[Greenbuilding] Measuring DesiccantHumidity

[conservation architect]

I am interested in dabbling with desiccants this summer to dehumidify in the 
house and dry externally with a heat source.  This is primarily a learning 
exercise more than proposing a specific apparatus.

Can you recommend a device or method to to measure relative humidity in the 
dessicant.

My plans are to purchase bentonite clay.  ASHRAE says a desiccant will 
absorb moisture until it meets the same relative humidity as the ambient 
air.  Therefore the quantity needed has to be sufficient to absorb in the 
range of relative humidity I might anticipate.  If I am dehumidifying 75F, 
80% RH to 75F, 50% RH, The dessicant must be a lower RH, say 20% to increase 
to 50% relative humidity.  Of course the real amounts will depend on the 
relative size of each.  Those numbers would work if both desiccant and air 
hold the same amount of moisture at saturation.

Eli