could it be a simple calculation of:
stp = 15psi; What is "stp" ?
1/150psi= 0.0066psi Yes, so what?
a chart i found implies that 0.0066psi is between 750 - 100 microns, What? That is nonsense
There is no relationship between PSI and Microns.
PSI is Pounds per Square Inch, a unit of pressure.
A Micron is on millionth of a Metre, a unit of length.
There is no equality between the two.
As for whether an air tank can withstand a vacuum, it is a bit difficult to say.
But as a 100% vacuum is no more than 15PSI and the tank is rated to 150PSI, it is a risk worth taking, the worst it could do is implode.
Having said that, I would not want my fingers near to it when it did implode.
The stresses involved in a vessel under vacuum are somewhat different from those on a vessel under pressure, it needs someone with a bit more stress experience than me to analyse it.
As for cutting a hole for a viewing port, that will raise the stresses quite a bit.
As for an adhesive to hold on a plexiglass window, I would suggest silicone rubber, something with a bit of elasticity.
The window should of course be placed on the outside of the tank so that air pressure forces the window onto the tank tightening the seal when there is a vacuum.
JimB