ASTM D831 Standard Test Method for Gas Content of Cable and Capacitor Oils
5. Apparatus
5.1 Degassing Chamber - Degassing chamber, A, made of heat-resistant glass (with calibrated oil well at bottom), having a fixed total space volume of about 175 to 300 mL. The oil well shall have a maximum capacity of 50 mL and shall be calibrated in 0.2-mL divisions.
5.2 Stopcocks - Glass stopcocks, B and C, which shall have large-diameter barrels and a mirror finish to ensure against leakage. Use stopcock grease on all stopcocks and ground-glass joints.
5.3 Atomizer - Glass pipet, D, placed to drop oil on the side of the degassing chamber, or
5.3.1 Fritted Disk (Alternative) - Capacity 30 mm, medium-porosity.
NOTE 2 - Some experience has shown improvement in the atomization process, particularly for oils of medium viscosity above 95 cSt at 40°C, if a 30-mm medium-porosity fritted disk is substituted for the pipet.
5.4 Pressure Gage - Pressure gage, E, of modified McLeod type. Include the volume of this gage in the over-all volume of the apparatus (Note 3). This is essential and must also include the volume of the gage-connecting tubing.
NOTE 3 - The volume of the gage may be obtained from the manufacturer or measured.
5.5 Oil Trap, F, having a capacity of 250 mL.
5.6 Thermometer, T, room ambient.
5.7 Cold Trap, J, employed to eliminate possible error due to presence of condensable vapors.
5.8 Oven, K, employed to enhance the atomization process. The oven shall enclose the degassing chamber, A, between stopcocks B and C, and point L. Provide suitable means for reading, maintaining, and regulating temperature in a range from 30 to 150°C. Measure temperature by means of a thermocouple fastened to the oil chamber at the 25-mL mark and suitably shielded to eliminate radiation errors.
6. Sampling
6.1 When convenient, connect the degassing chamber of the measuring equipment directly to the container from which the oil is to be sampled. This is usually not convenient and is often impossible. The method of sampling described in 6.2 is recommended as an alternative.
6.2 Use the sample container as shown in Fig. 2, which consists of a stainless steel cylinder 2 1/4 in. (5.7 cm) in inside diameter and 9 1/2 in. (24 cm) in length, closed at the bottom. An aluminum piston, accurately machined for an easy sliding fit, is inserted in the bore of the cylinder. Two nipples, diametrically opposite each other, are inserted at the extreme bottom of the cylinder. Each nipple has a screw plug at the end with a gasket for sealing. Make all connections to the measuring equipment from the sample container using glass or metal tubing. Connect butted joints using short sections of heavy-walled rubber tubing. Coat the tubing thoroughly with suitable sealing compound. Take all samples under slight oil pressure, with the following sequence of operations: Remove plugs from both nipples. Push the piston to the extreme bottom of the cylinder. Hold the cylinder so that the nipples point in a vertical direction. Using a rubber tube connection, force oil in through the nipple in the lowest position and flush a few millilitres out the opposite nipple to remove any trapped air bubbles. Then insert the plug in the outlet nipple and allow oil to push the piston up to fill the cylinder. Hold the piston at the top of the cylinder with one hand and plug the inlet nipple. The aluminum piston "floats" on the oil as the level varies due to temperature changes or removal of test specimens and prevents contamination by absorption and diffusion. Fit the piston accurately so it moves down freely with decreasing oil level to prevent voids forming under the piston which would allow rapid absorption of air by the top oil. Draw the test specimen continuously from the cylinder, weight the piston to ensure maintenance of contact with the oil. Wide variations in the result are possible in two test specimens from the same source unless the greatest care is taken in the sampling procedure. This phase of the test is so involved with the details of what constitutes correct practice that ability to procure consistent representative test specimens depends, to a great extent, on wide experience. The chief precaution to the procurement of representative test specimens involves a complete flushing of all piping and hose between the sample container and sample source, such as pothead, joint, cable, oil reservoir, etc., immediately preceding collection of the sample. Eliminate all long sampling pipe lines. After taking the test specimen, ensure that the piston always remains in contact with the oil prior to and during withdrawal of the test specimen.
