Multimeter 

Fluke 175 Digital Multimeter Multimeters are undoubtedly the most common item of electrical test equipment in use.  Often it is the first piece of equipment people will turn to when trying to visualize and obtain data on an electrical system.

As the name suggests, a multimeter is an single meter with the ability to measure several electrical quantities. Nearly all multimeters will include the measurement of at voltage, current and resistance.  Depending on the type and sophistication of the meter, other measurable quantities would include frequency, capacitance and even temperature.  In addition to measurement, some meters include functions such as data logging, diode and continuity testing, instrument loop testing and wireless remote displays.

Commercially, there is a massive range of multimeters available to suit various technical requirements and cost budgets.  Meters vary not in only what the can measure, but in how accurately they make the measurements and the range over which the measurements can be made.   Meters offer different levels of operational safety, varying levels of usability and construction quality.  Making a good selection, is a trade of between all these various elements.

Analogue and Digital Meters

Multimeters come in two types - analogue in which a moving needle displays the measured quantities on a printed scale and digital, where the measured quantity is read on an LCD display.

In industry, digital meters tend to dominate due to there ease of use.  Some users prefer analogue, particularly if quantities are varying as it can be more easy to get a feel for ranges.  On analogue meters, measurements can be peripherally viewed  by noting the approximate location of the needle on the scale.  

Parallax Error: in an analogue meter, the needle is above the scale and if the reading is not taken by looking directly on to the needle a visual error can occur.  Better quality meters will include a mirror on the scale, which if the user aligns the needle with its image  will avoid any parallax error.

Average or True RMS

Sanwa SH-88TR Analogue
Multimeter In a perfect sine wave, the average and RMS (root mean square) values of current are related to the peak by constant factors (2/pi =0.636  and 1/√2 = 0.707 respectively).  Many meters will measure the average value of an a.c. waveform and calculate the peak and RMS values by applying these scaling factors.

In real life situations, an a.c. waveform is often very different from that of a perfect sine wave.  Using an averaging meter in this instance would lead to errors.  True RMS meters will take into account the non-pure sinusoidal nature of the waveform being measured and return accurate RMS values of any measured quantity.  True RMS meters tend to be more expensive, although in many applications this extra cost is justified.

Tip:  many electrical systems involve the use of variable frequency drives.  In these devices, a synthetic sine wave is generated by adjusting the width of high frequency pules.  Many multimeters are designed, rated and calibrated for frequencies around 50 or 60 Hz.  Within a variable frequency drive, while the output frequency is stated as being around these levels, the reality of it's high frequency generation, may give unreliable readings on multimeters not designed for this. Multimeters intended for use with variable frequency drives should have an appropriate frequency resolution.

Calibration

The accuracy of any measuring device will drift over time.  This is the same for multimeters.  The object of calibration is to adjust the meter to ensure it is performing within its designed accuracy limits.  Calibration is normally performed in a laboratory, where the meter readings are compared to a known reference.

In industry and commercial use, multimeters are subject to being calibrated at periodic intervals (typically yearly).  In these applications before trusting any measurements, it is important to verify the accuracy of the meter by reviewing the calibration certificate and ensuring it is still in date.