Proof Level from Dusts (Indicated by the first X)
0: Without protection
1: Free from large solid objects
2: Free from medium-size solid objects
3: Free from tiny solid objects
4: Free from tiny solid objects whose diameters are bigger than 1mm
5: Preventing the gathering of harmful dusts and motes
6: Free from dusts and motes completely.

Proof Level from Water (Indicated by the second X)
0: Without protection
1: Free from negative influences when water drops hit the outside of the instrument.
2: Free from negative influences caused by water drops when the outside case slants to an angle of 15 degrees
3: Free from negative influences caused by water or rain from an angle of 60 degrees
4: Free from negative influences caused by liquid splashes from any directions.
5: Free from negative influences when flushed with water
6: Can be used in such circumstances as ship cabin.
7: Can be soaked deep 1m in water for a short time
8: Can be soaked in water under certain pressure for a long time.