THE ELECTRICAL RESISTIVITY METER 

 IN FISHERY I^A^ESTIGATIONS 



INTRODUCTION 



An electric resistivity (or conductivity) 

 meter is an instrument which deserves wider 

 use in fishery research and management be- 

 cause of its multiple applications. An inexpen- 

 sive, portable meter has been employed in 

 research on southeast trout streams in the fol- 

 lowing ways: (1) in making easy and rapid 

 estimates of the total dissolved solids in streams; 



(2) in facilitating the selection of proper AC and 

 DC electrofishing gear for various waters; and 



(3) in measuring the velocity, stretch-out, dilu- 

 tion, and effective range of a solute over miles 

 of a stream in conjunction with chemical recla- 

 mation operations. 



There are a number of new or improved 

 instruments on the market which were developed 

 for industrial, soils, and limnological work and 

 are lightweight, compact, line- or battery-pow- 

 ered, vacuum tube, Wheatstone bridges. In- 

 cluded are instruments which are applicable in 

 fishery investigations in the laboratory and field. 

 Models are equipped with visual or audio in- 

 dicators or both. Some meters indicate 

 resistivity in ohms, microhms, or megohms; 

 others indicate conductivity in mhos, micromhos, 

 or megmhos . Conductivity is the reciprocal of 

 resistivity and it is of little consequence whether 

 measurements are obtained in ohms or mhos 

 since they can easily be converted from one to 

 the other. 



The resistivity meter thaj: we have used 

 during the past four years on trout streams is a 

 Model RC-7, portable, battery-powered unit 

 manufactured by Industrial Instruments, Inc. 

 This instrument with its 6 -volt A battery and 

 108 -volt B battery weighs 25 pounds. It is 

 equipped with a "tuning-eye" null indicator 

 which permits readings to be made quickly and 

 easily. It measures resistivity (resistance in 

 ohms of a centimeter cube of any substance) 

 over a range of . 2 to 2,500,000 ohms and its 

 rated accuracy range is -»■ 1 percent of the read- 

 ing except at extreme ends of the scale. Immer- 

 sion electrodes for liquids axe incorporated 



into a flexible tube dip cell and the model used 

 on clear, soft water trout streams has a cell 

 constant of . 1 . Any instrument, however, 

 capable of measuring resistivity or conductivity 

 over a wide range with reasonable accuracy 

 could be used in fishery work. Most have the 

 advantage in that only a few moments are needed 

 to obtain a reading. 



METHODS 



The measurement of resistivity or con- 

 ductivity with a suitable meter is a simple 

 procedure but one which varies somewhat with 

 the type of meter. Specific instructions are 

 provided with meters and they should be followed 

 to insure accuracy in measurements. Since our 

 meter indicates resistivity of water samples, we 

 have elected to express all results and inter- 

 pretations in terms of resistivity (specific 

 resistance) rather than convert measurements 

 to the reciprocal, conductivity (specific conduct- 

 ance). 



The temperature of the water is taken 

 and recorded each time a measurement of re- 

 sistivity is made since it influences resistivity 

 significantly and in an inverse direction. The 

 relationship between the two is considered linear 

 for all practical purposes and amounts to a 2- 

 percent decrease in resistivity with each centi- 

 grade degree increase in temperature (Gustafson 

 and Behrman 1939). It is desirable therefore iu 

 comparing the resistivities of various waters to 

 correct the actual measurements to a selected, 

 standard temperature. It is common practice 

 in limnology and in industry to correct measure- 

 ments of resistivity or conductivity to 77° F . 

 (25° C .). Haskell (1954) presented a formula 

 for the correction of resistivity at one Fahrenheit 

 temperature to another as follows: 



R 1 _ 4 -I- T2 in which R = resistivity 



4-1- T, 



T = temperature in degrees 

 F. 



