ASTM D117 guide for sampling, test method and specification for insulating oil
ASTM D117 standard guide for sampling, test methods and specifications for electrical insulating oils of petroleum origin
PHYSICAL PROPERTIES
4. Aniline Point
4.1 Scope - This test method covers the determination of the aniline point of petroleum products, provided that the aniline point is below the bubble point and above the solidification point of the aniline-sample mixture.

4.2 Summary of Test Method:
4.2.1 Test Method D611 - Equal volumes of aniline and test specimen or aniline and test specimen plus n-heptane are placed in a tube and mixed mechanically. The mixture is heated at a controlled rate until the two phases become miscible. The mixture is then cooled at a controlled rate, and the temperature at which the two phases separate is recorded as the aniline point.

4.3 Significance and Use - The aniline point of an insulating oil indicates the solvency of the oil for some materials that are in contact with the oil. A higher aniline point implies a lower aromaticity and a lower degree of solvency for some materials.

5. Coefficient of Thermal Expansion
5.1 Scope - This test method covers the determination of the coefficient of thermal expansion of electrical insulating liquids of petroleum origin.

5.2 Definition:
5.2.1 coefficient of thermal expansion - the change in volume per unit volume per degree change in temperature. It is commonly stated as the average coefficient over a given temperature range.

5.3 Summary of Test Method - The specific gravity of insulating oils is determined at two temperatures below 90°C and separated by not less than 5°C nor more than 14°C. Test methods used may be D287, D1217, D1298, or D1481. The calculation of average coefficient of thermal expansion over this temperature range is given in Test Method D1903.

5.4 Significance and Use - A knowledge of the coefficient of expansion of a liquid is essential to compute the required size of a container to accommodate a volume of liquid over the full temperature range to which it will be subjected. It is also used to compute the volume of void space that would exist in an inelastic device filled with the liquid after the liquid has cooled to a lower temperature.

6. Color
6.1 Scope - This test method covers the visual determination of color of a wide variety of liquid petroleum products, including mineral insulating oils.

6.2 Summary of Test Method:
6.2.1 Test Method D1500 - The test specimen is placed in a glass sample jar (an ordinary 125-mL test specimen bottle is satisfactory for routine tests). The color of the sample by transmitted light is compared with a series of tinted glass standards. The glass standard matching the sample is selected, or if an exact match is not possible, the next darker glass is selected. The results are reported numerically on a scale of 0.5 to 8.0.

6.3 Significance - A low color number is an essential requirement for inspection of assembled apparatus in a tank. An increase in the color number during service is an indicator of deterioration or contamination of the insulating oil.

7. Examination
7.1 Scope:
7.1.1 Both visual examination and qualitative infrared absorption are described in this section. The test methods are:
7.1.2 Test Method D1524 - This is a visual examination of mineral insulating oils that have been used in transformers, oil circuit breakers, or other electrical apparatus as insulating or cooling media, or both. This test is intended for use in the field.

7.1.3 Test Method D2144 - The infrared absorption from 2.5 to 25 µm (4000 to 667 cm(-1)) is recorded as a means of (a) establishing continuity by comparison with the spectra of previous shipments by the same supplier, (b) for the detection of some types of contaminants, (c) for the identification of oils in storage or service. This test method is not intended for the determination of the various constituents of an oil.

7.2 Summary of Test Methods:
7.2.1 Test Method D1524 - Estimate the color of the oil by use of an oil comparator, matching the oil test specimen with tinted glass color standards. Note the presence of cloudiness, particles of insulation, metal corrosion products, or other undesirable suspended materials in the oil.

7.2.2 Test Methods D2144 - The infrared spectrum is recorded from 2.5 to 25 µm (4000 to 667 cm(-1)) either as the absorption spectrum itself, or as the differential between the test specimen and reference oil. The spectra are compared with reference spectra to establish the identity of the oil.

7.3 Significance and Use:
7.3.1 Test Method D1524 - The observation of the color and condition of the oil in a field inspection permits a determination of whether the sample should be sent to a central laboratory for full evaluation.

7.3.2 Test Methods D2144 - The infrared spectrum of an electrical insulating oil indicates the general chemical composition of the sample. Because of the complex mixture of compounds present in insulating oils, the spectrum is not sharply defined and may not be suitable for quantitative estimation of components. The identity of the oil can be quickly established as being the same or different from previous samples by comparison with the reference spectra.

8. Flash and Fire Point
8.1 Scope:
8.1.1 This test method covers the determination of flash and fire points of all petroleum products except fuel oils and those having an open cup flash below 79°C (175°F).

8.1.2 This test method should be used solely to measure and describe the properties of materials in response to heat and flame under controlled laboratory conditions and should not be used for the description, appraisal, or regulation of the fire hazard of materials under actual fire conditions.

