EN
Density Test
Density test of insulation oil: base oil type, degradation and contamination screening with standard methods and accredited measurement, with reports.
Density testing measures the mass per unit volume of insulation oil at a reference temperature (generally 15°C or 20°C). Although it looks like a plain physical parameter, oil density is a screening tool to investigate the base oil type, accumulation of oxidation products, solvent-contaminant mixing (e.g. cleaning residues) and, on ester based oils, water-alcohol interaction. New mineral insulation oils typically sit in the 0.85-0.90 kg/m³ band; deviation from the manufacturer value raises sample-mix or degradation suspicion.
Method and Measurement Discipline
- Measurement runs with a pycnometer (density flask) or digital density meter (oscillating U-tube); the result is reduced to the reference temperature with temperature compensation.
- Samples are drawn into clean dried vessels; air bubbles and oil homogeneity are checked before measurement.
- Comparison against past values (and, where available, the previous batch sample) matters more than a single reading; it is read with viscosity and refractive index.
- Post-filtration-drying changes (e.g. ester-water separation) shift density; therefore a repeat reading after maintenance also joins the programme.
Interpretation
A density deviation alone is not a fault diagnosis, but produces clues within the panel: rising density with increasing acidity points to oxidation build-up; a falling value can mark solvent or gas bubble. On ester oils, water presence and hydrolysis tendency are combined with moisture determination (conceptually with the Dissolved Gas/Water Analysis approach). Hence the test is interpreted within the Insulation Oil Test panel and together with Dielectric Strength results.
Legislation and Standards
The test is within the oil panel under the maintenance-test duty of Law No. 6331 and the Regulation on High Current Electrical Installations. Method references: density determination for mineral oil ASTM D4052 / IP 357 (oscillating U-tube), the pycnometric method and its TS EN transpositions, oil supervision IEC 60422 and acceptance-trend criteria per manufacturer specification with the IEC 61125 approach. Results are reported by the accredited laboratory and entered into the plant file.
Outcome
The report presents the measured value, reference temperature compensation, manufacturer band and comparison against previous trends together with the suitability/assessment note. Where a density drift appears without a matching change in acidity or moisture, the first suspicion is cross-contamination during oil delivery or a mixing error at the filling station; such cases are followed with a retest from a fresh sample point before any intervention on the transformer. Density testing is an oil panel item in our Transformer Tests and Analyses group; to build your item set and period plan reach our expert team via the contact page.
FAQ on Density Test
Density (and the related viscosity) is measured to verify the oil type, detect mixture-wrong oil-separation, read cooling-circulation behaviour and the degradation-contamination trend. The value is strongly temperature dependent; therefore the measurement is corrected to a reference temperature (generally 20 °C) and reported. While not confirming degradation alone, within the table it completes the chemical state of the oil when read with moisture-acidity-particles.
It is done temperature-controlled with a digital density meter (hydrometer-oscillator) or a viscosity bath; the result is given per the relevant ASTM/IEC oil test standards. Before measurement the sample is brought to a homogeneous-ambient-temperature state and air bubbles are removed. Tracking temperature-corrected density-viscosity on the same oil series early shows oxidation-sludge formation and wrong replenishment.
A deviation from the expected band can indicate wrong oil-replenishment, mixture, heavy oxidation-degradation or dirt-suspension. A concurrent change with moisture and acidity strengthens degradation. Viscosity rise in particular lowers cooling efficiency and raises the thermal hot spot. This finding forms an input for an oil-change-flushing decision and cooling system control.
When read with density-viscosity, breakdown strength, moisture, acidity, furan and DGA together, both the electrical and the chemical-thermal health of the oil emerges. For instance, if high viscosity-low breakdown-high moisture appear together, drying-filtration-cooling maintenance is suggested. The report gives temperature-corrected values with a limit table and enters them into the next year baseline; the slope, not a single measurement, is the basis.