Battery Slide the battery cover off as indicated. In this Articel you can find all the neccessary information for changing a battery on you power2max powermeter. Please ensure the correct polarity. There are four possibilities the indic tor may show, full, with 2 blacks, with 1 black and empty. To replace the batteries, if the unit is not. OPM interface: insert the fiber to be tested, test the optical power. REF/dB key: Short press the dB to switch unit, click once nW/dBm/dB to enter the upper clear data, press and hold until REF is displayed on the screen, and set the current optical power as reference value, enter the relative. at -22 (or 25 with tone on)).