Laserfiche WebLink
10 BIOLOGICAL REPORT 11 <br />High <br />a <br />c7 <br />Low <br />?a <br />a <br />0 <br />X/2 <br />Distance <br />X <br />Fig. 10. Generalized voltage gradient profile for two <br />identical electrodes. <br />design will develop high voltage gradients close to <br />the surface of the electrode and will electrify only a <br />small volume of water. In contrast, a U-curve that <br />decays more slowly will produce significant levels <br />of voltage gradient at distances farther from the <br />electrode and thereby extend the effective elec- <br />trofishing range. Voltage gradient profiles obvi- <br />ously provide another means for comparing the <br />electric fields generated by different electrodes. <br />Neither the voltage nor voltage gradient profiles <br />change significantly with the values in water con- <br />ductivities normally encountered when elec- <br />trofishing. Nearly identical profiles will result for <br />a given array regardless of whether the electrodes <br />are measured in low conductivity (50 µS/cm) or <br />high conductivity (5,000 µS/cm) water, provided <br />that the applied voltage is kept constant on the <br />electrodes. Therefore, it is necessary to record only <br />one voltage profile for an electrode, and these data <br />are then transferable to any water condition. How- <br />ever, the amount of power dissipated by the elec- <br />trodes is directly proportional to the conductivity <br />of the water, doubling the water conductivity dou- <br />bles the power and vice versa. Voltage profiles are <br />almost independent of water conductivity and <br />power dissipation, which verifies that voltage <br />measurements cannot be used to standardize elec- <br />trofishing operations. <br />The voltage profiles and voltage gradient profiles <br />can be measured at any convenient electrode volt- <br />age (Vs). Both types of profiles canbe linearly scaled <br />to other operating voltages. For example, if the <br />profiles were measured at 100 volts, the ordinate <br />values of the profiles could then be scaled by a factor <br />of 4 when operating the system at 400 volts. <br />Method 2: Direct Voltage Gradient <br />Measurements <br />It is possible to make direct in-water measure- <br />ments of voltage gradient by connecting a special <br />probe to either a cathode ray oscilloscope (CRO) or <br />digital voltmeter (DVM). This instrumentation is <br />probably more complex and expensive than the <br />voltmeter previously described, but it has the ad- <br />vantage of being able to make on-site readings for <br />timely equipment adjustments. <br />The voltage gradient probe (Fig. 11) consists of <br />a pair of insulated wires attached at one end to the <br />input terminals of a CRO or DVM and mechani- <br />cally supported on the other end by an electrically <br />insulated rod. The rod serves as a handle for the <br />probe, and its length is determined by the depth of <br />the water being measured. The pair of wires ex- <br />tends past the end of the rod, and each wire is <br />stripped of its insulation to expose a length of about <br />2 mm of bare wire with a diameter of about 1 mm. <br />When immersed into an electrical field, this probe <br />allows the differential voltage existing between the <br />two bared wires to be measured with the CRO or <br />DVM. It is convenient to separate the bared wires <br />by 1 cm and thereby measure voltage gradient <br />directly in volts per centimeter. <br />Cathode ray <br />Oscilloscope or <br />O <br />Digital voltmeter 0 0 <br />Rotation <br />/Bared wires <br />1 <br />------------------------ <br />---------------- - 1cm F*-- <br />Fig. 11. Diagram showing the configuration of a voltage <br />gradient probe.