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D2140-24 – Standard Test Method Technical Guide
🧪 Overview and Scope of D2140‑24
ASTM D2140‑24 is a standard practice for calculating the carbon‑type composition of mineral insulating oils of petroleum origin. The method correlates basic physical properties—specifically the Viscosity‑Gravity Constant (VGC) and the Refractivity Intercept (ri)—to the carbon distribution in the oil without requiring complex chemical separations.
The composition is expressed as the percentage of aromatic carbons (% CA), naphthenic carbons (% CN), and paraffinic carbons (% CP). These values are obtained from the correlation chart (Fig. 1 of the standard) once VGC and ri are known. The practice is applicable to oils with average molecular weights from 200 to above 600 and containing 0 % to 50 % aromatic carbons.
💡 Technical Note: By relying on standard physical tests for viscosity, density, and refractive index, this practice eliminates the need for complex fractional separation and purification procedures for routine analytical characterization of new insulating oils.
📏 Experimental Data Requirements and Calculated Parameters
The practice requires three fundamental experimental measurements: kinematic viscosity, density (or relative density/specific gravity), and refractive index. From these data, the Viscosity‑Gravity Constant (VGC) per Test Method D2501 and the Refractivity Intercept are calculated.
| 📐 Required Input Property | 🧪 Designated Test Method(s) | 📝 Measurement Details |
|---|---|---|
| Kinematic Viscosity | D445 / D7042 | Determined at the required reference temperature |
| Density / Relative Density | D1298 / D4052 / D1481 | Hydrometer method or digital density meter |
| Refractive Index | D1218 | Measured at 20 °C for hydrocarbon liquids |
⚠️ Important Limitation: This practice is primarily intended for new mineral insulating oils, either inhibited or uninhibited. It is not designed for used oils without validation of the correlation.
| ⚙️ Calculated Parameter | 🔬 Calculation Basis / Range |
|---|---|
| Viscosity‑Gravity Constant (VGC) | Calculated from density and viscosity per D2501 |
| Refractivity Intercept (ri) | ri = nD – (ρ/2), where nD is refractive index and ρ is density |
| % Aromatic Carbons (CA) | 0 % to 50 % |
| % Naphthenic Carbons (CN) | Determined from the correlation chart (Fig. 1) |
| % Paraffinic Carbons (CP) | Determined from the correlation chart (Fig. 1) |
⚙️ Application and Significance in Insulating Oil Analysis
D2140‑24 is widely used in the electrical power industry for the characterization of transformer oils and cable impregnates. The carbon‑type composition provides critical insight into the oil’s oxidation stability, gas‑absorbing characteristics, and overall chemical behavior under electrical and thermal stress. All values stated in the standard are regarded as SI units.
This standardized practice ensures consistent reporting of oil composition between laboratories, aiding in specification, quality control, and comparative analysis of mineral insulating oils.
❓ Frequently Asked Questions
🔍 What is the primary output of D2140‑24?
The practice provides the percentage of aromatic (% CA), naphthenic (% CN), and paraffinic (% CP) carbon atoms present in a mineral insulating oil.
💡 How do I calculate the Refractivity Intercept (ri)?
The Refractivity Intercept is calculated using the formula ri = nD – (ρ/2), where nD is the refractive index at 20 °C and ρ is the density at 20 °C.
⚡ What tests are required to implement this standard practice?
Kinematic Viscosity (D445 or D7042), Density or Relative Density (D1298 or D4052), and Refractive Index (D1218) are the three required experimental measurements.
📌 Is this standard applicable to all types of petroleum oils?
It is specifically designed for mineral insulating oils used in electrical equipment such as transformers and cables. It is applicable to oils with molecular weights from 200 to above 600 and 0 % to 50 % aromatic carbons.