Great! Now you understand the concepts of Voltage, Current, Resistance, and Power, we will now look at how we can measure these values in practice.


  1. Understand how to use a standard multimeter.
  2. Understand how to use a Clamp Meter
  3. Understand how to use both devices on a small craft electrical system.


  1. Set a multi meter to the correct setting with the leads in the appropriate sockets.
  2. Set up and use a clamp meter to measure current.
  3. Know how to use the mV setting to find port connections.


  1. Introduction to The Multimeter (Video Lesson)
    1. Measuring voltage
    2. Measuring resistance
    3. Measuring current
  2. The Clamp Meter (Video Lesson)

1 The Multimeter

A multimeter allows us to measure Voltage, current, and resistance, but it's important that we set the multimeter to the correct settings and and use the correct lead connection points for the task in hand. Watch the lesson below to see how we measure Voltage and resistance with a standard multimeter.

Please note video lesson below may take a while to load up...

1a Measuring Voltage

How to use a Multimeter (Voltage DC V & mV)

In the video above we covered how to set up a standard multimeter for 2 very different voltage tests:

  • 20V DC Testing in parallel (between the positive and negative terminals). Such as a 12v Battery Positive and Negative, or a 12v load such as a lamp. We selected a range close to and above that to which we need to measure, however, many meters are auto ranging, so you are only required to select the DC volts setting.
  • 2000mV DC Testing over terminals. Such as finding faulty connections between a battery post and the battery clamp. We are expecting voltages less than 20mV for a good connection. We chose a range close to and above that to which we need to measure, however some meters are auto ranging so you just select mV.

1b Measuring Resistance

How to use a Multimeter (Resistance Ohms Ω)

The multimeter is great for measuring resistance, and this is very useful for testing engine sensors, as we saw in the lesson above. Sensor manufacturers publish tables of resistances corresponding to the various values the sensor is required to measure, this can be confirmed by measuring the resistance of the terminals of the sensor / sender unit. When measuring resistance the meter runs a small current through the device to see how well it conducts, so the device must be disconnected from the circuit to be tested.

1c Measuring Current

How to use a standard Multimeter (Current Amps A)

When measuring currents with a standard multimeter, up to 10A, we have to interrupt the circuit and measure in series, with the positive test lead placed in the AMPS socket, and the black as always in the COM socket. The full circuit current actually flows through the meter. The big problem with this is we can only measure up to 10 A and if more current flows it can damage the meter, or blow an internal fuse, preventing any further current measurements. So for this reason I do not recommend measuring current with a standard DC multimeter, it's much easier and safer to use a DC Clamp meter, see below...

2 The DC Clamp Meter.

The DC clamp meter can measure voltage and resistance just like a standard multimeter, but it can also measure DC and AC currents without the need to interrupt the circuit.

How to use a Clamp Meter (Current Amps A)

I strongly recommend investing in a DC clamp meter as this will allow you to measure currents up to 200A without the need to interrupt, or make any electrical connection, it simply clamps around an individual conductor and measures the magnetic field to deduce current.

Please note video lesson below may take a while to load up...

The clamp meter measures the magnetic field around the cable and uses this value to calculate the current.

This device can be used to:

  • Check for parasitic loads on your batteries; when everything should be off.
  • Measuring alternator performance.
  • Diagnosing starting issues
  • Measuring the consumption of a particular device
  • Measuring the consumption of an entire circuit

Throughout this course you'll learn how to put this knowledge to good use for general testing, maintenance, fault prevention, and fault finding.

Tip - Recommended equipment.

For general use on board I recommend the UNI-T 210D as shown in the video lesson.

Complete and Continue