Recently, we addressed the question “what is calibration?” and provided a general overview of the subject. But calibration itself is not the end of the matter. In fact, after a measurement device has been calibrated, you’d be mistaken to imagine everything is good to go without any more attention required.
First, it’s a matter of devices
After calibration comes recalibration; basically, the process of ensuring that everything remains in order over the course of the device’s lifespan. Of course, it (recalibration) doesn’t apply to all devices. Once a tape measure has been calibrated and manufactured, that’s that. The markings aren’t going to shift. But to use another common example, what about a weighing scale based on the action of a spring?
That’s a different story. Over time, the tension of the spring will change and that will impact the measurement results the scale yields. If you want to ensure the scale remains accurate, periodic recalibration will be necessary. This obviously matters since, if you don’t recalibrate, the results will become less and less accurate. If you’re selling a valuable commodity (for instance, gold) you’d hardly want to use a scale that undervalued the amount being sold.
Equally, if you were using a machine tool to manufacture a part that needed to fit into another part within a larger machine, using a tool whose calibration wasn’t maintained would eventually lead to producing parts that didn’t fit. That would be an expensive and time-consuming problem to correct.
The point is, recalibration ensures that the calibration table is updated with current values, and the outcome of the process is that measurement results should again be correct.
Candidates for recalibration
In general, every measurement device that contains moving parts or relies on mechanical, electric, optical, or chemical properties that are known to change over time will give variation in the measurement results it presents and therefore must be periodically recalibrated. There are also particular aspects beyond these standard characteristics can trigger a requirement for recalibration. They include:
- Device workload: the heavier its usage, the more likely it is a measurement device will need regular recalibration.
- Environmental conditions: a device used in extreme conditions is more likely to need recalibration than a device used in stable conditions.
- Stability: An instrument that’s moved around (as opposed to being used in situ) is more likely to require recalibration.
- Shocks: An instrument that’s received a shock (for instance, being dropped) is an obvious candidate for recalibration.
The interval between recalibrations will vary depending on the device and can be regular (e.g., annually) or based on use frequency (e.g., every 1000th operation). Or it could be a combination of the two. It is also possible to verify calibration by measuring a set of reference objects with a certain interval, and only recalibrate when these verification measurements are outside a set specification value.
Generally, the following points can be observed as guidelines for the timing of recalibration:
- Consistent with the recommendation of the manufacturer.
- After any mechanical or electrical shock.
- Periodically as noted above (annually, quarterly, monthly)
- Criticality of the measurement in question
- Stability history of the instrument
- Regulatory requirements and quality systems
- Consequences and costs of a failed calibration
Let’s look at just a couple of those points in a little more detail. Manufacturer’s recommendations vary, but there is often a recommended calibration period for a device and, given that the manufacturer probably knows its own equipment’s behavior better than anyone else, at a minimum this is a good starting point for considering when to recalibrate.
Also, with some specific industry measurements, there can be standards-based, regulatory requirements that order the length of the calibration period. Here, even if it’s inconvenient to recalibrate, you’re bound to adjust to meet such demands.
What about validation?
Now, let’s talk about validation introducing first a term, “stability history”, that we haven’t used so far. Validation is also a critical element to consider when thinking about recalibration.
When recalibration indicates a device requires adjustment, the exact results and nature of that adjustment should be recorded. If you only adjust the instrument and achieve a renewed calibration certificate without recording any detail, it will appear that the instrument was stable with some possible drift. This doesn’t tell the whole story and may not be the case, thus having the potential to wrongly influence subsequent recalibration schedules.
The more calibration and recalibration information you collect over time, the greater the stability history of the measurement device you’ll be able to build, and this will enable you to set more accurate recalibration periods via validation. If an instrument drifts too much and frequently fails to meet the tolerance in recalibration, then you would make the recalibration period shorter. On the other hand, if a device consistently meets the tolerance limit in every recalibration without any need for adjustment, you will lengthen the period between recalibrations.
To summarize, the availability of stability history contributes to “validation”, something that should be done on a more regular basis than recalibration itself. Validation is done using a set of reference materials of which stability history is a part, and it can provide a trigger for recalibration (or, indeed, confirm whether calibration is still valid). Regular validation can also tell you if you’re measuring in a stable way (or not). When a system cannot be recalibrated to measure within specification, then it requires adjustment.
Taking cost and risk into account
Lastly, you might also consider undertaking a formal risk analysis to contribute to determining a recalibration schedule. The cost and consequences of a failed calibration could be significant, so you will want to determine the right balance between the costs of the calibration program and the risks of inadequate recalibration schedules. It’s essentially a matter of answering the question, “what are the consequences if this instrument gives us the wrong data?”
If the instrument or device is non-critical, then you may be willing to accept the odd calibration failure in favor of longer recalibration intervals. If the device/measurements are critical then the consequences of a failure to recalibrate adequately can be enormous and (see above) may even result in regulatory problems as well as loss of customer confidence, reputational damage, and the like. The most common consequence, perhaps predictably, is increased cost. In industries like pharmaceuticals and food, the margin for error is obviously minimal and the consequences of a lack of regular calibration are unacceptable. The same applies to parts made for aircraft or other transportation – and, indeed, for many areas.
Actually, this is well worth considering further. A part may only be small or be one of thousands that constitute a completed object, but they are all important to the structural integrity or function. Even failure of the smallest part can have massive consequences (remember the O-rings?), so the overall risk should be taken into account, as well as the resilience that’s built into the product. If a wire breaks, what happens?
Importance
It should be obvious that recalibration of a measurement device is a mission critical procedure. Without it, the values obtained in a measurement would be little more than random numbers. Of course, you could make the case that recalibration is only necessary when control measurements tell that the system is becoming inaccurate, but that doesn’t contradict the general assertion and as we’ve demonstrated, there’s more than to recalibration than just that.
About Conoptica
Conoptica is the market leader for measurement equipment in the wire & cable industry and has been providing high tech camera-based measurement solutions for the metal working industry since the 1993. We make sure that the metal working industry has access to key quantitative data about their products and tools.