Multimeters made easy, Part 1June 6, 2018
Getting the most out of your DVOM.
The only way to accurately diagnose an electrical circuit is with a volt/ohm meter (VOM). This device can usually check amperage and some may read frequency. There are two different styles – analog (needle) or digital (DVOM). Since the DVOM is the most prominent it will be the subject of our review.
Back to basics
Electricity is a resource used to do work. To harness this power a completed circuit is required. A circuit needs three components: a source of electricity, a load or device to operate, and two paths for the electricity to follow called conductors. Most if not all agricultural equipment employs direct voltage (D.C.), thus a power supply and ground are necessary.
Electricity has three distinct metrics that can be measured:
Electrical pressure is called voltage
Electrical flow is called amperage
Opposition to electrical flow is called resistance and is measured as ohms
The law that governs electricity in a circuit is called Ohm’s Law.
Electricity can be:
A.C. voltage (alternating current)
The DVOM will be used to diagnose a problem by checking the circuit for either:
Frequency (if applicable)
There are two main concerns when buying a meter: the internal resistance and the scaling or ranging. If you are going to use the meter on both old and new equipment then it is important for it have 10 million ohms (10 megohm) internal resistance. Any modern circuit that incorporates a microprocessor control such as on an engine, combine, sprayer, etc. will require a 10 megohm meter. With this internal resistance it does not add any load to the circuit. It is also backward compatible to older non-computer-controlled circuits.
Every meter has a series of range selections for what is being measured (ohms, volts, amps). The simplest way to understand this is to think of an analog pressure gauge on a sprayer. If you want to spray at 40 psi but the gauge goes from 0-1000 psi it would be very hard to accurately identify the desired setting. The range of the gauge is too great (resolution). If you had a gauge that reads from 0 to 100 psi then 40 psi would be easy to identify. For the same reason a DVOM also has range selections that are called scales. For example, if you are checking a 12-volt circuit you want a scale that will read accurately such as 0 to 20 volts and not one that is to 1,000 volts.
A meter can either have manual range selection or a function called auto-ranging. The difference being the placement of the decimal on the display is done automatically along with the resolution selection. More expensive meters are auto-ranging but I do not recommend this feature. It can become very confusing and cause you to misdiagnose a circuit. Let me explain.
When the meter makes a scale or range change it is identified on the display but may not be easily recognized. For instance, if you manually select the range of 2K (2,000 ohms) and the display read 1.5 you know it equals 1,500 ohms. If the meter auto-ranged to the 200K (200,000 ohms) and it reads 150 that means 150,000 ohms. Many prefer an auto-ranging meter but I have found them burdensome to use and distracting especially when dealing with a circuit of unknown value. I suggest a 10 megohm manual range selection meter, especially for beginners in electrical diagnostics.
Regardless of the type of DVOM or analog meter you have some basic tenets apply.
Depending on what is being tested the meter’s leads can be placed either in series or parallel. When in series the circuit is invaded and the meter is placed in line. Parallel placement means that the circuit is not disturbed and the leads are placed across it such as when checking a battery.
To check the resistance of a circuit or part it must be unpowered from its voltage supply. The meter has an internal battery that measures the ohms of what is being tested. The meter will be in parallel.
Unless the meter has an inductive clamp that reads a magnetic field in an operating circuit, to check amperage (current draw) the meter must be placed in series.
If voltage is being confirmed the meter will be placed in parallel.
Choosing a scale or range
“K” means kilo or thousands while “M” is for mega or million. Milli when attached to volts (mv) or amps (ma) is one thousandth. Thus, 500 mv is equal to 0.5 or ½ volt.
Use these steps to find the proper scale (ohms are being used as the subject).
- Start on the highest scale. If “1” is displayed it means open. Anything else means continuity.
- Scale down until the meter reads over range. Then go back up one scale.
- If you reach the lowest scale without over range, then use that setting.
- The scale will determine the decimal point placement. “K” is three places and “M” is six places.
Also read: Part 2: Checking a circuit