As a physical chemist and mechanical engineer, who has for nearly 40 years worked deeply in fluid and thermal sciences, the ultrapurification of fluids (including exotic 'oils', dielectric oils, etc. etc. for the electric power generation, semiconductor, ultrapure chemical, bio-pharma industries, etc. ), etc. etc. etc. .... Mr. Pascoe is essentially correct, not totally correct but close. Sorry to state this; but, most of you who somehow 'intuitively' oppose Mr. Pascoes statement are not correct.
1. Most oil made from cracking or distillation are almost completely dehydrated by the heat of the process - water content essentially nil.
2. When in CONTACT WITH **ATMOSPHERIC** AIR, water (as vapor and as free gaseous molecules) enters the mass of oils, etc. due to ***vapor pressure equilibrium drive (partial pressure of the water vs. the partial pressure of the oil)***.
3. Water in oil exists in various 'states':
a. molecular water (azeotropic mixture) ... insensible without using 'instrumentation', etc.
b. emulsified water (water that is held 'in suspension') ...insensible without using instrumentation, etc.
c. free water ... sensible water that can be observed *without* instrumentation, etc. In typical oil mixtures, free water is the only 'phase' of water able to 'gravimetrically settle out'.
Water that is 'condensing' on tank walls, etc. is the result of the ~'end stage' of the equilibrium changes to the oil being SATURATED with water ---- the oil is now becoming **fully SATURATED** with water (molecular water, emulsified water, AND free water). The same physical laws of vapor pressure 'equilibrium' which is temperature dependent can reverse the equilibrium direction so that the relatively warm oil, etc. begins to lose it water content ... and the water (as vapor) begins to condense on adjacent cooler surfaces. The greater the water saturation of the oil the greater the equilibrium drive now in the 'opposite direction' if the adjacent surfaces, etc. are cooler (and according to the partial pressure of the water, etc.) ... and the water vapor in the oil NOW condenses on the cooler tank walls ... only to gravimetrically fall/slide back down into fluid until it settles on the bottom.
If you took the same amount of oil and poured into an open 'pan' and exposed it to atmosphere .... it would 'pick-up' essentially the SAME amount of water as if it were in a 'tank' exposed to atmosphere ... and there would be NO evidence of 'condensation' !!!!
The water that gravimetrically settles to the bottom no longer becomes a 'true' part of the mixture (a 'liquid-liquid phase boundary' forms between the oil and free water) .. and this influences to the sum of the partial pressures 'above' the phase boundary ... but leaving the oil (+water mixture) above the boundary free to re-equilibrate and accept vapor migration from additional (humid) atmospheric air in contact with the oil surface. .... and now what is occurring (again by equilibrium) could be described as a 'pump with out moving parts' being operated solely by physical & chemical 'equilibrium'.
Water that is 'condensing' on the walls of a tank is primarily a SYMPTOM that the oil is SATURATED with water - chemical equilibrium (partial pressure equilibrium) moved the water vapor in the atmosphere into the oil.
Dont want 'water' in your oil, .... keep the MINIMUM amount of oil in your tank, put a desiccant trap (silica gel, activated alumina, etc.) on the vent line
.... or if you have a 'vacuum rated' tank simply close the vent valve when the engine is off; the minimum mass/volume of oil will 'pick-up' the minimum amount of water vapor ... so DONT keep your tank 'topped-off' especially if you're not using the oil quickly !!!!! Its the MASS of oil in the tank and the sum of partial pressures (equilibrium) of oil AND water vapor that is causing the 'transfer' .... not a tank wall !!!!! Condensation is a SYMPTOM that oil is ALREADY becoming fully saturated with water.
In 'industry', ultrapure oils that have become saturated with water are typically placed into a vacuum chamber, a strong vacuum is applied (to change the partial pressures which reverses the equilibrium) ... and THEN, the oil is run through water absorbing 'filters' (filters that contain the starch hydroxymethylcellulose (the same stuff in "pampers") to achieve less than 1 ppm water content. If water saturation is your problem you can buy filters that contain a water absorbing starch (for gasoline fuels
) from the 'typical' suppliers.
Question? If your 'intuitive' condensation hypothesis are correct, then why don't EMPTY tanks (exposed to atmospheric air) automatically fill up with water ????? :-)
BTW- My 'fuel system' includes a constant recirculation filtration stage (including a free water 'knock-out' trap), the tank VENT line
includes a bio-blocking filter to retard **fungal spores**, and the vent includes a desiccant chamber to remove incoming water vapor down to -40 deg. dewpoint ....
I gots no water, gots no biological fouling, gets my 'filters' free but hardly ever change them, .... gots NO 'fuel problems' in a 100 gallon black iron fuel
tank ... and I never have to down into the bilge during a heavy seastate to change out filters .... and 'power-puke' at the same time. My system also has a 2 gallon 'day tank' that stores filtered oil ... if there is ever a 'problem' I can simply open the 'weir valve' and vent valve on the day tank and let the oil 'drain by gravity' down to the engine .... for about ~2-3 hours run time - time enough to 'motor-away' from any danger, etc.
Water enters oil/fuel by *chemical equilibrium* (the sum of partial vapor pressures of the 'constituents'); the MORE oil, the MORE water. Condensation is a *symptom* that liquid is now FULLY SATURATED with water. Empty tanks (with atmospheric vents) dont automatically fill with water.