Probe Placement (Depth)

[CDougherty]

ASTM F2170 states that the RH probe should read the RH at 40% of the total depth of the slab (or 20% for a slab drying on two sides). While the standard probes are perfect for placement in the 1-1/2"-1-3/4" depth range to measure RH in a 4" slab, we often run into much thicker slabs. Are there other probes, or a longer reader to be inserted into a deeper hole, in order to measure the RH at 40% depth in an 8" or 12" thick slab? If not, how do you propose measuring the RH accurately in such a slab?

Thanks for the help.