8.2 Definitions:
8.2.1 flash point - the temperature at which vapors above the oil surface first ignite when a small test flame is passed across the surface under specified conditions.

8.2.2 fire point - the temperature at which oil first ignites and burns for at least 5 s when a small test flame is passed across the surface under specified conditions.

8.3 Summary of Test Method - Fill the test cup to the specified level with the test specimen. Heat the sample initially at 14 to 17°C/min (25 to 30°F/min) until the temperature is 56°C (100°F) below the expected flash point. Reduce the rate of temperature change to 5 to 6°C/min (9 to 11°F/min) and apply the test flame every 2°C (or 5°F) until a flash occurs. Continue heating and testing every 2°C (or 5°F) until the oil continues to burn for at least 5 s. The procedure is described in Test Method D92.

8.4 Significance and Use - The flash point and fire point tests give an indication of the flammability of an oil. They may also be used to provide a qualitative indication of contamination with more flammable materials. In the latter context, the flash point test is more sensitive.

9. Interfacial Tension
9.1 Scope - These test methods cover the measurement, under nonequilibrium conditions, of the interfacial tension of insulating oils against water. These test methods have been shown by experience to give a reliable indication of the presence of hydrophilic compounds.

9.2 Definition:
9.2.1 interfacial tension - the molecular attractive force between unlike molecules at an interface. It is usually expressed in dynes per centimetre or millinewtons per metre.

9.3 Summary of Test Methods:
9.3.1 Test Method D971 - Interfacial tension is determined by measuring the force necessary to detach a platinum wire upward from the oil-water interface. To calculate the interfacial tension, the force so measured is corrected by an empirically determined factor which depends upon the force applied, the densities of both oil and water, and the dimensions of the ring. The measurement is completed within 1 min of the formation of the interface.

9.3.2 Test Method D2285 - Interfacial tension is determined by measuring the volume of a drop of water that the oil will support. The larger the drop of water, the higher the interfacial tension of the oil. The instrument used to measure the volume of the drops of water is calibrated to read approximately in dynes per centimetre interfacial tension. For better accuracy, the reading can be corrected by a factor that depends on the density of the oil. The drop is allowed to age for 30 s and to fall between 45 and 60 s after formation.

9.4 Significance and Use - Interfacial tension measurements on electrical insulating oils provide a sensitive means of detecting small amounts of soluble polar contaminants and products of oxidation. A high value for new mineral insulating oil indicates the absence of undesirable polar contaminants. The test is frequently applied to service-aged oils as an indication of the degree of deterioration.

10. Pour Point
10.1 Scope - The pour point is applicable to any petroleum oil.

10.2 Definition:
10.2.1 pour point - the lowest temperature, expressed as a multiple of 3°C at which the oil is observed to flow when cooled and examined under prescribed conditions.

10.3 Summary of Test Method - After preliminary heating, the test specimen is cooled at a specified rate and examined at intervals of 3°C for flow characteristics. The lowest temperature at which movement of the oil is observed within 5 s is reported as the pour point. The procedure is described in Test Method D97.

10.4 Significance and Use:
10.4.1 The pour point of an insulating oil gives an indication of the temperature below which it may not be possible to pour or remove the oil from its container.

10.4.2 In connection with oil for use in cable systems, the pour point may be useful to indicate the point at which no free movement will take place in the cable or to indicate the temperature at which partial separation of wax may occur.

10.4.3 The pour point of a transformer oil is important as an index of the lowest temperature to which the material may be cooled without seriously limiting the degree of circulation of the oil. Some materials are sensitive to temperature cycling or prolonged storage at low temperatures, and their pour points may not adequately predict their low temperature flow properties.

11. Refractive Index and Specific Optical Dispersion
11.1 Scope:
11.1.1 Test Method D1218 - Describes a precision method for determining refractive index accurate to 0.00006 and refractive dispersion accurate to 0.00012. The liquid must be transparent, no darker than ASTM 4.0 color (see Test Method D1500) and have a refractive index between 1.33 and 1.50. The specific optical dispersion is calculated by dividing the refractive dispersion value by the specific gravity of the liquid.

11.1.2 Test Method D1807 - Describes a routine method for measuring refractive index accurate to three units in the fourth decimal place, measuring refractive dispersion, and calculating specific optical dispersion accurate to three units in the fourth decimal place. The oils must be transparent and light colored.

11.2 Definitions:
11.2.1 refractive index - the ratio of the velocity of light in air to its velocity in the substance under test.

11.2.2 specific optical dispersion - the difference between the refractive indexes of light of two different wave lengths, both indexes measured at the same temperature, the difference being divided by the specific gravity also measured at the test temperature. For convenience, the specific dispersion value is multiplied by 10.

