GB/T 11142-2025 Determination of gassing tendency of insulating oils under electric field and ionization

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1. Method number / full name / English name / replacement relationship

• Standard number: GB/T 11142—2025
• Chinese name: Determination of gassing tendency of insulating oils under electric field and ionization
• English name: Determination for gassing properties of insulating oil under electrical stress and ionization
• Replacement relationship: Replaces GB/T 11142—1989 "Determination of gassing tendency of insulating oils under electric field and ionization"; this document was first published in 1989, and this is the first revision.
• Custodianship: National Technical Committee for Standardization of Petroleum Products and Lubricants (SAC/TC 280)
• Publication/implementation: published by 2025-10-05, implemented by 2026-05-01
• Main technical changes compared with the 1989 edition (foreword):
  • Changed the scope (see Chapter 1, Chapter 1 of the 1989 edition);
  • Changed the specification requirements for the gas burette (see 4.1.4, 4.1.1.3 of the 1989 edition);
  • Added temperature control accuracy requirements for the heating device (see 4.2, 4.1.2 of the 1989 edition);
  • Changed the specification requirements for the syringe (see 4.6, 4.1.6 of the 1989 edition);
  • Changed the cleaning solvent (see 5, 6.1, 6.2, Chapter 5 of the 1989 edition, 6.1, 6.2);
  • Added sampling requirements (see 7);
  • Changed the hydrogen flow rate (see 8.4, 7.4 of the 1989 edition);
  • Changed the parallelism check and calculation method for gassing tendency (see 8.10, Chapter 9, Chapter 8 of the 1989 edition);
  • Changed the precision requirements for gassing tendency, and added deviation statistics for test results (see Chapter 11, Chapter 9 of the 1989 edition).

2. Scope of application and non-applicability

• This document describes a method for determining the ability of insulating oil to release or absorb gas in a gassing cell of specific geometry under the action of an electric field (or ionization) of sufficient strength to cause discharge at the oil-gas interface.
• Applicable to testing the gassing tendency of mineral insulating oils, natural ester insulating oils, synthetic ester insulating oils, and silicone-based insulating oils.
• Not applicable: not provided in the available source.

3. Method principle

• After saturating the insulating oil with high-purity hydrogen, the oil and the hydrogen layer above the oil are subjected to a voltage of 10 kV, a frequency of 50 Hz, an oil temperature of 80 ℃, an electrode gap of 3 mm, and continuous ionization for 60 min; under the action of the radial electric field, the oil-hydrogen interface undergoes discharge reactions, causing the oil itself to absorb or release gas. Its gassing tendency is expressed as the volume of gas absorbed or released by the test specimen per unit time. (Introduction)
• Definition of gassing tendency (3.1): the ability of insulating oil itself to absorb or release gas under the action of an electric field or ionization whose strength is sufficient to cause discharge at the oil-gas interface.

4. Reagents and materials (purity / preparation)

• 5.1 Dibutyl phthalate: chemically pure.
• 5.2 Isopropanol: chemically pure.
• 5.3 n-Heptane: analytically pure.
• 5.4 Vacuum silicone grease.
• 5.5 High-purity hydrogen: ≥99.999% (volume fraction).
• 5.6 Soft silk cloth: clean, dust-free, antistatic.
• 5.7 Qualitative filter paper.
• Annex B additionally involves: distilled water, n-heptane (used for determining the gas burette constant K).
• Preparation method: not provided in the available source (apart from the gas burette constant determination procedure in Annex B, there are no other solution preparation instructions).

