Understanding Meters and Their Use

There are many meters and all have their place.

By Daniel Featherstone

Our industry has seen so many changes in the last 20 years. Think about it: How many of you now install
variable speed drives? What about automation in the irrigation system? Use Bluetooth-connected devices and over-the- air communications?

And so too have changed the tools we use every day. Just look at meters. Ask yourself if you’re still using the good old Simpson 372?

Now, do you understand the variety of uses and functions for meters? Let’s explore and see what we really need on the rig.

The Basics

Figure 1. Meter symbols.

Let’s begin by discussing the various basic types of meters before we get into specific functions. The analog meter, the one with the swinging needle, was the standard for years. But do not brand it as an antique. This is still a great tool for the primary reason it has no memory; the needle is reading voltage or amps or whatever you have it tuned to measure in real time. This can be handy when looking for voltage drop or amp variance. And, yes, they are still available for purchase.

So, does that mean digital meters are bad because numbers do not change with a variance from the source you are reading? No, not necessarily; you simply need to understand how the meter functions.

Many digital meters read peak to peak, meaning a nontrue RMS (root means squared) meter is calibrated for sinusoidal waveforms, which can lead to inaccurate readings on distorted waveforms (common with VFDs).

This is where you need to understand how your meter reads. Other models may have a test button that reads true as long as the test or load button is engaged. More common today, though, are models featuring the RMS function and make a long subject short by calculating the effective heating value of the waveform. So, you may miss a “drop.”

Another difference between analog and digital meters is a manual set range vs. self-ranging. For example, self-ranging types will measure ohms, but you need to pay attention to any letters reported with the number output. For instance, “3.5K” is signaling to take 3.5 and multiply it by a thousand, where the letter M means multiplying by a million.

Finally, what does “OL” mean? “Overload,” right? No, “Out of Limit.” This signifies the meter is reading a number above the upper limit of the scale the meter is set for or can read. For a self-ranging meter, that is usually quite a big number. For meters where you can change the range, often by going up to the next setting, the meter may then report a number.

Often, though, it is not important to find the upper limit. For example, if a motor winding measures 3.8 to 4.3 and the meter is set to read 0 to 200 ohms, do we really care how far out of limit we are at this scale?

As for the settings and reading a digital meter properly, it is best to read the manual or search for instructional videos for the make of your meter.

AC and DC

Figure 2. Megohm meter.

AC meters measure live alternating voltage; so let me first ask this: How many of you are afraid or intimidated by live voltage? It’s okay; so am I.

For those who are not concerned with measuring voltage, you better respect it. Always wear your PPE, make sure your meter is safe and includes probes that are in good working order, and be sure the meter can safely measure the voltage with which you are working.

From the chart represented in Figure 1 you can see that AC voltage is designated by the symbol of a sign wave (∼). Most standard meters read up to 600 volts. I always set my meter for the highest reading to be sure not to damage the meter.

The key reminder when using an AC meter is safety. Turn off the power before setting the meter up. Use alligator clips or spring-loaded J-hook probes to keep your hands free. Once the probes and the meter are properly set, use the kickstand or hanging hook for the meter to keep your hands free once again. Turn on the power, get your reading, and then turn off the power to disconnect the meter.

Figure 3. Example of a multimeter settings.

DC meters measure direct current. Remember direct current only has two states, on and off. It is also “directional.” If you get a strange reading or no reading, reverse how the probes are set, as the reading should be steady when voltage is present.

A typical application for a DC setup would be for a relay. It should be closed when voltage is present and in the proper range for the relay. When power is not present, it should open.

Now a quick note: A variable speed drive output is DC but well out of the range of a DC meter, nor is it safe to measure with a DC meter. VFD output should be measured with an AC meter only.

Finally, milliamps (mA) are also on the chart but not used much in the pump industry. Yes, a transducer measures in milliamps, but we do not need a meter to test the transducer. Typically, transducer testing is done by disconnecting the transducer and looking for different fault conditions or simple continuity testing.

The Standard

Now, let’s talk ohms, often designated by the omega symbol (Ω). This is the meat of our discussion. There are two types of ohm meters: the standard ohm meter we have and will continue to use in our industry, and the other one, the megohm meter.

What is the difference between the two? An ohm meter tells the event that happened: The short to ground is present, the wire is damaged. This is because the measurement voltage is not that great. Even the Simpson 372 is pushing only 30 volts.

So why a megohm meter? It can push up to 1000 volts and evaluate the integrity of the insulation of the pathway. This can be critical when working with VFDs and other technologies. Small imperfections or damage to wire insulation can be proofed by pushing more voltage.

So how do you set up the meter to read the ohms? First, set the output voltage to twice the operating voltage of the equipment. You may have heard to go right to max voltage output, but that is not advisable. For example, a 230-volt wire often is only rated to 600 volts, and by going to the max setting output, which is often 1000 volts, you can discover some issues that may not be relevant to the running voltage.

Where megohm meters are extremely useful is testing buried wire. Have you ever assumed the wire is good and then had issues retrofitting to a VFD in the system? Or have you been concerned the pump you just sent down may have damaged wiring? Megohm meters can see what the standard ohm meter cannot.

So, do you need a megohm meter? Yes, and they are often incorporated into a multimeter and affordable.

Other Functions

Another function of a multimeter is the diode test. A diode is simply an electrical check valve, allowing the flow of voltage in only one direction. Diode testing is not commonly used in our industry.

Continuity tests can be useful or can be just a pass-fail. This is one in which you need to read the manual or watch a video because some are just checking to see if the pathway for voltage is there—tone or no tone.

So completely impractical? No, not at all. For example, it can be a good quick test for a transducer cable. But continuity tests are now tied to an ohm’s test. They may range from 0 to 200, or other ranges. This is why you should read the manual.

Another feature of some meters is to measure the hertz (Hz) of an electrical output. Personally, this is one I have not used in my 25-plus years, but it would be practical if you install and work regularly on VFDs.

The last test I want to discuss is the capacitance test. This is for capacitors, and it is easy to find meters featuring this function. Many of you may be thinking the Simpson 372 (or almost any analog meter) can do this. To that I will say, “Yes but no.”

By that I mean, yes, the test tells me the capacitor can capture voltage and store it, but it does not tell me the microfarad output. Understand a start capacitor is typically rated +20% to –0% for the microfarad output.

So, can an analog test proof a start capacitor as good and have it still fail? Absolutely. Picture this: The start capacitor is rated for 86 microfarads, but it is a touch weak. Say it is 83, so is it not feasible it will pass the analog test? Yes.

Now you put it back in and it fails. . . . Bang! Hopefully, no one was hurt or has hearing loss. There is no excuse for this with modern meters. Multimeters with all of the above features are affordable and easy to use. Be sure the capacitance meter can handle up to 200 microfarads. No excuses.

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So, let’s review. Do you need a megohm meter? Yes, for so many reasons, but also for simply testing buried wire integrity. A good digital meter will cover many of these tests—if not all—and being much more affordable, you should be able to purchase a megohm meter and a smaller multimeter that can measure capacitance with reasonable spending.

Remember, analog meters still have their place in the industry, and they are still available too. I highly recommend an analog amp probe, especially if you are working with three-phase power.

Daniel Featherstone is a technical training leader for Pentair Flow Technologies. He has been with Pentair for more than 25 years and training for more than 20. His experience includes a variety of pumps from standard centrifugal and submersibles to vertical multistage and vertical line shaft turbines, and more.