A digital multimeter (DMM) is one of the most commonly used pieces of test and measurement instrumentation. Quality processes depend on its continual proper operation. However, time, environment, and physical use (or abuses) change a DMM’s characteristics. That’s why it’s important to periodically calibrate or verify the performance of a DMM.
Calibration confirms that the multimeter’s performance meets the required specifications. The specifications can be the standard figures published by the DMM’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.
Choosing the right calibrator
The calibrator you choose for the job should have these qualities:
- Its specifications should be at least four times better than the specification of the DMM it is testing.
- Its functions need to cover the functions of the DMM it calibrates. DMMs 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.
Often the DMM’s manual will recommend functions and minimum specifications necessary to calibrate the DMM properly. The manufacturer should provide all the test points and pass/fail limits as evaluated against their specification.
There are also 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 DMMs” provides guidelines on terminology, procedures, measurement uncertainty, and calibration certificates.
Fluke Calibration publishes a variety of materials to help you choose the best calibrator for your workload. For example, the “Electrical Calibrator Workload Matrix” summarizes the functions, accuracies and targeted workload for every Fluke Calibration electrical calibrator.
Another application note, “A generic DMM test and calibration strategy,” provides more details on how calibration relates to a DMM’s performance, the rationale of testing and strategies for quality calibration.
Sequences of calibration
Depending on the “as received” calibration state of a DMM and the customer’s requirements, there are three different sequences of operation that could be performed. The third sequence is the most common and most valuable for calibration customers.
- Calibration only (no adjustments made)
- Adjustment first; post-adjustment calibration (as left data)
- Pre-adjustment calibration (as found data); adjustment; post-adjustment calibration (as left data)
The calibration certificate
“EURAMET CG-15: Guidelines for Calibration of DMMs” recommends this information be included in a report of calibration:
- General information about the calibration
- Identification of the procedures employed
- Measurement uncertainty