For the correct execution of analyses of transformer oils, in part for the determination of gas in oil, oil sampling is enormously important. Sampling can only be carried out with a special sampling set, in accordance with standards. In order to obtain a representative sample, the insulating oil cannot come into contact with air or other impurities from the environment during sampling. During transport as well, the absolute impermeability of the specimen containers must be guaranteed. OELCHECK analyzes the gases dissolved in the oil directly from the 3-way stopcock, with a gas-proof 20 ml glass syringe, with which the sample was extracted. For the other investigations, the oil sample is filled into a gas-proof 1,000 ml aluminium bottle. Each of the prepaid OELCHECK ISO 1 to ISO 5 analytical sets contains, in addition to a test data sheet, a self-adhesive laboratory address for the return of the syringe, a shipping carton lined with plastic foam, in which there is a 20 ml glass syringe with a firmly attached 3-way stopcock in a recess designed for this purpose. An optional sampling tube, which provides the connection between the transformer and the injection connection, is available. In addition to the syringe, which is required in filled form for each ISO set for the DGA analysis, the analytical sets ISO 2 to ISO 5 contain an aluminum bottle with a filling volume of 1,000 ml. With this large container, sufficient volume of sample is available for the analyses contained in the extended set volume. All sample containers should be filled to the brim so that no air can influence the measurement results. A requirement is that all equipment coming into contact with the oil must be clean.