Symbols.
Voltmeter
Ammeter
Ohmmeter
Subsection 4.4.1 Measuring Instruments
Here are some electric measuring instruments likely to be of interest to marine engineers.
Digital Multimeters.
A digital multimeter (DMM) is a versatile electronic tool that combines multiple measurement functions in one device. Most digital multimeters can measure AC and DC voltages, AC and DC currents up to a few amps, resistance, and perform continuity tests. Some meters can additionally measure capacitance, frequency, check diodes, and measure temperatures with an optional thermocouple. A good digital multimeter is a valuable tool for troubleshooting electrical and electronic devices.
When using a DMM, always ensure that the test leads are correctly connected to the meter; the black lead goes in the common (COM) jack, but the red lead position depends on the measurement you intend to make. Next, select the desired measurement mode with a selector switch to configure the DMM’s internal circuitry, and finally connect the test leads to the circuit and make the measurement. The DMM displays the measured values on a digital screen, showing the numerical value and unit. Some models have auto-ranging, which selects the appropriate measurement range automatically, and data hold functions to freeze the displayed value.
To measure voltage you connect the DMM probes to the circuit across two points of interest. An analog-to-digital converter (ADC) detects the voltage difference between them and converts it into a digital format which is then processed and displayed. When measuring large voltages, a potential divider circuit proportionally reduces the voltage to a suitable lower value.
To measure current, the DMM is connected in series with the circuit under test and the voltage drop across a precision internal resistor is measured to determine the current. However, to use a DMM to measure current you must disconnect and open a portion of the circuit to introduce the meter. For this reason, DMMs are rarely used for current measurement.
Resistance measurement involves applying a stabilized voltage from the meter’s battery to the component being tested and measuring the resulting current. Using the applied voltage and measured current, the DMM calculates the resistance.
A continuity test is just a resistance measurement used to determine if two points are electrically connected. Often the meter will produce an audible “beep” if the resistance is below a threshold value of 50-100 ohms.
Voltage testers.
Voltage testers are simple and rugged tools used to quickly check whether voltage is present or absent. They should always be proven on a known live voltage source before and after making voltage tests for lock-out/tag-out purposes.
Clamp-on ammeters.
Clamp-on ammeters are hand-held instruments used to measure AC currents. A plier-like pick-up coil is closed around the conductor in which current is to be measured, and the movement responds to current induced in the pick-up coil by the changing magnetic field surrounding the conductor. Clamp-on meters are preferable to digital multimeters for measuring currents because the don’t require the circuit to be disturbed to use, and they can measure larger currents than a DMM, however, because they depend on a changing magnetic field they do not work with DC currents.
Megohmmeters.
A Megohmmeter or megger is a electrical meter specifically designed to measure very large resistances, particularly the resistance of electric insulation, resistance which is too large to be measured accurately with a DMM. It consists of an internal high voltage source (up to 1000 V), an accurate ammeter, and a display scale marked in megohms.
Before use, the equipment under test must be secured, locked out, and grounded to discharge any residual capacitive static charge. The meter is connected between an insulated conductor and ground, and then the test voltage is applied for 60 seconds or until the resistance reading stops changing, whichever comes first. The meter measures the current flow through the insulation, and using ohm’s law, calculates and displays the insulation resistance.
