One drum of oil sampled at the top of the tank, in the middle and at the outlet can produce three different readings at the same laboratory. That is not necessarily a problem with the testing; the three positions represent three different states. The top is the gas phase above the oil surface, the middle is the main phase surrounded by a large volume of oil, and the outlet is what the oil looks like after it has been heated, atomised, filtered and pressurised. To let the data answer a process question, you first have to know which section the bottle came from.
A sample taken at the top of the tank holds the gas phase
When insulating oil is degassed in the vacuum chamber, the gas released from the oil collects at the top of the tank. The target gas content is ≤0.01% (GB/T 423) and that figure measures the concentration of dissolved gas in the oil phase; with the sample taken at the top, the air that gets into the bottle is that gas phase itself, and the reading has no correspondence with the gas content of the oil phase. The same applies to a water content test on a top sample: the humidity of the air in the bottle takes part in the equilibrium directly, and no amount of sealing slows that down to the point where it disappears. The top of the tank is therefore not a sampling point, it is an observation point.
A top sample carries one more source of interference: the oil surface itself is moving during degassing, and the gas-liquid interface rises and falls. Foam and fine oil droplets are carried into the sample bottle with it. The gas content measured this way usually comes out high and varies at random, and taking the sample again does not reproduce it.
A sample taken in the middle reflects the main oil phase
A sample from the middle gives the main oil phase, and that is the state most oil quality figures should correspond to. It still has two limitations. First, the depth of the sample matters: the water phase and the sediment close to the tank floor get into the bottle with it. Second, the state of the oil flow at the moment of sampling affects the result, because the contents of the tank are churned up during vacuuming and the depth and angle at which the sampling tube or dipper is held changes the reading. The position and the method therefore have to be fixed for a middle sample as well, and only then can two results be compared.
The outlet is the only valid sampling point for the result of the process
What comes out of the outlet is the result of the process: what this batch of oil actually is after heating, atomised degassing and fine filtration. The breakdown voltage of ≥65 kV required by GB/T 507, the water content of ≤3 ppm required by GB/T 7600 and the cleanliness of NAS 1638 class 5 are all figures given for purified oil, so they can only be sampled at the outlet. The degassing capacity and the filtration accuracy of the machine both have to show up in the bottle taken at the outlet.
| Sampling point | What is taken | What it can answer | What it cannot answer |
|---|---|---|---|
| Top of the tank | The gas phase above the oil surface | Whether degassing is taking place | The gas content and the water content of the oil phase |
| Middle of the tank | The main phase surrounded by oil | The state of the oil itself in the tank | The result after the filtration stage |
| Outlet | The oil after heating, atomising and fine filtration | Whether the process has met the acceptance figures | Which stage inside the machine is at fault |
- Take acceptance samples at the outlet, never at the top or in the middle of the tank instead.
- Record the oil temperature and the sampling position before and after sampling; without one of the two the next set of figures cannot be compared.
- When comparing the oil before and after purification, use the same position and the same containers for both.
- If a reading looks wrong, confirm the sampling point before looking at the machine; the other way round wastes maintenance time.
What a bottle of oil can tell you depends on which section it was taken from. If the position is wrong, the reading is accurate about a different state.
