t a t i o n A Montreal, Que. CANADA, H3C 3.47 I I NTRODUCT I ON The p r e c i s e measurement of the p e r m i t t i v i t y and moisture content i n granular products has been d i f f i c u l t t o achieve i n the past mainly due t o u n c o n t r o l l a b l e v a r i a t i o n s i n the geometrical packing f a c t o r of the granular m a t e r i a l . T h i s paper presents moisture r e s u l t s obtained by computer-aided measurements of the q u a l i t y factor and the resonant frequency o f a microwave c a v i t y containing a group o f three t o s i x t e e n g r a i n s o f the t e s t product. The t e s t g r a i n s are suspended from a g r a v i m e t r i c balance and are d r i e d over a short p e r i o d o f time i n s i d e the t e s t c a v i t y by microwave power. The bone d r y weight o f the samples used t o c a l c u l a t e the moisture level i s accurately determined i n these measurements: a sudden change occurs i n the d i e l e c t r i c losses o f the t e s t sample when the g r a i n s begin t o char as combustion i s j u s t i n i t i a t e d . I n the case o f granular ceramic products ( z e o l i t e s ) , more s t a b l e r e s u l t s are obtained. I1 METHOD AND MEASUREMENT SYSTEM A microwave system u s i n g close-loop techniques measures simultaneously the weight and the complex d i e l e c t r i c constant o f sample m a t e r i a l s heated by strong e l e c t r i c RF f i e l d s i n s i d e a resonant c a v i t y . The uses o f a micro-computer permits t o r e l a t e , i n real time, the moisture content of sample m a t e r i a l s exposed t o RF f i e l d s t o the dynamic changes observed i n the complex d i e l e c t r i c constant a t the test frequency [ l ] .F i g u r e 1 shows a b l o c diagram o f the measurement system. I t i l l u s t r a t e s how the sample weight i s monitored w i t h automatic balance and i t s complex d i e l e c t r i c constant measured as the sample m a t e r i a l undergoes weight and p e r m i t t i v i t y changes produced by strong RF f i e l d s i n s i d e the microwave c a v i t y . The resonant frequency and the q u a l i t y factor o f the c a v i t y are r a p i d l y measured by an a c t i v e frequency technique already reported [ 2 ] . I n the present system however, a 20-W TWT a m p l i f i e r i s used so t h a t a large signal can be fed i n t o the c a v i t y . Such measurements are o f extreme i n t e r e s t i n the study o f "hot spots" observed d u r i n g i n d u s t r i a l microwave heating and d r y i n g systems. The RF s i g n a l s are generated a t the nominal ISM ( I n d u s t r i a l , S c i e n t i f i c and Medical) frequency of 2450 MHz, i n s i d e the c a v i t y . The measurement system i s completely operated w i t h d i g i t a l c o n t r o l s of HP m i n i computer 9816. I t c o n t r o l s d i r e c t l y the IEEE 488 bus f o r instruments and c a l c u l a t e s the percent moisture from the sample weights and the m a t e r i a l p e r m i t t i v i t y from c a v i t y resonance frequency and q...