Understanding the basics of digital multimeter calibration

In this article, you’ll learn:

• Why calibrate a digital multimeter
• How often to calibrate
• How to choose the right multimeter calibrator
• Sequences of calibration
• How to calibrate – step by step procedure
• What’s on the digital multimeter calibration certificate
• Links to other more specific content on digital multimeter calibration

Why calibrate a digital multimeter?

A digital multimeter (DMM) is one of the most used test and measurement tools. Quality measurements and processes depend on the continual, proper operation of a digital multimeter. However, time, environment, and physical use (or abuses) change a digital multimeter’s characteristics. That’s why it’s important to periodically calibrate or verify the performance of a digital multimeter.

Calibration confirms that the multimeter’s performance meets the required specifications. The specifications can be the standard figures published by the digital multimeter’s manufacturer, or they can be customized based on an organization’s needs. In general, the more precise and accurate a multimeter is, the more complex the specifications will be.

How often should you calibrate a digital multimeter?

The most common DMM calibration interval is yearly. Some specifications may have a different calibration interval, or period in between calibrations. The time in between calibrations can also change depending on time, environment, and use.

How to choose the right multimeter calibrator

When selecting a calibrator for DMM calibration, keep an eye on these qualities:

• Specifications at least four times more precise than the specification of the DMM under test.
• Calibrator functions need to cover the same functions of the DMM it’s calibrating. Digital multimeters have at least five functions, with many ranges per function; the calibration procedure needs to confirm that the DMM is suitable to use through this full range of capabilities.

Fluke Calibration publishes a variety of materials to help you choose the best calibrator for your workload. The Electrical Calibrator Workload Matrix summarizes the functions, accuracies and targeted workload for every Fluke Calibration electrical calibrator.

Finding the digital multimeter calibration standard

Often the digital multimeter’s manual will recommend functions and minimum specifications necessary to calibrate it properly. The manufacturer should also provide all the test points and pass/fail limits as evaluated against their specification.

There are published guides by recognized organizations which provide recommended practices for calibrating digital multimeters and other instruments. For example: EURAMET cg-15: Guidelines for Calibration of Digital Multimeters provides guidelines on terminology, procedures, measurement uncertainty, and calibration certificates.

Sequences of calibration

Depending on the calibration state you receive a DMM in, and the customer’s requirements, there are three different sequences of operation that could be followed. The third sequence is the most common and most valuable for calibration customers.

1. Calibration only (no adjustments made)
2. Adjustment first; post-adjustment calibration (as left data)
3. Pre-adjustment calibration (as found data); adjustment; post-adjustment calibration (as left data)

How to calibrate a digital multimeter – calibration procedure

Using a multi-product calibrator you’ll want to calibrate each function the device under test (DUT) is capable of. When using a Fluke Calibration multi-product calibrator like the 5522A, you’ll find the front panel is arranged to make it easy to use, even for first time operators.

For these examples, we are using a 5522A Multi-Product Calibrator, a Fluke 87V Digital Multimeter and a pair of compatible test leads. The following will walk through, step-by-step, how to calibrate direct voltage, alternating voltage, resistance, capacitance, and direct current.

Calibrating direct voltage

The 5522A can output direct voltage from 0 to ± 1000 V.

1. Connect the V and COM terminals of the digital multimeter to the NORMAL outputs of the 5522A.
2. Set the meter to direct voltage mode.
3. Select a 10-volt DC output from the 5522A by pressing:
   
   10 V is indicated on the output window.
4. Turn the edit knob to vary the 5522A’s output. Notice how the control window now displays error, showing the reference voltage and the difference between it and the current output.
5. Turn the edit knob to return the output to 10 V.
6. The cursor keys above the edit knob and on either side of the EDIT FIELD key allow you to select which decades on the output display to edit. Press the cursor key until the cursor is under the most significant 0 on 10.00000 V.
   Then turn the edit knob clockwise until the output display reads 33 V. Notice that the 5522A dropped into standby mode. This safety precaution protects the operator from inadvertently slewing the output to a potentially hazardous level.
7. Now press the NEW REF key, located in the keypad to the left of the edit knob. Notice that the error display disappeared from the control window. The indicated output is now set as the reference and errors can be calculated with respect to the new reference value. This simplifies certain kinds of troubleshooting or flatness tests.
8. Press the RESET key. This resets the output to 0.0000 mV and puts the 5522A in standby mode.

Calibrating alternating voltage

Alternating voltage is available from 1 mV to 1000 V over a frequency range of 0.01 Hz to 2 MHz. Operation is the same as with volts DC. Just enter the value and the units.

1. Put the digital multimeter in AC voltage mode.
2. Connect the V and COM terminals of the digital multimeter to the NORMAL outputs of the 5522A. Then select a 10 volt 60 Hertz output by pressing:
   
   Notice how the display in the control window has changed, providing you with the opportunity to change the waveform of the volts AC signal or to set a DC offset for duty cycle and sensitivity checks.
3. Press the up arrow under WAVE sine. Notice the menu changes to WAVE tri for triangle wave. Also notice the output display has changed from a conventional rms voltage reading to peak-to-peak.
4. Press the up arrow under WAVE tri and the display changes to WAVE square. Note you are now offered a softkey choice for setting duty cycle, making it easy to directly check the duty cycle display on the digital multimeter.
5. Press the RESET key when done.

Calibrating resistance

Resistance values are available from 0 to 1.1 GΩ.

