Question QA012 :
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 ?

Answer :
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 permeability values.
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.

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