Understanding Digital Multimeter DMM Specifications / Specs

- summary, tutorial or overview of the basics of the digital multimeter DMM, specifications or specs and understanding and using the specification to choose the right DMM.

This DMM Digital Multimeter tutorial is split into several pages each of which addresses different aspects of DMM, Digital Multimeter technology and operation:

It is often necessary to understand the capabilities of a digital multimeter. It is obvious when buying one, where as new test equipment or used test equipment, or for test equipment rental, that its performance will be of great importance. So too, its specification or spec is important when choosing a particular instrument to be included in a test specification or test schedule for an electronics test of a particular unit under test. It is important to know a variety of aspects of the performance of the digital multimeter, the exact parameters required depending upon the need for the data.

Manufacturers issue specifications for their products and these give all the important parameters relating to the performance of the digital multimeter, DMM. This can be used where buying the equipment as new, or as used test equipment, or even for test equipment rental. The supplier will be able to provide the specification, or often it can be viewed on the Internet.

For many different types of product, the different manufacturers will specify their products in different ways, making it difficult to compare like for like. Fortunately in the case of digital multimeters, the specifications are normally defined in a way that enables different DMMs from different manufacturers to be compared easily.

Basic digital multimeter features

One of the key parameters of any digital multimeter, DMM is the types of measurement that can be made by the particular digital multimeter and the ranges over which it will operate. Most DMMs will offer a variety of measurements. The basic measurements will include:

• Current (DC)
• Current (AC)
• Voltage (DC)
• Voltage (AC)
• Resistance

However, using integrated circuit technology, most digital multimeters are able to offer additional test capabilities. These may include some of the following:

• Capacitance
• Temperature
• Frequency
• Transistor test - hfe, etc
• Continuity (buzzer)

It is worth bearing in mind that these additional test capabilities may not offer the same levels of accuracy that a specialised meter for the particular measurement may be able to offer .

When choosing a particular digital multimeter it is necessary to look at not only the maximum, but also the minimum readings that can be taken.

True RMS vs average

One important element of a digital multimeter specification is whether the DMM is able to measure true RMS values. Today many high end digital multimeters include this facility, although the lower end instruments will typically not incorporate it. True RMS measurements provide a measure of the actual heating effect of the waveform. The reading will therefore take account of the AC waveform, although with any harmonics and distortion, as well as any DC components. It is worth looking carefully at the DMM spec to ascertain whether it does provide a true RMS measurement, and if so whether it is able to incorporate any DC components that may be present.

Those digital multimeters that do not provide a true RMS measurement measure the average of the absolute value of AC voltage and are calibrated so that the readings are corrected to that of the RMS value for a sine wave. This normally works well but errors occur if harmonics are present with the effect becoming progressively worse as the harmonic content increases. For example, if a triangular waveform is measured this will give an error of nearly -4%, while for a square wave signal the error is just over +11%. The presence of any DC will then introduce further errors. Pulse waveforms will also be difficult to measure and the error will depend upon the duty cycle of the signal.

In many instances an average reading calibrated for RMS values is quite satisfactory. However it is necessary to choose the correct DMM to buy new or as used test equipment, or when choosing test equipment rental. It also helps when interpreting the results that are obtained if the limitations of the digital multimeter are fully understood.

DMM resolution

The resolution of a digital multimeter is normally specified in terms of the number of digits displayed. Typically this will be a number consisting of an integer and a half, e.g. 3 ½ digits. By convention a half digit can display either a zero or 1. Thus a four and a half digit meter could display up to 19999.

Occasionally a three quarters digit may be used instead of the half. When this is seen, it indicates that the DMM additional numeric can display a number higher than one, but less than nine. Often the range is extended to 399, 3999, etc.

When choosing a digital multimeter for a particular application, it is worth remembering that increased levels of resolution do not come without a penalty. Longer settling times are required for the far right digits to reach their final value. Thus the time between readings is longer. It is therefore normal to use ranges that give sufficient resolution and no more to ensure the fastest update rate is achieved.

Uncertainty specifications

No item of test equipment, including a digital multimeter is able to offer 100% accurate measurements. There is always a level of uncertainty associated with the measurement. As a result the capability of the DMM needs to be specified accordingly. Normally the error is quoted in the form:

Sometimes the spec may be quoted in terms of counts rather than digits. The two terms can be used interchangeably in this case they indicate the value of the least significant digits for a particular range - representing the resolution of the digital multimeter for that range.

To see how this works out in practice, take the example of a digital multimeter set to a 19.99 range. In this case one digit or one count will be equal to 0.01. Then taking this further, when measuring a voltage of 10 volts on the 19.9999 volt range. If the uncertainty is quoted as ± (0.002 % + 2 counts) then the total uncertainty for this reading can be calculated as:

An alternative method used by some DMM manufacturers is to specify the uncertainty in terms of:

It is a simple matter to then multiply the maximum reading for the range by a percentage to get the second term. The second term often results from factors such as noise and various offsets that occur within the measurement system within the digital multimeter.

Environmental and other external specifications

A digital multimeter will only be able to meet its specifications when it is within a certain environment. Conditions such as temperature, humidity and the like will have impact on the performance. Also conditions such as line voltage can affect the performance. In order to ensure that the digital multimeter is able to operate within its uncertainty specification, it is necessary to ensure that the external conditions are met. Outside this range the errors will increase and the readings can no longer be guaranteed.

A further element to be considered is the calibration period of the digital multimeter. As all circuits will drift with time, the DMM will need to be periodically re-calibrated to ensure that it is operating within its specification. The calibration period will form part of the specification for the DMM. The most usual calibration period is a year, but some digital multimeter specifications may state a 90 day calibration period. The 90 day period will enable a tighter specification to be applied to the digital multimeter, allowing it to be used in more demanding applications.

When looking at the calibration period of the digital multimeter, it should be remembered that calibration will form a significant element of the cost of ownership and after some years will be significantly above that of any depreciation. A long calibration period for the digital multimeter is normally to be advised, except when particularly demanding testing is required.

Additional DMM features

In addition to the basic measurement features that the digital multimeter, DMM may offer, there are a number of other facilities that enable measurements to be made more easily. With digital multimeters making good use of integrated circuit technology, a variety of features can be included that would not have been possible when using analogue multimeters.

One of the features present on many high end DMMs is auto-ranging. As the name implies, this allows the DMM to choose the correct range for the value appearing at its input. Manual selection of the particular type of measurement to be made is still required.

A further feature that most digital multimeters incorporate is auto-polarity. This enables the DMM to indicate the polarity of the reading with respect to its input connections without the need for the meter leads to be connected the correct way round.

Summary

The digital multimeter specification is an important item associated with the DMM. Whether the DMM is to be bought as new or used test equipment, or whether it is to be obtained through a test equipment rental agreement, the DMM specification is a vital element in ensuring that the right digital multimeter is obtained.

Further pages from this tutorial
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