MEINOPA (1968) and MEINOPA II (1980)
The MEINOPA system with a towed gear carrying electrodes and a seawater conductivity measurement cell is used for the automatic determination of the electrical characteristics of the seawater and seabed. Knowledge of these characteristics is necessary to determine the effectiveness of mine-sweeping gear equipped with current-carrying electrodes.
MEINOPA is a Dutch acronym for Environmental Parameters Measuring Unit (MEetINstallatie OmgevingsPArameters). The MEINOPA II system is a continuation and extension of a previous measurement system that was developed and used by TNO’s Mine Group in support of the Royal Netherlands Navy. The system is used to determine the effective sweeping width of the electrical minesweeping gear of minesweepers. This requires that the electrical properties of the seawater and the seabed are measured. Magnetic mines are swept by generating a magnetic field that is large enough to trigger the mines. That magnetic field is generated with a so-called open two-electrode gear.
The minesweeping gear consists of two insulated conductors of unequal length with at the end copper electrodes with which the (electric) sweeping current is brought into the water. The sweeping width now depends on the generated field, and thus on the size of the electric current and the field distribution in the water and the seabed. To calculate the sweeping width, the current distribution in the vicinity of the gear must be known. We can determine that distribution by measuring the voltages generated by the sweeping current at known locations near the gear in the water. This is done by also towing a measuring cable with measuring electrodes at specific distances. With the measured voltages, the current through the gear, and the measured electrical resistance of the water calculations can be made. The MEINOPA installation gathers the data, performs the calculations and gives the results on a strip of paper. With these data and the depth measurements of a Fathometer, the effective minesweeping width can be determined.
Before use, the dry end of the measuring cable is connected to the signal processing box on the half deck (aft deck) of the minesweeper. Here the measured voltages are converted to frequencies to increase the immunity to electromagnetic interference. A cable connects this cabinet to the processing cabinet in the navigation room at the front of the ship. The sweeping current of about 400 Amps is also fed through this cabinet. The measured current value is also converted to a frequency before being sent to the processing cabinet. The electrical resistance of the seawater is determined with a measuring cell placed in the seawater cooling water inlet of the diesel engines of the minesweeper. This information is also converted to a frequency and fed to the processing cabinet. The processing cabinet finally performs a number of operations on all this data and outputs the results via a printer.
Some technical details
The conductivity of a liquid is measured with a pipe through which the liquid flows. Seven copper rings are placed in this pipe. An alternating current source is connected to the middle ring on the one hand and the two outer rings on the other hand. The supplied current will be distributed over both halves of the measuring cell. The liquid conducts this current flow to a certain degree as the conductivity of seawater depends on the temperature and the salt content. The currents between the middle and the outer rings generate an electrical field in the water which is measured with rings 2-3 and 5-6. The size of the voltage is a measure of the conductivity.
The measurement of large currents without galvanic contact is done with a so-called transformer shunt. The principle is as follows: the current to be measured goes through one winding of a transformer. This creates a magnetic field in the transformer. There is a sensor in the transformer, which samples this field. The output signal of this sensor is fed to a circuit that is connected to the secondary windings of the transformer (winding ratio 1: 5000, for example). The circuit now generates a current which counteracts the field in the transformer. The circuit is tuned in such a way that the secondary generated field completely compensates for the primary field. Thus, the current through the secondary winding is a measure of the field, and in turn a measure of the primary current. Calibration is done by passing the primary current through a resistor with a known value and measuring the voltage over the resistor (A = voltage/resistor value).