Why don’t Vaisala’s moisture-in-oil instruments offer a ppm output for measuring moisture in lubricating oil?

Vaisala’s moisture-in-oil instruments directly measure water activity in lubricating oil. Although it is possible to calculate ppm from water activity, the calculation is based on the water solubility of the oil, a characteristic that changes over the working life of the oil. Calculation models that are correct for new oil would be incorrect for oil that has been in service or has been treated with additives. A majority of lubrication oil problems are caused by water contamination, and especially free water in the system. Water activity measurement provides direct indication of free water formation whereas ppm measurement does not.