Thanks for you comments.
Although the moisture is in very low concentrations I am still interested in what happens to it as the equipment I am designing must not suffer from icing over an extended period (2 - 5 years. To prevent icing I intend to place the dewar in a very dry environment (-60 to -70°C dewpoint)
At this level the water in the air will be approx 2vpm.
I have done some tests at much higher dewpoints (-30°C) observing the icing rate and have extrapolated to estimate the required DP for extended use.
Another tricky problem is moisure vapour travelling up the nitogren vent connected to the sealed dewar.
I intend to prevent this by having a long small bore tube to maximise the gas velocity. I am trying to calculate the vapour flow up the pipe under static conditions using Fick's law and then intend to estimate the mass flow rate of nitrogen required to maintain equilibrium.
Does anyone know the diffusion rate of water vapour in dry nitrogen? Or indeed a better way of approximating the required dimensions of the vent pipe?
Are there any surface coatings that inhibit ice build up (or at least adhesion) and what is "acunolate residue"?
All help gratefully received
