[PIC] How to measure level of liquids and sense relativehumidity?
Robert A. LaBudde email (remove spam text)
<x-flowed>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
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: lcfltd.comral
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"
See also: www.piclist.com/techref/microchip/devices.htm?key=pic
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