5. Instruments and equipment

• 4.1 Gassing apparatus
  • 4.1.1 Composed of a gassing cell, a gas burette, a hollow high-voltage electrode, etc.; the structural schematic is shown in Figure 1.
  • 4.1.2 Gassing cell: made of hard borosilicate glass; the part subjected to the electric field is a thin-walled glass tube with an inner diameter of 16 mm±0.2 mm and an outer diameter of 18 mm±0.2 mm (dimensions see Figure 2); the exterior is coated with solvent-resistant silver paint as the outer electrode (grounded), with a grounded copper conductor; the outer electrode shall have a vertical slit to observe the reaction at the oil surface.
  • 4.1.3 Hollow high-voltage electrode: machined from a stainless steel rod conforming to steel grade 06Cr19Ni10 in GB/T 1220—2007, with a polished surface; outer diameter 10 mm±0.1 mm (dimensions see Figure 2); a stainless steel capillary with an outer diameter of 2 mm and an inner diameter of 1 mm is inserted inside as the hydrogen passage; a precisely machined 24/29 conical polytetrafluoroethylene stopper with a central hole supports the electrode so that it is aligned with the center of the gassing cell; after repeated use, the sealing performance of the conical polytetrafluoroethylene stopper shall be checked; a metal three-way stopcock is installed on the hydrogen inlet tube at the top of the hollow high-voltage electrode.
  • 4.1.4 Gas burette: a borosilicate glass U-shaped tube with an outer diameter of 7 mm and graduations (unit: mm), connected to the gassing cell by a 14/25 conical glass ground joint, and also provided with a bypass stopcock and three glass bulbs, see Figure 1.
• 4.2 Heating device: a glass bath equipped with a thermostatic controller, containing silicone oil or transparent insulating oil, and equipped with a stirring device to maintain temperature uniformity of the oil bath, with a temperature control accuracy of ±0.5 ℃; a support is installed in the oil bath to fix the gassing cell and the gas burette.
• 4.3 High-voltage generator: a step-up transformer, voltage transformer, or other step-up equipment capable of generating a test voltage of 50 Hz±0.2 Hz, capable of controlling the test voltage within 10 kV±0.2 kV; the difference between the peak factor of the test voltage (ratio of peak value to average effective value) and the peak factor of a sine wave shall not exceed ±5%; the electrical stress of the insulating oil sample during the test may be calculated with reference to Annex A.
• 4.4 Transparent safety shield: to prevent operators from contacting equipment and components carrying high voltage.
• 4.5 Thermometer: measuring temperature range 0 ℃~100 ℃, scale division 0.1 ℃.
• 4.6 Syringe: volume 10 mL, scale division 0.1 mL.

6. Recommended operating conditions

• Test voltage: 10 kV±0.2 kV (4.3, 8.7)
• Frequency: 50 Hz±0.2 Hz (4.3, 8.7)
• Oil temperature: 80 ℃±0.5 ℃ (6.5, 8.7); if the sample releases/absorbs too much gas or the equipment operating temperature is relatively low, the oil bath temperature may also be set to 60 ℃±0.5 ℃ for testing (6.5)
• Electrode gap: 3 mm (8.2, the distance from the spherical tip and cylindrical surface of the electrode to the bottom and inner wall of the gassing cell is 3 mm)
• Hydrogen flow rate: introduced at a stable flow rate of about 3 bubbles/s (8.4)
• Saturation time: bubble in the sample for 10 min, then continue bubbling for 5 min, with a total of 15 min hydrogen introduced (8.4)
• Energization time: energize for 10 min and record B10; then continue energizing for 50 min and record B60; continuous ionization for 60 min (8.8)
• Test time used for calculation: t = (60−10) min = 50 min (Chapter 9)

7. Sampling and specimen preparation

• 7 Sampling: sample in accordance with the requirements of GB/T 7597; the sample shall be sealed, protected from light, and protected from heat.
• 8.1 Specimen preparation: filter about 10 mL of specimen using dry, clean qualitative filter paper, draw 5 mL±0.1 mL of specimen with a syringe, and immediately inject the specimen into the gassing cell.

