A moisture meter is used to measure the moisture of a certain substance. There are two main reasons for using a moisture meter. The first is testing soil, which can be very important for various types of farming, since moisture levels need to be maintained at certain levels throughout the year based on the crop and growth desired. The second reason pertains to sealing or painting a surface like wood, concrete, or fibreglass. If moisture has entered the material then sealing it can be destructive until the surface has dried out completely.
If you want to create a soil moisture meter, you will probably need to make a two-pronged electrical resistance measurer. This allows you to stab two metal prongs into the soil and attach the ends to an ohmmeter, which measures electrical resistance. You can easily make this system by using two nails (or other strips of metal) suspended about 15 millimetres apart from each other in a plaster of Paris solution (or a similar, inert substance), with the prongs open at the bottom so they can be plunged into the soil. A drinking straw or similar device works well as a mould and casing for the plaster and the metal prongs.
Fortunately, you do not need to know exactly what ohmmeter readings indicate high or low soil moisture content--not at first. You can simply calibrate your system by testing the meter when it is completely dry, and then when it is immersed in water. These provide the 0 and 100 per cent markers for your moisture tests.
Measuring the moisture content of floors or siding is more difficult. You are usually better off buying an electrical moisture meter rather than trying to make one for these projects, although your metal-prong device may work for wood or carpet readings. When it comes to thick surfaces like fibreglass, there is not a very reliable homemade option.
Concrete, fortunately, is a more porous substance and you can perform a simple test to see if it is dry. Take several pieces of poly film or similar material and duct tape them down at various locations on the concrete slab, making sure there are no air pockets. Leave them there for one to two days and then peel the duct tape back. If the concrete is at all damp or darkened with water content, there is too much moisture for sealing or other projects.