11.3 Summary of Test Method:
11.3.1 The two methods differ in the accuracy of the refractometer used. After adjusting the instrument temperature to 25°C, apply the test specimen to the refracting prism, read the refractive index, and read the compensator dial reading. From the correlation tables supplied with the instrument obtain the refractive dispersion. Calculate the specific optical dispersion by dividing refractive dispersion by the specific gravity of the oil.

11.4 Significance and Use:
11.4.1 Refractive Index of an insulating liquid varies with its composition and with the nature and amount of contaminants held in solution. Where the refractive index of an insulating liquid when new is known, determinations made on the same liquid after periods of service may form a basis for estimating any change in composition or the degree of contamination acquired through solution.

11.4.2 Specific Optical Dispersion serves as a quick index to the amount of unsaturated compounds present in an oil. As the dispersion values for paraffinic and naphthenic compounds are nearly the same and are essentially independent of molecular weight and structural differences, values above a minimum of about 97 bear a direct relationship to the amount of aromatic compounds present in insulating oil.

12. Relative Density (Specific Gravity)
12.1 Scope:
12.1.1 The methods used to measure relative density (specific gravity) may use a hydrometer, pycnometer, or an oscillating tube.

12.1.1.1 Test Method D287 - Uses an API hydrometer and is limited to liquids having a Reid vapor pressure of 180 kPa (26 psi) or less.

12.1.1.2 Test Method D1217 - Covers the use of a pycnometer to measure the relative density (specific gravity) of petroleum fractions.

12.1.1.3 Test Method D1298 - Covers the use of a hydrometer to measure relative density (specific gravity) directly or the measurement of API gravity followed by conversion to relative density (specific gravity). This test method is limited to liquids having a Reid vapor pressure of 179 kPa (26 psi) or less. This test method is most suitable for use with mobile transparent liquids, although it can also be used with viscous oils if sufficient care is taken in the measurement.

12.1.1.4 Test Method D1481 - Covers the determination of the densities of oils more viscous than 15 cSt at 20°C. The liquid should not have a vapor pressure greater than 13 kPa (100 mm Hg) at the test temperature. To measure the density of less viscous liquids more accurately than permitted by the hydrometer method, Test Method D1217 is available.

12.1.1.5 Test Method D4052 - Covers the measurement of relative density (specific gravity) by the measurement of change in oscillation frequency of a vibrating glass tube filled with test liquid.

12.2 Definition:
12.2.1 relative density (specific gravity) - the ratio of the mass (weighed in vacuum) of a given volume of liquid at 15.6°C (60°F) to the mass of an equal volume of pure water at the same temperature. When reporting results, explicitly state the reference temperature, for example, specific gravity 15.6/15.6°C.

12.3 Summary of Test Method:
12.3.1 API gravity may be measured at the oil temperature using a hydrometer (Test Methods D287 or D1298) and converting to 15.6°C using Guide D1250.

12.3.2 Relative density (specific gravity) may be measured at the oil temperature using a hydrometer (Test Method D1298) and converted to 15.6°C using Guide D1250.

12.3.3 Test Method D1481 - The liquid is drawn into the bicapillary pycnometer through the removable siphon arm and adjusted to volume at the temperature of test. After equilibration at the test temperature, liquid levels are read; and the pycnometer is removed from the thermostated bath, cooled to room temperature, and weighed. Density or relative density (specific gravity), as desired, is then calculated from the volume at the test temperature, and the weight of the sample. The effect of air buoyancy is included in the calculation.

12.4 Significance and Use:
12.4.1 Electrical insulating oils are usually sold on the basis of volume delivered at 15.6°C (60°F). Delivery is often made on the basis of net weight of product in drums, and the specific gravities often are measured at temperatures other than 15.6°C. The values of relative density (specific gravity) at 15.6°C must be known to calculate the volume at 15.6°C of the oil delivered.

12.4.2 The relative density (specific gravity) of a mineral insulating oil influences the heat transfer rates and may be pertinent in determining suitability for use in specific applications. In certain cold climates, ice may form in de-energized transformers exposed to temperatures below 0°C, and the maximum specific gravity of the oil used in such equipment should be at a value that will ensure that ice will not float in the oil at any temperature the oil might attain.

12.4.3 When making additions of insulating liquid to apparatus in service, a difference in relative density (specific gravity) may indicate a tendency of the two bodies of liquid to remain in separate layers rather than mixing into a homogeneous single body of liquid. Such conditions have caused serious overheating of self-cooled apparatus. Suitable precautions should be taken to ensure mixing.

13. Specific Heat
13.1 Scope - This test method covers determination of the specific heat of electrical insulating liquids of petroleum origin.

13.2 Definition:
13.2.1 specific heat (or heat capacity) of a substance - a thermodynamic property that is a measure of the amount of energy required to produce a given temperature change within a unit quantity of that substance. The standard unit of heat capacity is Joules/Kg°C at some defined temperature; specific heat is dimensionless as it is the ratio of the substance's heat capacity relative to that of water.

