The normal method of measuring effective humidity in wood, concrete, sand, etc., is by conductivity. Use a 1 kHz square-wave from the PIC through two stainless steel probes spaced a fixed distance apart. For wood and concrete, use needles. Measure the conductivity as an unknown resistance in a bridge and read it with an ADC input. The method is not a priori specific for water, but it can be calibrated on materials in a specific environment. The official method for moisture is to weigh the sand, cook for 4 hrs @ 125 C (=270 F), and weigh again. The weight loss is crude moisture. Conductivity is also used as a surrogate for other measurements, such as pH in pool water or plant soil. Tap water usually has a conductivity of 1-200 uS/cm. The water in the sand would be probably 100x larger than this, up to 20,000 uS/cm. Therefore the effective resistance of 1 cm apart probes would be expected to be 50 ohm or more. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causas scire" ================================================================