It seems that this topic is only directed to an above level concrete slab w/pan? It is somewhat silly to even do an RH reading of this type slab. Moisture levels in these above level cement poured substrates will continently fluxuate to unknown severities depending on what is happening environmentally below. To try to find a liquid application of a vapor barrier, based on the maximum tolerances of any product is ludicrous. All above crete type slabs must be sealed with a barrier which should meet or exceed the permeation rating of the 6mil plastic sheet. That of lesser perm value is rather a gamble in any sense.