500 95 9500 95 43000 15000 150 15000 150 69400 20000 200 20000 200 92500 30000 300 30000 300 138500 40000 400 40000 400 185000 50000 500 50000 500 231000 60000 600 60000 600 277500 70000 700 70000 700 323500 80000 800 80000 800 370000 90000 900 90000 900 415500 100000 1000 100000 1000 462000 125000 1250 125000 1250 578000 150000 1500 150000 1500 694000 175000 1750 175000 1750 810000 200000 2000 200000 2000 925000