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Moisture meters are ineffective or outright useless when used with bonsai soil. They can provide inaccurate or even opposite readings when used with bonsai soil.

The meters work by measuring electronic conductivity between soil particles and the moisture between them. Since bonsai soil is supposed to have relatively large irregular granules, the moisture between them can make moisture meters intended for use with finer potting soils always read "dry" even if the soil has just been watered.

In short, don't waste your money on a moisture meter for bonsai. Use the "chopstick" method to measure moisture. It's cheaper and more effective. Simply insert a chopstick, or any other ligthly colored sliver of wood, about two inches into the bonsai soil. When you think the tree needs water, lift the stick out. If the stick is dark or wet an inch or so down, wait another six hours or so...