An ultra low frequency bridge for dielectric measurements

Abstract

The bridge desc ribed is capable of measurin g t he p arallel capacitance and resista nce of dielectric specim e ns ill t he fr eque ncy r a nge of 0.008 to 200 cps. It employs no ear thing device a nd is directly con nected to a t hree termin al oscillator. The su bstit ut ion m ethod is employed. The capabilities of the bridge ar e experime ntall y tested by measure men ts upon known capacitors a nd resistors and by co mpa riso n with results obtained usin g a Schering bridge near 100 cps. Th e apparatus is capable of acc uracies previously unattained at these Jow frequcnci es. Cap acita nces between zero a nd 100 pf may bc measured to an acc uracy of ± (0.05 %+ 0.00 2 pf) at frequencies " bove 5 cps. Below 5 cps t he acc uracy becomes ± [0.05 %+ (0.002 + 2 X 10 5 /jR n)pf], wh ere f is the freque ncy i n cps a nd RD is the equi valent resistance in o h ill S ac ross the detcctor te rmin a ls. Co ndu ctances between 10-9 a nd 10-15 mh os m ay be measured to an accuracy of abo ut ± (1 %+ 3f X 10-15 mhos) when J~O.l cps. \Vhen f < O.l cps the accu racy is abo ut ± (l %+ 2 X lO-IO mhos). The dielect ric constant « ') of a spec im e n m ay be determined to an acc uracy proportio nal to that of capacitance measu re me nts. The loss ind ex «") may be determin ed to a n accuracy of about ± (1 % + 5 X 10-4 /C ,) when f ~0.1 cps and ± (1 %+ 3 X 1O-5/jC ,) when .r< 0.1 cps. H 1'e C , is t he vacuum capacita nce of t he specimen ex prcssed i n picofarads. Th e eff cct.s of stray impedances shunting the bridge ratio a nns, a rc investigated. Useful modi fi cations of Lho bridge are disc ussed.

Electromagnetic moisture measurement

“…Low-frequency frequency domain techniques to measure the complex permittivity of materials are often based on refinements [302][303][304] of sensitive methods for impedance compensation as originally designed as, for instance, Wheatstone [305,306], Wien [307], Schering [308], and Scheiber [309] bridge circuits. The principle of operation at the basis of all bridge methods is illustrated by Figure 54.…”