Nitrogen and Ambient Air

I agree, you should pull a vacuum on the mast before you put in the nitrogen. As you correctly suspect, you're trapping moisture in the mast currently, the nitrogen will do nothing more then keep dirt and contamination out. But what is in, stays in.

That said, when you pull a vacuum on the mast, what will happen is the act of pulling the vac will dry out the mast. Moisture hates a vacuum and cannot survive. So if you have moisture it may take up to 8 to 16 hours (depending on the diameter of the mast) to fully vacuum the interior. If there is water in the mast, it will take even longer..

But your question was should you pull a vac before inserting the nitrogen, most certainly.

Yes on the vacuum. Do you think it would be wise to maintain a slight positive pressure of N₂ after vacuum/purge/fill?

YES ! That IS the solution! Talk to any telephone lineman. See my comment (important) to Labyguy in Charlotte.

You are incorrect; nitrogen DISPLACES moisture.

Pumping down on the void of heliax cabling seems like a good idea, but only if you have perfect seals that can take a pressure difference in both directions. The seals in a heliax cable expect a slight positive pressure. You will likely just be drawing in air from the wrong direction and thus make poor seals for when you re-pressurize. Instead I would first purge the installed cable with nitrogen flowing in at one end and an open relief valve at the other. Then seal the relief valve and maintain a charge to the cable with nitrogen from a cylinder. If you add a simple bead flowmeter (Dwyer makes some nice ones) you can assure a complete purge by knowing the amount flowed through the cable and the volume of the cable. Also the flowmeter will help you to detect a leak before the bottle discharges.

Excellent/ perfect. I obviously agree.

Yes, purge w/nitro, pull vacuum, break vacuum w/ nitro and hold nitro if possible. I'm assuming you can hold nitro if you can pull a vacuum.

A few additional thoughts. If you get the mast off the ground, it won't get as cold at night and less moisture will condense as liquid. There is nothing magic about nitrogen. Dry air works just as well. So if you get an air dryer and flow dry air through the mast, it will keep your dew point lower than the air outside the mast. If you have spare electrical power, you might heat the air inside the mast. This will lower the relative humidity. The most cost effective solution will depend on how well the mast is sealed.

You could also consider building replaceable silica jell cartridges into your cable duct once you have sealed the duct. You can get moisture indicators behind glass windows to monitor the moisture level. The cartridges can be re-energised easily by cooking at the recommended temperature. You would always be sure of the humidity level if any by simple observation.

This mast or tube, how well is it sealed where the cables enter and leave? Is it capable to keeping a sealed dry internal nitrogen atmosphere? If it cannot be sealed you need a cable capable of being wet run for it's life.

These cables, are they what is called hard line or heliax, or are they the common satellite black vinyl RG6, 8, 9, 59, 59, 212,213 etc?

Some of these are double insulated, some now.

Many of these are only good for 4-5 years in a moist environment, so they might have already exceeded their usable life. You can buy long lived RG cable, Teflon etc, $$.

I often see satellite installations 10 years old with RG59 or the like, 10 years old in a buried garden hose that has degraded badly over that time. Usually replacing the cable with a waterprrof one does the trik.

Drill down in this search

http://www.google.ca/#hl=en&source=hp&q=%22rf+cable%22+%2Bwaterproof&btnG=Google+Search&meta=&aq=f&oq=%22rf+cable%22+%2Bwaterproof&fp=4bd26c1f505ba07b

Absolutely! The purpose of the vacuum is to eliminate moisture laden air and replace it with a dry gas, like nitrogen. The gas doesn't need to be under pressure. It can be slightly above atmospheric pressure, but not much. Pulling a vacuum is the only way to fully evacuate moist air. The nitrogen should be at 100% concentration; no air.

Some responses to this post imply that the identity of the gas inside the mast are important with regard to moisture issues. We should understand that if the internal gas obeys Dalton's law, the only critical factor is the partial pressure of water vapor. The remaining gas can be nitrogen, oxygen, carbon dioxide, helium, any gas mixture, or even a vacuum. The only thing that counts are the number of water molecules inside the mast. It doesn't matter what other species the water molecules are mixed with, or whether there are other types of gas molecules in the mast.