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Electrical Fields from Model Anode Arrays

Electrical fields around electrofishing anodes are critical to fish capture effectiveness. The size, shape and intensity of those electrical fields are determined by the anode design and deployment in the water as well as by the electricity applied to them.  There were two main questions to answer in this little study: (1) Could accurate electrical measurements be made from approximately ¼ scale model electrodes in a small body of water, and (2) Would those measurements provide useful information about the effect of anode ring size on their electrical fields?

For this study, model anodes were constructed basically in the shape of Wisconsin rings with six droppers each. Normally, droppers are flexible cables for movement through the water and through brush or vegetation, but the model droppers were constructed of stiff metal rods in order to maintain their shape and their relative position in the water.  The three rings had maximum distances between rod centers of 20.5, 13.4 and 9.3 cm. The rods were 15 cm long immersed 11.8 cm in the water while the rings themselves were above the water. Thus, the immersed surface area of each anode array was the same.  Each anode array was placed on a non-conductive circular plate resting on the test tank (kiddie pool) bottom to prevent puncture by the stiff droppers.  A 24 x 28 cm aluminum plate served as the cathode. The cathode plate lay flat on the bottom of the test tank such that only the top half was exposed to the water for conducting electrical current.  The test tank water inside dimensions were approximately 190 x 118 cm, filled to a depth of 12.5 cm. Ambient water conductivity was 156 µS/cm at 27 C. An ETS ABP-3 backpack electrofisher served as the power supply for 50 volts of continuous direct current, and voltage at distance from the anode array center was measured with a UNI-T UT81B scopemeter set on meter mode.

3 rings

Model “Wisconsin” ring anodes of 9.3, 13.4 and 20.5 cm max distances between dropper rod centers.  Rods were of 3/16 inch stock (4.8 mm) immersed 11.8 cm. Anodes were graciously built by Mr. Rocky Carney of Magnolia Steel of Natchitoches, Inc., Hagewood, Louisiana. If can consider these as ¼ scale, at full size the rings would be 14.6, 21.1 and 32.3 inches in diameter of ¾ inch stock for the dropper rods.

 

3 rings b

The three ring anode arrays shown together to emphasize their relative proportions.

 

Test setup b

Test setup in kiddie pool. Pool water area approximately 190 x 118 cm, filled to a depth of 12.5 cm. Ambient water conductivity was 156 µS/cm at 27 C. An ETS ABP-3 backpack electrofisher served as the power supply for 50 volts of continuous DC and measured at distances from the anode center with a UNI-T UT81B scopemeter set on meter mode.  If metal plate cathode represents an electrofishing boat and the model anode array represents a full-size anode array which typically is suspended from a boom off the boat bow, then the voltage measurements were made in the fore direction from the anode array, i.e. away from the boat.

Voltage measurements by distance from anode ring centers were fit to a power regression with a vertical offset.  The first derivative of each power regression was used to calculate field intensity (voltage gradient) profiles for each size of anode array.

V profiles

Voltage by distance data for all three ring sizes, each with 50 volts applied to the system of both electrodes. The smaller ring produced more voltage at a given distance from the anode.

VG profiles b

In contrast, the voltage gradients from ring centers were basically the same for all thee ring sizes. Note that 50 volts were applied to each, and each had six identical rods as droppers. Current was 0.05 A for small ring, 0.06 A for other two. Droppers for the smallest ring were relatively close together, so the resistance may have been slightly higher due to electrical interaction among the dropper fields associated with like charges repelling.

Conclusions

Could accurate electrical measurements be made from approximately ¼ scale model electrodes in a small body of water? Whereas my intent is to repeat this study in a larger tank with deeper water and to make measurements in other directions from the anode array, the voltage by distance data were excellent fits to the power regressions. In turn, that allowed calculation of the first derivative to provide curves of voltage gradient by distance.  Affirmative.

Would those measurements provide useful information about the effect of anode ring size on their electrical fields? The top graph clearly shows a difference in voltage profiles for the three ring anode sizes.  The smallest ring produced a higher voltage, and the largest ring a lower voltage, at a given distance from the anode array center.  However, the anode ring size basically had no effect on the voltage gradient profiles.  The applied voltage was held constant at 50 volts and the immersed surface area was the same for each anode array despite the difference in ring size because each ring had six droppers of the same stock diameter immersed to the same depth.  The smaller ring likely had slightly higher resistance because of more competition among the closer droppers in terms of like charges repelling one another. The current was measured only with the ETS ABP-3 meter which reads to 0.01 ampere.  The meter values were 0.06 amps for the two larger rings and 0.05 amps for the smallest ring. If the study is redone, a current probe should be used to more precisely measure current, i.e. to more significant figures.  However, these ammeter readings support the notion that resistance was slightly higher for the smaller ring due to interaction among the closer droppers. Given that, these results indicate that applying the same voltage, and basically the same current, to anode arrays of different ring diameters results in similar voltage gradient profiles, at least if the immersed surface areas are the same for the anode arrays. This is useful information.  Affirmative.

Many thanks to Mr. Rocky Carney for constructing the anode arrays and to Debra Dean for recording the voltage by distance data. This study would not have been possible without either contribution.

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