8. Analysis procedure

• 8.1 Filter about 10 mL of specimen using dry, clean qualitative filter paper, draw 5 mL±0.1 mL of specimen with a syringe, and immediately inject the specimen into the gassing cell.
• 8.2 Apply a thin layer of specimen to the surface of the conical polytetrafluoroethylene stopper supporting the hollow high-voltage electrode, then insert the hollow high-voltage electrode into the gassing cell so that the distance from the spherical tip and cylindrical surface of the electrode to the bottom and inner wall of the gassing cell is 3 mm.
• 8.3 Ground the outer electrode, connect the hydrogen inlet tube, and discharge the outlet gas directly or through a fume hood to the outside.
• 8.4 Close the bypass stopcock at the upper part of the gas burette, open the needle valve, and introduce hydrogen at a stable flow rate of about 3 bubbles/s for saturation, bubbling in the specimen for 10 min; then open the bypass stopcock and allow hydrogen to continue bubbling in the specimen for 5 min; after a total of 15 min hydrogen has been introduced, first close the needle valve, then immediately close the bypass stopcock, so that the liquid levels on both sides of the gas burette are balanced at the same horizontal plane.
• 8.5 Connect the high-voltage lead to the hollow high-voltage electrode.
• 8.6 Place the transparent safety shield in the specified position, check the bath temperature, and record the initial reading of the gas burette liquid level.
• 8.7 Turn on the high-voltage power supply, adjust the voltage to 10 kV, with a frequency of 50 Hz, and simultaneously start the stopwatch, recording the voltage, temperature, and gas burette liquid level reading at this time; monitor the reaction in the gassing cell through the vertical observation slit of the outer electrode; under normal conditions, the oil-gas interface appears to be boiling.
• 8.8 Energize for 10 min, record the test voltage, temperature, and gas burette liquid level reading (B10); then continue energizing for 50 min, record the test voltage, temperature, and gas burette liquid level reading (B60). For specimens with excessive gas absorption, when the dibutyl phthalate liquid level is about to exceed the gas burette graduation line, pause the test and record the liquid level, empty the gas in the gas burette, restore the dibutyl phthalate liquid level to the initial state, and then continue the test; the total amount of gas absorbed by the specimen is the sum of the amounts of gas absorbed before and after venting.
  • Note: To ensure normal operation of the equipment, it is recommended to read the gas burette liquid level every 10 min until the end of the test. The change in reading value over time usually shows a good linear relationship. If the data are too scattered, check the equipment and repeat the test.
• 8.9 Cut off the high-voltage power supply and the heating power supply.
• 8.10 Repeat steps 8.1 to 8.9, determine each sample at least twice, and calculate the gassing tendency according to the method in Chapter 9. The difference between the two determination results shall not exceed the calculation result of formula (1); otherwise, resample and determine again until two sets of data among the determination results meet the requirement.
  • Formula (1): 0.3 + 0.26 X
  • Where: X—— the absolute value of the arithmetic mean of the two determination results, in microliters per minute (μL/min).
• Preparation work (Chapter 6):
  • 6.1 Sequentially use isopropanol and n-heptane to separately clean the gassing cell, connecting tube, gas burette, and syringe. If there is dirt or waxy substance in the gassing cell, inject isopropanol into the gassing cell, gently scrub with a hard nylon brush, and then rinse clean with n-heptane. Finally, purge the above cleaned components with clean, dry compressed air to remove residual solvent.
  • 6.2 After cleaning the hollow high-voltage electrode with isopropanol and blowing it dry, wipe the surface with a soft silk cloth.
  • 6.3 Evenly apply a layer of vacuum silicone grease at the joint between the gassing cell and the gas burette, taking care not to let the vacuum silicone grease enter the gassing cell, then assemble the gassing cell and the gas burette, place them on a fixed support, and at this time do not insert the hollow high-voltage electrode into the gassing cell. After assembly, check the airtightness of the apparatus.
  • 6.4 Inject dibutyl phthalate to about half the scale into the gas burette. For samples with a relatively large amount of gas absorption or gas evolution, the height of the injected dibutyl phthalate should be adjusted. For samples with a relatively large amount of gas evolution, the injection amount should be lower than half the scale; for samples with a relatively large amount of gas absorption, the injection amount may be higher than half the scale.
  • 6.5 Set the oil bath temperature to 80 ℃±0.5 ℃, and record it after the temperature stabilizes. If the sample gas evolution/gas absorption amount is too large or the equipment operating temperature is relatively low, the oil bath temperature may also be set to 60 ℃±0.5 ℃ for testing.

