Doesn't look like anyone else is going to reply so I'll give you my two'pennorth
These cheap 2 pin meters may give you a slight indication whether the surface is dry enough for a sealer coat. However they can give you eronous readings. This can be due to salts contamination of the material your're testing or a high carbon content. You would also have to interpret the readings to decide what they mean.
The correct way to measure the moisture coming off a surface, which is what you require, is to use a Hygrometer. This measures the amount of moisture in the air. This is called the Equillibrium Relative Humidity. This is the critical measurement which tells you if a surface is suitable for a particular coating. The measurement is taking in the air above the surface of the material by using a humidity box or by taping a square of polythene over it. It should really be left in place for 72 hours before a reading is taking. Normally the reading needs to be below 75% although some coatings specify 65%.
This is all a bit scientific for your needs. There is plenty of info on the net and York Survey Supplies do a resonable meter for around 100 quid.
Of course you can avoid all this by simply using a water based coating.