What knocks a digital instrument “out of calibration?”
Three common causes of calibration problems are:
- Component shift. First, the major components of test instruments (for example, 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; this shifting is typically what calibration finds and corrects.
- Drops. 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.
- Overloads. Exposing a digital multimeter (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.)