⛽ Viscosity index (VI) of oils
Calculation of viscosity index (VI) of lubricating oils based on measured kinematic viscosity values at 40°C and 100°C. Measure of viscosity stability as function of temperature.
Overview
Viscosity Index (VI) is a dimensionless number defining the change in oil viscosity as a function of temperature. The higher the VI, the smaller the viscosity change with temperature change — the oil maintains more stable lubricating properties at both low and high temperatures. This parameter has fundamental significance in selection of lubricating oils for applications operating over a wide temperature range.
The VI calculation method was developed by Dean and Davis in 1929 and is based on comparing the test oil viscosity with viscosities of two reference oil series — with low (VI = 0, L series) and high (VI = 100, H series) viscosity index — having the same viscosity at 100°C. ISO 2909 standard defines two procedures: A (for VI ≤ 100) and B (for VI > 100), with different calculation formulas.
Typical VI values: naphthenic oils (group I) — 80–100, refined paraffinic oils (group II) — 95–110, hydrocracked oils (group III) — 120–140, PAO (group IV) — 130–160, synthetic esters (group V) — 130–170. Modern multigrade engine oils (e.g., 5W-30) achieve VI > 150 thanks to polymeric viscosity index improvers (VII — Viscosity Index Improvers).
Viscosity index is a calculated parameter — it does not require a separate test, but results from kinematic viscosity measurements at 40°C and 100°C according to PN-EN ISO 3104 (ASTM D445). The laboratory calculates VI based on tables included in the standard or using mathematical formulas.
Method principle
Viscosity index is calculated from two measured kinematic viscosity values: v40 (at 40°C) and v100 (at 100°C).
Procedure A (VI ≤ 100): VI = [(L - U) / (L - H)] × 100, where L = viscosity at 40°C of oil with VI = 0 and the same v100, H = viscosity at 40°C of oil with VI = 100 and the same v100, U = v40 of test oil. Values L and H are read from standard tables.
Procedure B (VI > 100): VI = [(antilog N) - 1] / 0.00715 + 100, where N = (log H - log U) / log v100. This formula extends the VI scale above 100 without an upper limit.
Applications
- Classification of base oils (API groups I–V)
- Selection of lubricating oils for multigrade applications (engine, gear)
- Quality control of oils in production — compliance with TDS cards
- Evaluation of viscosity modifier (VII) effectiveness in formulations
- Comparison of synthetic and mineral oils
- Testing thermal stability of oils after aging (VI change)
- OEM specifications (e.g., VW, MB, BMW require VI > specified values)
Key parameters
| Parameter | Value |
|---|---|
| Input data | Kinematic viscosity at 40°C and 100°C [mm²/s] |
| Range v100 | 2–70 mm²/s (tables), above 70 — formulas |
| Mineral oil (gr. I) | VI = 80–100 |
| Synthetic oil (PAO) | VI = 130–160 |
| Engine oil 5W-30 | VI > 150 (typically 160–170) |
| Viscosity measurement method | PN-EN ISO 3104 (ASTM D445) |
Standard
- Standard number
- PN-EN ISO 2909:2002
- Title (PL)
- Przetwory naftowe — Obliczanie indeksu lepkosci na podstawie lepkosci kinematycznej
- Title (EN)
- Petroleum products — Calculation of viscosity index from kinematic viscosity
Step-by-step procedure
1. Viscosity measurement at 40°C
Measure kinematic viscosity of oil sample at 40 ± 0.02°C according to PN-EN ISO 3104. Perform min. 2 determinations, check repeatability.
2. Viscosity measurement at 100°C
Measure kinematic viscosity of same sample at 100 ± 0.02°C. Use appropriate capillary (usually smaller diameter than for 40°C).
3. Applicability range check
Ensure v100 is in range 2–70 mm²/s (tables). For v100 > 70 mm²/s use formulas given in standard.
4. Reading L and H values from tables
Based on v100, read from standard tables the values L (VI=0) and H (VI=100) corresponding to viscosity at 40°C of reference oils.
5. VI calculation — procedure A
If v40 ≥ H: VI = [(L - U) / (L - H)] × 100, where U = measured v40. Round result to integer.
6. VI calculation — procedure B
If v40 < H (VI > 100): N = (log H - log U) / log v100, VI = [(10^N - 1) / 0.00715] + 100.
7. Result verification
Compare calculated VI with expected value for given oil type. Large deviations may indicate viscosity measurement error.
8. Reporting
Report VI result as integer (without unit). Include v40 and v100 values and calculation method (A or B).
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Capillary viscometer (Ubbelohde) | SI Analytics / Schott-Geräte, Cannon-Fenske, sizes 50–600 | 300–1,500 PLN / pc |
| Thermostatic bath 40°C | Julabo VISCO Bath, Tamson TV 4000, Lauda Proline PV | 10,000–30,000 PLN |
| Thermostatic bath 100°C | Julabo, Tamson, Lauda — oil or silicone bath up to 100°C | 12,000–35,000 PLN |
| Stopwatch / automatic timer | Digital stopwatch or automatic flow time measurement | 100–5,000 PLN |
| Reference thermometers | Calibrated Pt100 thermometers, accuracy ±0.01°C (40°C) and ±0.02°C (100°C) | 500–3,000 PLN |
| Automatic viscometer | Anton Paar SVM 3001 (viscosity + density automatically) | 80,000–150,000 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Viscosity reference oil (CRM) | — | Certified viscosity standard e.g., Cannon, Paragon Scientific, for calibration |
| White spirit | 8032-32-4 | For rinsing viscometer capillaries before and after measurement |
| Acetone | 67-64-1 | For final capillary rinsing and drying |
| Silicone oil for bath | 63148-62-9 | Thermostating medium for 100°C bath (instead of water) |
Health and safety (OHS)
- Oil bath 100°C — risk of burns, use thermal gloves
- White spirit — flammable, work under fume hood
- Glass capillaries — fragile, risk of cuts during assembly
- Oil samples — avoid prolonged skin contact, use gloves