So there is a annual calibration for E-perm? You say it's included in the equipment. Does that mean there is no charge for the calibrations? John, You send the reader and the references to Rad Elec annually for a certified check of the calibration. There is a fee charged for this. The reference electrets give you the ability to keep a check on the calibration throughout the year. These are included with reader. So, if for some reason the reader becomes faulty, you are not taking inaccurate readings for the remainder of the year. You would send it in early for repair/recalibrating. The E-perm, although it does not give hourly readings, it still gives a total integrated calculation for the entire time of the test. Like Scott said, there is no requirement for power at the test site. Scott, Tamper resistance is accomplished with zips, tamper tape, positioning, etc. There is also a temp./humidity device available that inserts inside the test box. I like the SN too. It would be my second choice.