Honorable Badon-

I don't like to keep bringing this up, but your inaccuracies must be corrected. Your quote of 14.7psi at seal level is for normal atmostpheric pressure, not vacuum (subatmostpheric) pressure. So even without any vacuum involvement, per your calculations, our fragile TPG slabs should surely be exploding at this very instant. But I have one in front of me...and it's not. Also, you state yourself that pressure is a measure of force *per unit area*. You can calculate the total force all you want; it is certainly a worthy arithmetic problem. However, confusing total force and pressure is not allowed. In this universe, pressure is the important variable, not total force. The physical vacuum (subatmospheric) pressure involved in sealing a slab would have no crushing effect whatsoever; even if the slab was sealed hermetically, the force exerted would actually be one of expansion (or 'negative pressure,' as much as I dislike that term); but they're not hermetically sealed, so this is a moot point-->no pressure gradient equals no work done to slab. The only new source of force or potential energy on the slab would be from the shrinkage of the vacuum seal wrapper. I've never actually checked this myself, but I feel it's a safe bet to assume millimeter thick mylar bags will rip long before the slab will crack. Ultimately, it all comes down to permutations of Ohm's Law. Bottom line: unless you are utilizing a specialized academic laboratory- or industrial-level vacuum sealer, the slabs are safe.

As I think about it, the 'outward' vector forces on the vacuum-sealed slab would actually be ameliorated by the 'inward' vector forces of the shrinking wrap. The stability and durability of the vacuum-sealed slab would likely be substantially higher than an untreated one. It's possible even a synergistic (and not just additive) effect could be observed. Anyone have a vacuum sealer and a hammer? I would be happy to volunteer two slabs for a test.

Yes, you are mostly right in your thinking about the balance between inward and outward pressure. However, that only applies to fully balanced forces. Since a slab has projections (ridges, etc), the plastic of the vacuum sealer stretches out over those projections, leaving areas that do not have balanced forces. There will be areas of the slab that will be loaded with much larger forces than my simple calculations demonstrate, just like the supports of a tent are loaded more than the fabric of the tent itself.

Total force and pressure can be the same things, depending on how you calculate "total force". Pressure has no meaning without area. You may have meant "net force". Since the inside and outside of the slab do not have the same pressure, the plastic faces are loaded with force according to the calculations. But, since the pressure inside is not zero, my calculations may not be accurate. But that does not mean my calculations are too high, it only means they're not a complete model of all the forces on the slab.

When you account for projections, the forces might be higher than my calculations indicate. More advanced mathematical modeling would be required to produce more accurate numbers (FEA). Also remember that there is no such thing as negative pressure in normal physics - there is in advanced and theoretical physics, but it's not relevant in this circumstance.

In short, the only thing we can be certain of is that there are forces on the slabs. These forces could be quite high, depending on the interacting geometry of the plastic vacuum sealer sheeting, and the slab. It also happens to be that the sonic sealing on the slab is a variable that can't be accounted for. So, it may not actually be possible to calculate whether the vacuum sealer puts enough force on the slab to exceed the yield strength of the sonic seal.

My solution? Don't bother calculating any of it - the risk is unnecessary, the reward is close to zero, so just use normal plastic bags (with balanced internal and external pressure) instead of a vacuum sealer. No need for calculations, and all forces are balanced, so the net force caused by the bag will be zero. And you still get a seal.

Unless you plan to drill a hole in the slab and suck the air out, a vacuum sealer won't help, but could hurt.