Why can't a slab be 100% airtight and watertight ?

I believe you stated plastic: "IS NOT USED IN THE CONSTRUCTION OF A PRESSURE VESSEL"! I beg to differ!

I believe a common application which a layman can relate to the use of plastics in the construction of a pressure vessel, is an automotive radiator. You will find that the radiator in an automobile of many vehicles have plastic sides designed to resist 2 atmospheres pressure at elevated temperatures in excess of 120 degrees C. (248 degrees F.) These components use a labyrinthical crimped seal to achieve pressure vessel capability.

Many of the General Motors products utilized this design. I would be pleased to send you a photo of a failed Fiero GT radiator that exceeded design temperatures, being replaced by an all metal custom designed/constructed Aluminum unit.

Just my learned experience!

 

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There is a DGG blister pack style holder mentioned in this topic...
http://www.cointalk.com/threads/poland-graded-1896-morgan-dollar.111874/page-2#post-1785143

Wouldn't that type of idea have the potential to be waterproof and airtight? Has anyone purchased an item in a blister pack and put it in water?

 

You're splitting hairs with the airtight. Watertight would do. It would be airtight enough.

 

I believe you'll find that because of high water surface tension and the relatively minimal effects of this solution, which generally hasn't the detrimental oxidizing et al effects of vapors. the sealing capability should be such to exclude vapors.

I would have been very pleased if my past design/fabrication customers requiring enclosures for noxious, toxic, explosive in any oxygen concentration [e.g. C(2)H(4)O meets all of these conditions], required only water-tight construction. I believe that oxidizing agents provide greater harmful effects to coins than a surface deposition agent.

JMHO

 

I'm actually happy with my stapled cardboard holders and for better coins, air-tites.

The butcher that I am...

 

Quick side note. The plastic end tanks on either side of my truck's radiator started leaking last week forcing me to replace it. Weird. I digress.

I apologize for not following up with more information (its kinda technical and doesnt make for the greatest read) on the concepts of pressure, vacuum, air tight and water tight. To imrich it sounds like you may have a different primary language so hopefully what I am putting down you can pick up. First, you are correct about the use of plastic side tanks on car radiators, a practice begun for, what else, to cut costs. So you are right that metal and some type of plastic can be bonded to one another in such a manner that the points of contact would not leak under pressure... BUT you and I are talking about two different things. Yes if you drive your car around for awhile and (DONT DO IT!) decide for some reason to take the cap off of your radiator while the engine is still hot... you will be greeted with a trip to the burn ward. So while there is most certainly pressure inside the radiator you must remember antifreeze (glycol) circulates by means of a water pump. This is a dead give away that a vehicles coolant system operates at relatively low pressures. If I emptied the system, plugged the opening and the drain and added compressed air up to 500psig, would the radiator leak? If I pulled a vacuum on the radiator, a deep vaccum between 200-500 microns, would the radiator pass a standing vacuum test? It sounds like you have at least thought about this whole issue and that you may perhaps be aware of different methods and materials than myself so I would be curious to hear your response. Thx

 

I believe we both know the answer to your hypotheses, and we apparently differ in semantics, as I believe a car radiator would be considered a liquid pressure vessel, albeit not an ASME rated (or even OSHA rated @ 15 psig) vessel. A car radiator isn't designed for diffusion pump or elevated (i.e. >760mmHg) gaseous pressure containment applications. I believe however, it is designed for saturated steam containment.

I believe that with minimal design modifications including hermetic sealing with barrier plastics, your definitive criteria (OSHA standard) for a pressure vessel could be met.

JMHO