13.3 Summary of Test Method - The specific heat is determined by Test Method D2766. The measurement is made by heating a test specimen at a known and fixed rate. Once dynamic heating equilibrium is obtained, the heat flow is recorded as a function of temperature. The heat flow normalized to specimen mass and heating rate is directly proportional to the specimen's specific heat capacity.

13.4 Significance and Use - A knowledge of the specific heat is helpful in designing adequate heat transfer properties for electrical apparatus. A higher specific heat value indicates a more efficient heat transfer medium.

14. Thermal Conductivity
14.1 Scope - This test method covers the determination of the thermal conductivity of electrical insulating liquids of petroleum origin.

14.2 Definition:
14.2.1 thermal conductivity - the ability of a substance to transfer energy as heat in the absence of mass transport phenomena. The standard unit of thermal conductivity is as follows:
W/(mK) (Cal/cm s °C)

14.3 Summary of Test Method - The thermal conductivity is determined by Test Method D2717. This test method measures the temperature gradient produced across the liquid by a known amount of energy introduced into the test cell by an electrically heated platinum element.

14.4 Significance and Use - A knowledge of thermal conductivity is helpful in designing adequate heat transfer properties for electrical apparatus. A high value indicates a good heat transfer efficiency property for the liquid.

15. Turbidity
15.1 Scope - This test method determines the amount of suspended particulate matter in electrical insulating oil of petroleum origin.

15.2 Definition:
15.2.1 turbidity, n - the reduction of transparency due to presence of particulate matter. The standard unit of turbidity is the nephelometric turbidity unit (NTU), which is defined as the intensity of light scattered by a known aqueous suspension of formazine.

15.3 Summary of Test Method - The turbidity is determined by Test Method D6181. This test method measures the scattered light at 0.5 p rad (90°) or 0.5 and 1.5 p rad (90° and 270°) angles to the incident beam using a nephelometer that has been calibrated with a standard aqueous suspension of formazine.

15.4 Significance and Use - Turbidity measures particulate contamination in electrical insulating oil that may not be apparent to the unaided human eye and could affect the performance of the dielectric fluid.

16. Viscosity
16.1 Scope:
16.1.1 Test Method D88 - Covers the empirical measurement of Saybolt viscosity of petroleum products using the Saybolt viscometer at temperatures between 23.1 and 98.9°C (70 and 210°F).

16.1.2 Test Method D445 - Covers the determination of the kinematic viscosity of liquid petroleum products by measuring the time for a volume of liquid to flow under gravity through a calibrated glass capillary viscometer.

16.1.3 Practice D2161 - Provides tables or equations for the conversion of centistokes into Saybolt Universal Seconds or Saybolt Furol Seconds at the same temperatures.

16.2 Summary of Test Methods:
16.2.1 Test Method D88 - The efflux time in seconds for 60 mL of test specimen to flow through a calibrated orifice in the Saybolt viscometer is measured under carefully controlled conditions, particularly temperature and liquid head. The time is converted by an orifice factor and reported as the viscosity of the sample at that temperature.

16.2.2 Test Method D445 - The time is measured in seconds for a fixed volume of liquid to flow under gravity through the capillary of a calibrated viscometer under a reproducible driving head and at a closely controlled temperature. The kinematic viscosity is the product of the measured flow time and the calibration constant of the viscometer.

16.2.3 Practice D2161 - The Saybolt Universal viscosity equivalent to a given kinematic viscosity varies with the temperature at which the determination is made. The basic conversion values are given in Table 1 of this practice for 37.8°C (100°F). Factors are given for converting units at other temperatures. The Saybolt Furol viscosity equivalents are given in Table 3 of this practice for 50.0 and 98.9°C (122 and 210°F) only.

16.3 Significance and Use:
16.3.1 The fundamental and preferred method for measuring kinematic viscosity is by use of Test Method D445. The Saybolt instrument in Test Method D88, being of all-metal construction, may be more rugged for field use, but values obtained are significantly less accurate than those obtained by the use of the capillary viscometers in Test Method D445.

16.3.2 Viscosity of electrical insulating oils influences their heat transfer properties, and consequently the temperature rise of energized electrical apparatus containing the liquid. At low temperatures, the resulting higher viscosity influences the speed of moving parts, such as those in power circuit breakers, switchgear, load tapchanger mechanisms, pumps, and regulators. Viscosity controls insulating oil processing conditions, such as dehydration, degassification and filtration, and oil impregnation rates. High viscosity may adversely affect the starting up of apparatus in cold climates (for example, spare transformers and replacements). Viscosity affects pressure drop, oil flow, and cooling rates in circulating oil systems, such as in pipe-type cables and transformers.