Typically, the water content of the insulation paper (WCP) is determined by measuring the water content in the oil (WCO), however, this method has a few inherent problems. Measurements are taken directly from the oil requiring sampling, transportation and lab testing to be done. WCP calculations are done indirectly from WCO measurements based on several assumptions. These can introduce significant inaccuracies if the assumptions are found to be incorrect.
Another method to determine WCP is using the water activity of the oil. This allows the direct calculation of the WCP unaffected by inaccuracies of WCO based calculations. Additionally, Aurtra’s algorithms allow for determining the hotspot temperature as well as the direct calculation of DP.
The main benefits of using water activity are that the method does not require moisture and temperature to be in equilibrium, which the oil sampling method assumes. It also assumes that the solubility of water in oil is the same for all oils and never changes whereas the water activity method is unaffected by changes in the solubility of water in oil.
 Saha, Tapan Kumar, and Purkait, Prithwiraj. Transformer Ageing: Monitoring and Estimation Techniques: Monitoring and Estimation Techniques. 1st ed., Wiley, 2017.