Moisture in transformer oil calculator: ppm, relative saturation and saturation temperature

Convert the water content of an oil between mg/kg and relative saturation at any temperature, and find the temperature at which free water would appear.

Water in mineral oil: ppm, relative saturation and saturation temperature

Convert water content between mg/kg (ppm) and relative saturation at the oil temperature, and check at which temperature that same water would saturate the oil.

Optional: the same water, at another temperature. Useful to see what happens when the unit cools down.

Indicative result. The calculation runs in your browser and is not sent to any server. This website may contain errors or omissions: before making any decision about an installation, the information must be checked and validated by a qualified engineer. Legal notice and terms of use

What this calculator does

It converts the water content of a mineral oil between milligrams per kilogram (what a Karl Fischer titration gives) and relative saturation in percent (what an in-line capacitive probe gives), at whatever temperature you enter. And it calculates two figures that rarely appear in the report: the solubility of the oil at that temperature and the saturation temperature, below which that same water starts to appear as free water.

Why ppm are misleading

Water dissolved in oil matters not for its absolute amount but for how close it brings the oil to saturation. And solubility depends very strongly on temperature:

log₁₀(Ws) = 7.42 − 1670 / T

with T in kelvin and Ws in mg/kg. That is about 53 mg/kg at 20 °C, 255 at 60 °C and 490 at 80 °C.

The practical consequence: an analysis of 25 mg/kg taken with the transformer hot looks reassuring — under 10% saturation — but that same water, once the unit cools to 20 °C, sits at nearly 50% saturation. This is why a sample without the oil temperature recorded is worthless, and why in-line probes report relative saturation rather than ppm.

The formula is for new mineral oil. Aged or oxidised oil dissolves more water, so the real saturation would be somewhat lower than the figure shown here: the result stays on the safe side.

The saturation temperature

This is the most useful number in the calculator and the one that is hardly ever worked out. Given the measured water content, it tells you at what temperature the oil would reach 100% saturation. Below it, free water can appear — and free water is what really destroys dielectric strength and what causes failures on cold starts or after a long winter shutdown.

A transformer at 40 mg/kg saturates at around 14 °C. If it sits in a room that drops below that, the risk is not theoretical.

Worked example

A 20 mg/kg sample taken with the oil at 60 °C, on a unit that settles at 20 °C when shut down.

  • Solubility at 60 °C: 255 mg/kg → saturation 7.8%: dry oil.
  • Solubility at 20 °C: 53 mg/kg → saturation 37.9%: very wet oil.
  • Saturation temperature: 8.6 °C.

Same oil, same water, two opposite readings. If the unit is shut down in winter and the room falls below 9 °C, free water will appear.

What this calculator does not do

It does not say how much water is in the paper, which is where most of it sits and what actually has to be dried out. Paper-oil equilibrium takes days or weeks to establish and depends on temperature and load history, so estimating paper moisture from a single oil sample is unreliable; equilibrium curves, and better still continuous measurement, are used for that. It does not cover esters or silicone fluids either, whose solubility differs greatly from mineral oil, and it is no substitute for a dielectric strength test.

What measures it

Water content is determined by Karl Fischer titration in the laboratory, and relative saturation by capacitive probes fitted in the oil circuit, which is what lets you see the trend rather than a single snapshot. The dielectric oil analyser and the rest of the oil measurement equipment cover the testing side; drying and degassing cover the treatment side. If the oil is wet, it is worth looking at the dissolved gas analysis too: paper degradation leaves its trace in CO and CO₂.

Frequently asked questions

Why are two oils with the same ppm of water not equally wet?
Because what degrades the insulation is not the absolute amount of water but how close the oil is to saturation, and solubility depends enormously on temperature: about 53 mg/kg at 20 °C against about 255 at 60 °C. Twenty mg/kg is 38% saturation in a cold oil and under 8% in one at service temperature.
What relative saturation is acceptable?
The most widespread maintenance practice treats an oil as dry below 5%, moderately wet between 5% and 20%, wet between 20% and 30%, and very wet above 30%. Limits in mg/kg by voltage class are given in IEC 60422, which is the reference to consult for a normative criterion.
Where does the water in a transformer come from?
From three places: ingress from the atmosphere through gaskets, breathers or during work on the unit; water released by the insulating paper as it ages, because cellulose degradation produces water; and water left inside since manufacture or the last treatment. Most of a transformer’s water is in the paper, not in the oil: the oil only shows the fraction in equilibrium with it.