9. Qualitative and Quantitative and Calculation Formulas

• Gassing tendency G is calculated according to formula (2):
  • G = (B60 − B10) K / t
  • Where:
    • G—— gassing tendency, in microliters per minute (μL/min);
    • B60—— the gas burette liquid level reading recorded at energization 60 min, in millimeters (mm);
    • B10—— the gas burette liquid level reading recorded at energization 10 min, in millimeters (mm);
    • K—— gas burette constant, known, or calculated with reference to Appendix B, in microliters per millimeter (μL/mm);
    • t—— the test time used to calculate the gassing tendency, in minutes (min), t = (60−10) min = 50 min.
  • Note: If G is a positive value, it indicates gas evolution; if G is a negative value, it indicates gas absorption.
• Parallelism test formula (1): the difference between the two determination results shall not exceed 0.3 + 0.26 X; X is the absolute value of the arithmetic mean of the two determination results (μL/min) (8.10).
• Appendix A (informative) Calculation of electrical stress:
  • Gx = 1/XK1 · V / [ln(d2/d1)/K1 + ln(d3/d2)/K2] (formula A.1, presentation form of source-text extraction, requires confirmation against the source standard (source-text extraction))
  • Where: Gx—— electrical stress at the test point (kV/cm); V—— applied voltage (kV); X—— distance from the test point to the axis of the hollow high-voltage electrode (cm); K1—— dielectric constant of the insulating oil; K2—— dielectric constant of the gassing cell; d1—— diameter of the hollow high-voltage electrode (mm); d2—— inner diameter of the gassing cell (mm); d3—— outer diameter of the gassing cell (mm).
• Appendix B (informative) Determination of the gas burette constant K:
  • B.1.1 Clean the gas burette with n-heptane, and dry it in an oven at 100 ℃ for at least 2 h.
  • B.1.2 Fix the gas burette vertically on a support, use a syringe with a plastic tube needle to add a certain amount of distilled water into the gas burette, and avoid water droplets adhering to the inner wall of the gas burette during addition. Weigh and record the mass of the gas burette and distilled water (m1); during weighing, avoid tilting the gas burette to prevent distilled water from wetting the inner wall of the gas burette.
  • B.1.3 Use another syringe to add about 3 cm of n-heptane into the gas burette; during addition, likewise avoid n-heptane wetting the inner wall of the gas burette, weigh and record the mass of the gas burette and the liquid in the tube (m2), and record the lowest liquid level height of n-heptane (SL) and the highest liquid level height (SH).
  • B.2 Calculation: K = (m2 − m1)×10³/ρ / (SH − SL) (formula B.1)
    • Where: K—— gas burette constant (μL/mm); m2—— mass of the gas burette and the liquid in the tube after adding n-heptane (g); m1—— mass of the gas burette and distilled water (g); ρ—— density of n-heptane (g/cm³); SH—— highest liquid level height of n-heptane (mm); SL—— lowest liquid level height of n-heptane (mm).
  • Repeat step B.1.3 3 times, and take the average of 3 values of K. If the difference between the highest value and the lowest value of K exceeds 5% of the average, discard the results and repeat the process.

10. Precision

• 11.1 The results of 10 repeated tests on 2 samples by the same operator, in the same laboratory, using the same instrument, according to the same method, are statistically calculated to obtain the sample average, repeatability standard deviation, and repeatability limit (95% confidence level), as shown in Table 1.
  • Table 1 Statistics of deviation of test results in the same laboratory (unit: μL/min)

    Sample	Average x	Repeatability standard deviation sr	Repeatability limit r
    Gas-evolving sample	32.86	0.582	1.63
    Gas-absorbing sample	−23.22	0.872	2.44

• 11.2 The results of 2 repeated tests on 14 samples by different operators, in different laboratories, using different instruments, according to the same method, through statistics from 6 laboratories, are used to obtain the sample average, repeatability standard deviation, and repeatability limit (95% confidence level), as shown in Table 2.
  • Table 2 Statistics of deviation of test results in different laboratories (unit: μL/min)

    Sample number	Average x	Repeatability standard deviation sr	Repeatability limit r
    1	30.75	2.39	6.69
    2	14.80	1.29	3.61
    3	28.97	1.60	4.48
    4	34.85	2.14	5.99
    5	28.22	2.18	6.10
    6	39.27	1.64	4.59
    7	37.26	1.16	3.25
    8	40.70	1.34	3.75
    9	−30.95	0.51	1.43
    10	−48.30	3.76	10.53
    11	16.30	0.90	2.52
    12	27.02	3.24	9.07
    13	26.32	2.44	6.83
    14	20.94	1.09	3.05

• Parallelism requirement (8.10): the difference between the two determination results shall not exceed 0.3 + 0.26 X.

11. Report

The test report shall contain at least the following contents (Chapter 10):

• a) Gassing tendency G (μL/min), take the arithmetic mean of the two determination results, accurate to 1 decimal places;
• b) Test voltage (kV);
• c) Test temperature (℃);
• d) Test duration (min);
• e) The saturating gas used.

12. Normative References

• GB/T 1220—2007 Stainless steel rods
• GB/T 7597 Sampling method for power oils (transformer oil, turbine oil)
• GB/T 1.1—2020 "Guidelines for standardization work Part 1: Structure and drafting rules for standardization documents" (mentioned in the foreword as the drafting basis)

13. Source and verification

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14. Quick Reference for Questions and Answers