When a tank or compartment has a deducting component, what
permeability should I use to get the a given net permeability of,
say, 0.85 ?
In order to arrive at an overall permeability of 0.85 the deductions
would normally be 0.85 also.
The default permeability for additional components (adding or deducting)
is the same as the permeability of the first component. This is
true whether a component is produced directly or by reference to
the component of another part.
The paragraph in the manual which reads "If the component is
a deducting one, it still is assigned a permeability; however in
this case ..." is only an apology for using the term "permeability"
for a deduction when in fact it would be more logically called "impermeability".
It was not intended to imply anything about the use of any particular
There are two ways that permeability can be used:
1) The formal, regulatory, or customary permeability values are
used without further consideration of the actual structure. This
is what we have been considering so far.
2) Calculation of the volume occupied by structure, machinery, fixed
cargo, etc. leads to an average permeability for the space. In this
case, the permeability used for a deduction will depend on the nature
of the object being deducted. If it is a smooth tank which does
not overlap any of the structure already deducted from the overall
space, then its permeability should be 1.0. It is in effect reducing
the molded volume but not the structure volume. Therefore, the average
permeability must decrease as a result of the deduction.
The PERM command in the GHS Main Program is a convenient way to
check the average permeability of a tank. It can also be used to
set the permeability of a tank (assigning the same permeability
to all components). Thus,
PERM (ENGRM.C) = 0.85
insures that ENGRM.C has the permeability of 0.85.