When isolating or ventilating a permit-required confined space, it is critical that a competent person on your team take meter readings with a 4-gas monitor for oxygen (O2), toxic gases and if applicable, combustible gases (VOCs) with a PID to determine if the processes have been effective. Of course you already know this if you or your company are performing this type of work. But are you measuring for the correct PEL levels?
PEL limits change with some regularity, and what may be right one year, may not be legal the following year. Further what is a legal level in Arizona may not meet the Cal-OSHA standards in California. As an example, in 2010 over 200 regulation changes were proposed for existing federal PELs and 164 new substances were proposed that hadn't been regulated in the past.
The suitable levels for each of the atmospheric hazards should be clearly listed on the confined space entry permit and it is the Confined Space Administrator that is obligated to know if any changes in the standards have occurred. To check the level of the specific chemicals you will be dealing with, you can search the OSHA PEL webpage. Additionally RAE Systems has a very good chemical PEL reference page that provides TWA (Time Weighted Average), STEL (Short Term Exposure Limits), and IDLH (Immediate Danger to Life and Health) generally given in parts per million (ppm), or sometimes in milligrams per cubic meter (mg/m3) for every many of the chemicals out there.
New gas monitors will always have the most up to date federal levels programmed, but these levels can and should be changed manually if there are changes in the laws or if your state has different requirements. If OSHA PEL or Cal-OSHA levels change, contact the manufacturer of your monitor for assistance or consult your manual for instructions on how to change the alarm levels.