Viscosity of Transformer and Process Oils

Severely hydrotreated oils can serve as base oils, insulating (transformer) oils, or process oils.


Severely hydrotreated oils can serve a variety of purpose:

  • Nytro 4000X is an insulating oil
  • T110 is a base oil
  • Nypar 315 and Nyflex 3150 are process oils

The given samples were provided by courtesy of Nynas AB, Sweden.

  • Insulating oils play an important role in power supply. They are applied in transformers as electric insulators and coolants.
  • Base oils - true to their name - are the basis for a variety of products. With different additives, base oils become formulated engine oils, gear oils or also insulating oils and heat transfer oils.
  • Process oils are also used in several fields. For example as plasticizers in the rubber and polymer industry (for automotive tires, sealants, or footwear), as textile auxiliary formulations in the production process of yarns, and - due to their very low aromatic hydrocarbon content - as component of cosmetics.

Hydrotreating minimizes undesirable components such as sulfur, nitrogen and aromatics. The process shall yield products, which are in accordance with standardized specifications. The carbon type distribution as quality parameter for oils allows to split the total composition of the heavier fractions of petroleum into paraffinic, naphthenic and aromatic composition. Oils with specific carbon type distribution are designed for various industries.

To know the carbon type analysis is important to improve product properties, the process efficiency and reliability. Standard ASTM D3238 defines how to calculate the carbon distribution and ring content of olefin-free petroleum oils from measurement of refractive index, density and molecular weight (by n-d-M method). ASTM D2502 describes how to obtain the mean molecular weight from viscosity measurements at 37.78 °C and 98.89 °C (100 °F and 210 °F). Kinematic viscosity values at 40 °C and 100 °C are extrapolated from the measured kinematic viscosities according to ASTM D341.

As an alternative method, carbon type composition is analyzed conforming to ASTM D2140. Further, density values at 15 °C are extrapolated according to API (American Petroleum Institute) functions, using product group 'D' - Lube oils. The Viscosity-Gravity Constant (VGC) is calculated following ASTM D2501.

Viscosity Tables – Measurement data

ASTM D3238: Carbon type distribution of severely hydrotreated oils

Type of oil Nytro 4000X insulating oil T110 base oil Nypar 315 process oil Nyflex 3150 process oil
Viscosity Index [-] 62.928 32.26 104.873 71.5
Kin.Visc. (D341) 40°C [mm²/s] 9.1459 107.99 20.174 142.85
Kin.Visc. (D341) 100°C [mm²/s] 2.372 9.0764 4.1279 12.452
API Density 15°C [kg/m³] 869.5 914.8 851.3 898.9
Refractive Index [nD] 1.4746 1.5035 1.4681 1.4949
D2502 Mean Molec. Mass [g/mol] 273.01 399.59 371.59 494.49
D3238 CA [%] 2.35 13.8 0.2 9.63
D3238 CN [%] 47.3 34.73 30.55 29.03
D3238 CP [%] 50.4 51.4 69.23 61.3
D3238 RT [-] 1.96 3 1.58 2.95
D3238 RA [-] 0.08 0.68 0.01 0.58
D3238 RN [-] 1.88 2.32 1.57 2.37

ASTM D2140: Carbon type composition of severely hydrotreated oils

Type of oil Nytro 4000X insulating oil T110 base oil Nypar 315 process oil Nyflex 3150 process oil
Kin.Visc. 37.78 [mm²/s] 9.8283 124.23 21.963 163.46
API Density 15°C [kg/m³] 869.50 914.83 851.30 898.93
Refractive Index [nD] 1.4746 1.5035 1.4681 1.4949
D2501 VGC [-] 0.8403 0.8551 0.8001 0.8285
D2140 CA [%] 2.00 14.40 1.13 9.25
D2140 CN [%] 46.25 33.23 30.43 29.93
D2140 CP [%] 51.70 52.35 68.43 60.88


Measured with SVM™ 4001 in combination with refractometer Abbemat 500.


Reference Measurement with SVM™ 4001 + Abbemat 500 refractometer
kin. Vis. yes
dyn. Vis. no
density yes
multiple temperatures no
Related carbon type, base oil, insulating oil, transformer oil, process oil, hydrotreating, ASTM D2140, ASTM D3238, ASTM D2501, ASTM D2501