Electrode Characteristics and Design

3

Session Purposes

• Participants will be able to better evaluate electrodes in regards to potential power allocation and power output.

• Participants will be able to improve their electrode design.

• A third consideration in electrode design, field patterns, will be covered in the following module.

Session Objectives• Measure and calculate electrode resistance• Determine power allocation among electrodes• Determine resultant output power across water

conductivity range of interest• Compare electric fields resulting from various

electrode designs• Contrast specific vs. ambient water conductivity

and calculate ambient water conductivity from specific conductivity

• Use information on resistance, power allocation and output demand, and electric field pattern for electrode design

Important Basics• Ohm’s Law: R = V ∕ I

• To measure electrode resistance, hook electrodes up to a circuit and place in water

• The circuit must have a power source and accurate metering; thus, the circuit could be electrofishing gear or another power source with volt and amp meters

• Energize the circuit, take volt and amp readings from meters; AC, DC, or pulsed DC will give the same results*

• Use Ohm’s Law to get total electrode resistance of unit**

Important Basics

• The larger the surface area of the electrode, the lower the resistance; this allows “more room” for charge carriers to leave and for greater current flow; Ohm’s Law shows this: resistance is inversely proportional to current

• EF gear with larger electrodes will have a greater amp and power demand• as shown by: I = V ∕ R and P = I2 x R

Important Basics

• The larger the surface area of the electrode, the lower the resistance; an electrofishing unit with larger electrodes will have an overall greater amp and power demand

Generator Currentvs Volts, Electrode Dia.

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Spherical electrodes of two different diameters

Important Basics• Within an electrode system wired in series, however,

smaller electrodes have more power dissipation

• for instance, a tow-barge shocker with one hand-held ring anode and one plate cathode attached to the barge bottom• the smaller anode has lower resistance and more

power dissipation

• for instance, a boat with 2 boom anodes and the hull wired as the cathode*• here, the anode system wired in parallel is

considered one electrode with one equivalent resistance; in the simplified circuit the booms and the hull are wired in series

Important BasicsField Coupling & Interaction

• For electrodes having opposite polarities (anode & cathode), they will have independent fields unless brought close together; proximity causes the field to couple and amperage draw to increase (lower resistance values than when independent).

• For electrodes of the same polarity (as anode & anode), independent fields again unless brought close together; with close proximity, resistance increases.

• When making electrode resistance measurements, typically you want independent fields.

Important BasicsWater Conductivity (σ)

• Two types, ambient and specific σ

• σ is a function of the concentration of ions and temperature

• Specific σ standardizes water temperature (usually to 25 °C)

• Specific σ is used in limnological or pollution studies where the ionic concentration is of interest

• In electrofishing, we are not interested in what σ would be at some standard temperature; rather we are interested in the actual water conductivity (“ambient σ”)

• Thus, we do not standardize by temperature

• Some meters allow you to measure ambient conductivity directly

• There is a formula for converting data taken from meters that only output specific conductivity or for historic specific conductivity data

Important BasicsWater Conductivity (σ)

• Equation to convert specific conductivity to ambient conductivity:

σa = σs ∕ (1.023(Ts – Ta))

Also, you can use the conductivity converter on the “Power Goals” tab in Electrofishing with Power or on any tab within EF Goal Power

Important BasicsWater Conductivity (σ)

• See “Considerations for Water Conductivity Meters” for a brief on factors to consider and example models

• You may view the water conductivity instructional video for measuring technique and additional details:

Water Conductivity

Important BasicsWater Conductivity (σ)

Measurement Procedures

• In Electrode Resistance Procedures.pdf, you’ll find detailed directions on simple methods to determine electrode resistance and % power to the anode(s).

• You also canview the“Electrode

Resistance”video

Practice with the Numbers

• Use the “Electrode Resistances” tab in the Electrofishing with Power Excel program and work through the examples found in the accompanying documentation

Some Rules-of-Thumb for Power Allocation

• Total electrode resistance for backpacks (ring anode and rattail cathode) typically is around 300 Ohms (at 100 µS/cm)

• Acceptable percent power to the backpack anode is at least 50% and a common number is 55%

• Total electrode resistance for boats (two Wisconsin array anode booms and the hull as the cathode) often ranges from 30 to 50 Ohms (at 100 µS/cm)

• Suitable percent power to the boat anodes is greater than 50% and commonly is around 55 – 70%

Evaluate Some Implications for Range of Successful Electrofishing

• Use Boat Power Excel

• Compare 2 EF boats with different resistances:– 4800W, 600V, 45A, 74Ω, 20% duty

– 4800W, 600V, 45A, 40Ω, 20% duty

• According to Boat Power, what are the water conductivity ranges that the units can successfully fish in?

Comparison of Electrode Electric Fields• We have covered two factors needed for

electrode design: power allocation to the anode and power demand.

• Another important consideration is the electric field generated- the effective zone of capture.

• This topic will be covered in detail in Module 4.

• Other considerations, as electrode dimensions and construction materials will be covered in the Electrofishing Systems module

(Module 6).

Comparison of Electrode Electric Fields• For your reference,

Kolz, A.L. 1993. In-water electrical measurements for evaluating electrofishing systems. Biological Report 11, U.S. Department of the Interior, Fish & Wildlife Service, Washington, D.C.

has graphs comparing horizontal field extent of various electrode arrays, including spheres.

also includes a discussion for

making electrode resistance

measurements

Next Step

“Electric Fields” (Module 4)