In daily oil condition monitoring, water content is the measurement everybody knows: the sample is taken and sent for testing under GB/T 7600 and the report gives a figure in ppm. Gas content is harder, its basis of judgement being GB/T 423, with the ZYD-100 reaching ≤0.01% after treatment. The difficulty is not in the instrument but in the fact that the sample itself starts changing once it leaves the machine.
Gas content measures the gas that has already escaped
Gas in oil exists in different forms: gas dissolved in the oil, free bubbles that can be seen, and vapour formed by the water. Degassing is the business of driving these out of the oil and measuring them. The object of a gas content test is precisely the gas that was driven out.
The problem is that dissolved gas moves out on its own at atmospheric pressure. A bottle that has just been filled can look clear, and after standing for a while a ring of bubbles may appear at the top. By the time it reaches the laboratory, what is measured is a state that has been redistributed, not the state at the moment of filling. Water content behaves differently: water does not leave a closed container on its own, so a water sample can wait while a gas sample cannot.
Temperature and standing time both change the result
The same drum of oil can give different gas content readings at different temperatures, because the solubility of gas in oil changes with temperature. How much gas space was left in the container when the sample was drawn, whether the oil was agitated, and how much air was displaced before the cap was tightened all become part of the result.
This is also why two tests of the same oil sample can return two figures: the two samples differed in temperature, in the way they were filled and in how long they stood before testing, so they were never the same state to begin with. When results differ, align the conditions first and the numbers second.
The conditions that must be reported with the sample
| What has to be recorded | Why it cannot be left out | What happens without it |
|---|---|---|
| The oil temperature at sampling | Solubility changes with temperature, and temperature is part of the result | Two results cannot be compared |
| The sampling point and the sampling method | Samples taken from the tank bottom, the tank top and the oil outlet are in different states | The result cannot be reproduced when it is disputed |
| Whether the sample was degassed, and to what degree | A degassed sample and a raw sample are not the same thing | A treated result is taken for the performance of the machine |
| The container type and the gas space left in it | The amount of gas space directly affects how the gas redistributes | Free bubbles cannot be explained |
| The time between sampling and testing | Dissolved gas keeps separating out over that period | The result drifts with time |
Which one to check first when both fail
Water content is judged under GB/T 7600 as ≤3 ppm and gas content under GB/T 423 as ≤0.01%. When both are out of limit, the usual order on site is to check water content first. The reason is that water content has a clear path to being reproduced: the sample bottle, the test method and the standard are all fixed, and the same sample can be checked repeatedly. A gas content that will not come down points more often to the degassing stage not having been done properly, which takes the investigation back to the vacuum system, the atomisation of the incoming oil and the residence time inside the tank.
In the opposite case, if water content passes and gas content stays high, stop doubting the sample and look at whether the working vacuum of -0.08 ~ -0.0999 MPa is held steadily, and whether the oil mist is genuinely being delivered into the vacuum zone.
Water content can wait for the sample to reach the laboratory, gas content cannot: it changes from the moment the bottle is filled.
