U.S. Patent No. 6,475,435, directed to an over-pressurized sterilization tunnel for food packaging.  “Sterilant concentration levels” of the claim is at any point in the tunnel rather than just in the volume of pressurized gas (pages 4-5).  The specification mentions levels of gas and residual concentration on lids of bottles (page 5).  There is nothing in the claims limiting the levels to gas and not residual on lids, so the broadest reasonable interpretation includes levels in gas and residual on a surface (page 5). 

Hindsight: Alternatives in the specification are good for broadening meaning, which is what happened here. For dealing with prior art, narrowed claims are good.  Where a measurement is involved, it may be best to include dependent claims specifically to the likely boundaries of the measurement.