Question 1

What is relative saturation (%RS) and why use it instead of ppm?

Accepted Answer

Relative saturation is the dissolved water in the liquid expressed as a percentage of the saturation level at the sample temperature: %RS = 100 · ppm / Ws(T). Because water activity is temperature-invariant for an equilibrated system, %RS lets you compare samples taken at different temperatures — the raw ppm cannot, since the saturation ceiling rises sharply with temperature.

Question 2

Which saturation coefficients does the calculator use?

Accepted Answer

The IEC harmonised averaged-literature coefficients (Table A.1), identical across IEC 61203:2025 Annex A and IEC 62975:2021 Annex A, for new and aged mineral oil, synthetic ester, natural ester and silicone. At 20 °C these give about 53, 1880, 925 and 192 ppm respectively. Vendor or product-specific data (e.g. some synthetic esters near 2700 ppm) can run 30–40 % higher and are the right denominator only for a specific named product.

Question 3

What is the free-water temperature?

Accepted Answer

It is the temperature, on cooling, at which the dissolved water reaches 100 %RS (saturation). Below it, water can come out of solution as free water. If the free-water temperature is at or above the operating top-oil temperature, the oil is at or near saturation — the calculator flags this.

Question 4

How reliable is the estimated paper moisture (%WCP)?

Accepted Answer

It is read off the digitised IEC 61203:2025 Annex A equilibrium figures, not from a table, so it is approximate. It is valid for Kraft (non-upgraded) paper only; ester equilibrium is a slow pseudo-equilibrium reached only at elevated temperature over long periods; and natural ester has only a single 50 °C curve, so the calculator returns no %WCP estimate for natural ester away from 50 °C rather than fabricating one.

Moisture & Relative-Saturation Calculator

Moisture & relative-saturation calculator

Worked examples

Mineral oil, Cat C

Synthetic ester

Natural ester (50 °C only)

Methodology