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Selecting a Volt Meter

One of the truly indispensable tools in any farm shop is a volt/ohm meter, or VOM. It is the only way to accurately diagnose an electrical circuit, and it can usually check amperage; some may read frequency.

Before going further, we need to review some basics. Electricity has three distinct metrics that can be measured. They include electrical pressure called voltage, electrical flow called amperage, and opposition to electrical flow called resistance (measured as ohms). The law that governs electricity in a circuit is called Ohm’s Law. Electricity can be DC voltage, AC voltage (alternating current), or Hertz (frequency).

For farm shop use
There are two different styles of VOMs on the market including an analog (needle) model or a digital (DVOM) model. Since the DVOM is the most prominent product available these days, it will be the subject of this review.

When buying a DVOM, check its 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, or sprayer) 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 noncomputer-controlled circuits.

Every DVOM has a series of range selections for what is being measured, such as ohms, volts, or 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 to 1,000 psi, it would be hard to accurately identify the desired setting as the gauge range is too great. 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 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.

A meter can either have manual range selection or a function called auto ranging. The difference is that the placement of the decimal is done automatically along with the resolution selection. More expensive meters are auto ranging. I do not recommend this feature. It can become very confusing and cause you to misdiagnose a circuit.

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 reads 1.5K, you know it equals 1,500 ohms. If the meter auto-ranged to 200K (200,000 ohms) and it reads 150, that means 150,000 ohms.

I recommend buying a 10-megohm manual-range selection DVOM.

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 (thousand) while M means mega (million). Milli, when attached to volts (mv) or amps (ma), is ¹∕1,000. 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.

Next, scale down until the meter reads over range. Then go back up one scale. If you reach the lowest scale without going over range, then use that setting. The scale will determine the decimal point placement. K is three places and M is six places.

 

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