In "Letter Re: Advice on Firearms Caching", Mark J. wrote "Should I simply use a Hot Hands hand warmer inside the mylar bag and then another one inside the PVC tube? I should not have to worry about moisture if it is vacuum sealed? right? "
Well, no--regardless of the chemicals in the heater. Putting any temporary heat source in a sealed container may actually cause corrosion or water damage that wouldn't have happened before.
This is why:
Heating air does not remove moisture from a confined environment; it simply increases the air's ability to absorb moisture from other objects in that environment. That sounds exactly like what we want--except, this only lasts as long as the air stays warm. If the warm, moisture-laden air isn't moved out of the environment, when that air cools back down it will no longer be able to hold the extra moisture, and the moisture it was holding will condense back out of the air--probably as droplets on the surfaces within the container. The galvanic action that causes corrosion is especially strong on the edges of formed water droplets, and is often why we see pitting of metal surfaces.
When using heat to remove moisture, either the heat must stay on, or the moisture-laden air should be able to circulate away from the item(s) being protected before the air can cool. Folks often think of the warming dehumidifiers used in gun safes--these work for two reasons: much of the warm, moisture-laden air is circulated out every time the door is opened, and when the door stays closed, the heater keeps the environment constantly warm.
These principles are true for any sealed environment, whether its a PVC tube or a CONEX shipping container.
Thanks, - Britt (A Mechanical Engineer with experience in the HVAC industry)