The 5522A uses a synthesis technique for simulating resistance values. So, rather than using discrete values, you may select values in ohms much the same way you would select a voltage. The 5522A’s unique resistance synthesis technique results in wide dynamic range. Unlike synthesized resistance techniques used in other calibrators, the 5522A’s may be used with a wide range of DUTs, including handheld digital multimeters; analog volt-ohm meters, which typically output relatively high current in low resistance ranges; and bench digital multimeters, which typically output relatively high voltage to make high resistance measurements.

1. Connect the test leads to the Ω and COM inputs of the digital multimeter and to the NORMAL terminals of the 5522A.
2. Put the digital multimeter in ohms mode.
3. On the 5522A, select a 10 ohms output by pressing:
   
4. Notice that the control window displays a range of lead compensation choices. Simply press the up arrow under COMP off to cycle through the choices—off, 2- wire and 4-wire. With compensation off, the 5522A does not compensate for external lead resistances. In 2-wire mode, the 5522A compensates for lead 9 resistance at the terminals of the DUT. 4-wire mode permits the 5522A to be used with DUTs capable of making 4-wire measurements.
5. Notice to the left of the edit knob a group of eight function keys, dedicated to miscellaneous functions. The bottom two, MULT X and DIV, permit you to step the output up or down in even decades of the reference output displayed in the output window.
6. Now the output reads 1 kΩ. Edit the output with the edit knob and keys to set the value to 1.42 kΩ and press the key NEW REF.
7. Try using the DIV key by pressing it three times.
   Now the output display indicates 1.42 Ω.
8. Press the RESET key when done.

Calibrating capacitance

Capacitance is available from 330 pF to 1 mF.

The capacitance function operates in much the same way as the resistance function; that is, simply enter the desired capacitance output. The design of the 5522A’s synthesized capacitance circuit permits it to be used with meters that use both the DC charge/discharge method currently used on handheld digital multimeters, as well as the AC technique used by RCL meters.

1. Connect the test leads to the Ω and COM inputs of the digital multimeter and to the NORMAL terminals of the 5522A.
2. Put the digital multimeter in capacitance mode (on the Fluke 80 Series, select resistance mode then press the blue button for capacitance).
3. On the numeric unit’s keypad, notice there are a number of units indicated in red. These units are entered by first pressing the SHIFT key. The 5522A keyboard will remain in a shifted state until another key is pressed. This eliminates the need to hold down the SHIFT key while entering the shifted value.
4. Microfarads are entered as shifted units. Give it a try. Enter 5 microfarads by pressing:
   
5. The same lead resistance compensation choices available in resistance mode are available in capacitance as well. You may also edit the output in the same way. Press the DIV key twice to reduce the output in even decades to 50 nF.
6. Now, edit the output. Select the most significant digit on the output display by pressing the key back arrow three times.
   And edit the output by turning the edit knob. As with all the other calibration modes, the 5522A calculates the error and displays it in the control window.
7. Press the RESET key when done.

Calibrating direct current (DC)

Direct current is available from 0 to ± 20.5 A.

Sourcing direct current is identical to sourcing direct voltage; however you have to change connections on the 5522A, and most likely also on the digital multimeter. Current is available from the AUX terminals of the 5522A.

Be sure to connect a UUT to the calibrator, or the 5522A will go over-compliance and trip out.

1. Connect the A and COM terminals of the digital multimeter to the AUX HI and LO terminals of the 5522A and put the digital multimeter in mA/A current mode.
2. Select 100 mA by pressing:
   
3. Then press the MULT X key once to change to 1 A, then again to change to 10 A.
4. Turn the edit knob to see the 5522A calculate the uncertainty.
5. Press the RESET key when done.

Calibrating alternating current (AC)

Alternating current is available from 33 μA to 11 A over frequencies from 0.01 Hz to 10 kHz.

Using the 5522A in alternating current mode is similar to using it in alternating voltage mode; however, you must change connections on both the 5522A and the digital multimeter. Current is available at the AUX terminals.

You also must be sure to connect a DUT, or the 5522A will go over-compliance and trip out.

1. Connect the A and COM terminals of the digital multimeter to the AUX HI and LO terminals of the 5522A.
2. Put the digital multimeter in alternating current mode by turning the digital multimeter knob to mA/A (DC) position and pressing the blue button (Fluke 80 Series).
3. Select 100 mA 60 Hertz by pressing:
   
   Notice that the control window display shows that the output is on the auxiliary terminals. Also notice that you have the same choice of waveforms available in alternating voltage mode. Each is selected by pressing the softkey under the display. As with all other modes, the output can be edited by selecting a digit on the output display and slewing it up and down with the edit knob.
4. Press the RESET key when done.

Multimeter calibration certificate

EURAMET cg-15: Guidelines for Calibration of Digital Multimeters recommends including certain pieces of information in a calibration report. “Besides the general information about the calibration certificate (such as the title, its univocal identifier, the number of page on each page with the total number of pages), the CL (name and address of the laboratory where the calibrations was carried out), the customer (name and address), the unambiguous identification of the DMM calibrated (such as its serial number), the date of its receipt and the date of performance of the calibration, the name, function and signature of person authorizing the calibration certificate, the following information is considered necessary in order to provide full knowledge of the operations performed on the digital multimeter:

• identification of the calibration method used (i. e. by the identifiers of calibration procedures used)
• evidence that the measurements are traceable (i. e. citing the standards used in the calibration)
• the conditions (i. e. environmental conditions such as temperature and humidity) that have an influence on the calibration results
• the calibration results with the units of measurement
• the measurement uncertainty

Keep Learning:

• Introduction to the Basics of Digital Multimeter Calibration (on demand webinar)
• A generic digital multimeter test and calibration strategy
• Formatting for Fluke Calibration Certificates of Calibration
• Understanding the Basics of Digital Multimeter Calibration | Back to Basics Series

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