Causes of calibration problems
What knocks a digital instrument “out of cal?” First, the major components of test instruments (e.g., voltage references, input dividers, current shunts) can simply shift over time. This shifting is minor and usually harmless if you keep a good calibration schedule, and this shifting is typically what calibration finds and corrects.
But, suppose you drop a current clamp — hard. How do you know that clamp will accurately measure, now? You don’t. It may well have gross calibration errors. Similarly, exposing a DMM to an overload can throw it off. Some people think this has little effect, because the inputs are fused or breaker-protected. But, those protection devices may not trip on a transient. Also, a large enough voltage input can jump across the input protection device entirely. This is far less likely with higher quality DMMs, which is one reason they are more cost-effective than the less expensive imports.
Calibration frequency
The question isn’t whether to calibrate — we can see that’s a given. The question is when to calibrate. There is no “one size fits all” answer. Consider these calibration frequencies:
Manufacturer-recommended calibration interval. Manufacturers’ specifications will indicate how often to calibrate their tools, but critical measurements may require different intervals
Before a major critical measuring project. Suppose you are taking a plant down for testing that requires highly accurate measurements. Decide which instruments you will use for that testing. Send them out for calibration, then “lock them down” in storage so they are unused before that test.
After a major critical measuring project.If you reserved calibrated test instruments for a particular testing operation, send that same equipment for calibration after the testing. When the calibration results come back, you will know whether you can consider that testing complete and reliable.
After an event. If your instrument took a hit — something knocked out the internal overload or the unit absorbed a particularly sharp impact — send it out for calibration and have the safety integrity checked, as well.
Per requirements. Some measurement jobs require calibrated, certified test equipment — regardless of the project size. Note that this requirement may not be explicitly stated but simply expected — review the specs before the test.
Monthly, quarterly, or semiannually. If you do mostly critical measurements and do them often, a shorter time span between calibrations means less chance of questionable test results.
Annually. If you do a mix of critical and non-critical measurements, annual calibration tends to strike the right balance between prudence and cost.
Biannually. If you seldom do critical measurements and don’t expose your meter to an event, calibration at long frequencies can be cost-effective.
Never. If your work requires just gross voltage checks (e.g., “Yep, that’s 480V”), calibration seems like overkill. But what if your instrument is exposed to an event? Calibration allows you to use the instrument with confidence.
One final note
While this article focuses on calibrating DMMs, the same reasoning applies to your other handheld test tools, including process calibrators.
Calibration isn’t a matter of “fine-tuning” your test instruments. Rather, it ensures you can safely and reliably use instruments to get the accurate test results you need. It’s a form of quality assurance. You know the value of testing electrical equipment, or you wouldn’t have test instrumentation to begin with. Just as electrical equipment needs testing, so do your test instruments.
