Features: High accuracy, stable performance, long service life. Suitable for use in interior decoration structure, can detect the location of metal and wood in wall, can also detect other foreign bodies in the same object. The maximum-detection depth is 19mm below- the surface- of the uniform object, and the detection range- error is 3mm. Test it several time-, and if it all sounds an alarm. That is, there may be supporting colum underneath- middle area. When there is no metal or other foreign object underneath-, it will not flash and sound, for accurate test result, can be test several time-. The instrument is placed on the surface- of the test, press power-switch, at this time- the first- indicator red light and green light on, and issued "di-" sound, and then the red indicator light-out, indicating success. When sampling, the first- red indicator light is-on and emits short "didi-" sound, indicating that the sampling is unsuccessful. Specifications: Material: Plastic Battery: 6F22 9V Battery(NOT Included) Measuring Depth: Approx. 20mm Size: Approx. 145x62x25mm/5.71x2.44x0.98in Note: The battery in NOT included. Please allow 1-2cm errors due to manual measurement, make sure that you do not mind before you order. Due to the difference between different monitors, the picture may not reflect the actual color of the item. Package includes: 1x Wall Scanner