This software is now included for FREE with RHT10.
Extech RHT10-SW  

RHT10-SW: GPP (g/kg) Software for RHT10

Calculates Grains Per Pound (grams per kilogram) up to 1120 GPP (160g/kg)

The Extech RHT10-SW is Windows®-compatible software for Extech model RHT10. The software calculates water vapor in air in grains per pound (gpp) and in grams per kilogram (g/kg).