Remove the battery cover and insert the 6F229V laminated battery into the battery compartment. Pay attention to the + and - polarity when installing. If there is no reverse, the display will have no digital display.
It can be carried around the coal mine.
3.After the instrument is turned off, the power must be turned on again to the atmospheric calibration. The instrument was used for a period of time and it was found that the instrument showed less than 21% after it was introduced into the atmosphere. Then you need to recalibrate.