GB625024A - Improvements in or relating to instruments for measuring moisture content in wool and other substances - Google Patents

Abstract

625,024. Resistance measurements ; moisture content measurements. MARCONI INSTRUMENTS, Ltd., BROCKELSBY, C. F., and TODD, W. A. H. April 15, 1946, No. 11544. [Class 37] The resistance R of certain hygroscopic materials, notably wool, cloth, timber and tobacco, varies at constant temperatures substantially in accordance with the expression log. R= A+B/m<SP>1</SP>/ 3 , wherein A and B arc constants depending on the material and the structure of the electrodes between which the material is placed for measurement and m is moisture content. If the material to be tested is a conductor at zero moisture content the relation is log. G - G 0 =A + B/m, where G is total conductance (i.e. with moisture present) and Go is inherent conductance. A moisture measuring resistance network according to the invention is so designed that adjustments therein may be effected to satisfy the constants A and B and such that one moisture scale serves for a range of materials. The moisture meter produces an indication proportional to log. R-A or in the case of inherent conductance log. (G-G o )-A. It is preferred to avoid maintaining constant temperature and under these conditions the relationships are log. R=A+B/Tm1/3 and log. G-G 0 =A+B/Tm1/3 where T is absolute temperature ; the indication is then also proportional to T/B. A preferred embodiment is a double bridge network, Figs. 2 and 2a (not shown) which is so constructed as to wholly electrically satisfy A and B and provide automatic correction for T and is as disclosed in Specification 625,023. Fig. 1 shows an alternative electro-mechanical arrangement in which the test specimen between its electrodes constitutes the arm R1. The arm P1 is equal to the third arm Q1 multiplied by 10A and the balance arm S1 is a logarithmically variable resistor. The adjustment shaft of S1 drives an indicating pointer N through a variable velocity drive VR which is set to the value B. The scale MS is moved automatically or manually so that it is displaced in proportion to the temperature T. A suitably shaped cam CD is interposed in the drive to give a log. m scale. If there is inherent conductance Go in R1, a balancing resistance is shunted across S1. Self-balancing bridges may be employed.