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Evolution of The Digital Multimeter

Evolution of The Digital Multimeter

Posted by Barry Ward ,May 10th 2016

Digital Multimeters, or DMM, or what used to be called DVM (digital volt meters), are very versatile tools and come in many levels of accuracy. A typical DMM for use in the field is between 3 and 5 digits of display. Some are listed as 3 ½ digits (meaning they can show between 0 and 1999 units), and some are listed as 3 ¾ units (they can show between 0 and 3999 units). In the laboratory you might have a 5 ½ digit or even more (I have seen 8 digit units). As the number of digits goes up, so does the price – typical 3-4 digit units cost between free and $1000 (depending upon Brand, resolution, and other features). 

Digital Multimeter

When you go to 6 digits the cost goes up to $2000, and they definitely are NOT field units. The higher resolution units, 8 digit or so, may cost $5000 and more, and typically need to be kept in a climate controlled environment in order to maintain their accuracy.

These units typically really only measure voltage, but buy using Ohm’s law then can seem to measure anything. Ohm’s law is E=IR where E is voltage, I is current, and R is resistance. In the DC case, the DMM can read voltage directly, current by passing the current through a known value of resistance and then measuring the resulting voltage, and resistance by passing a known current through the unknown resistor and measuring the voltage. AC is similar, but there are some minor math calculations that also need to be done.

Many of the field DMM will also measure capacitance, frequency, temperature, or duty cycle. These functions can be useful in different applications: capacitance is typically used to check starter caps on single phase motors, Frequency is typically used to ensure the line is 60Hz, temperature can be used for many things like knowing that a motor is being overloaded because it is warmer than expected.

Field units are typically placed in a shock absorbing rubber case. Many brands can be dropped multiple feet without being hurt, but it is not recommended that you try this. The lab grade units do not have this protection and are usually 110VAC powered instead of battery powered.

For measuring larger currents there are clamp meters. These devices have all of the normal DMM features plus a “clamp” or one-turn transformer that opens. This allows the unit to be opened to be placed around a wire to measure the current flowing in that wire. All clampmeters will work with AC and some will also work with DC. Air Conditioner technicians frequently use these so they can measure the current to the compressor to verify it is within limits.

One final comment – most modern field type of DMMs hold their accuracy very well. I have a meter that I have had for almost 25 years. I do not use it every day, but I do use it many times a year. I have replaced the batteries, but that is all I have done. I checked it the other day and it was still well within its factory specs. I also checked a free DMM I own and it was within its specs.

Not sure what you need? Call me and I will be glad to discuss it with you.