General Tools MP905 Deep Wall Moisture Sensor The General Tools and Instruments MMD905 Deep Wall Moisture Sensor is an accessory for General's MMD900 and MMD950 Deep Sensing Moisture Meters The MP905 extends the measurement depth of the instruments to 5 inches, the length of the MP905's twin probes. Like the pins on the MMD900 or any pin type moisture meter, the twin probes sense the resistance between them and convert that resistance to a voltage Circuits inside the MMD900 and MMD950 read that voltage as a proxy for the moisture level of the underlying wood or building material Utilize this attachment for measuring the moisture in deeper surfaces like walls, thick wood and other industrial tool applications