How Often Should I Calibrate My Equipment?
Calibration is important. We all know that. However, knowing when to get calibration can sometimes be a little tricky.
Unfortunately, one calibration is not good enough for a piece of equipment to remain in operation for years to come due to the wear and tear that everyday use puts on the equipment.
In fact, ISO 9001 dictates that calibration be performed at specific intervals. The reason behind this is that even the best maintained equipment in the world degrades over time.
For a good example of this, lets look at a Go/No-Go cylindrical plug gage. The purpose of the "Go" end of the gage is to slide through the dimension it is measuring. As the gage continues to be used, inevitably it checks dimensions that are rather "snug" (up against the low end of its tolerance). Each time this gage is used in this scenario, microscopic amounts of material wear off. When dealing with tolerances commonly in the 1/10,000th of an inch, it does not take much for a gage to wear past its acceptable tolerance limit. Regular, repeated calibrations catch this issue before it causes any of the serious quality issues presented above.
So how do you know how often you should calibrate your equipment?
Per Manufacturer's Recommendations
Many types of measuring equipment will come with an owner's manual giving an approximate recommendation for how often it should be calibrated. This recommendation should be treated as the baseline standard, with your own analysis determining whether something should be calibrated more frequently.
Following Mechanical or Electrical Shock
If a piece of measuring equipment undergoes any mechanical or electrical shock, it should always be recalibrated to ensure its accuracy. Even if nothing appears to be wrong with the instrument, the extremely precise tolerances employed by these instruments mean that even microscopic changes can alter its efficacy.
Periodically (e.g. Annually, Quarterly, Monthly)
This is far and away the most common approach to scheduling recalibration, as it is by far the easiest to maintain. One year time periods being the most common interval of this approach. Generally speaking, this will suffice in catching equipment before they fall out of tolerance.
Certain scenarios call for a shorter time window and some call for a longer time window. All decisions on this front must be evaluated from your end to determine what scenario is the best for you. A simple way of looking at this is if too many pieces of equipment are being found out of tolerance, the time cycle for recalibration may need to be shortened. Remember, you do not want your equipment failing calibration.
After a Certain Number of Uses
As technology continues to advance, it is becoming easier and easier to track how often a piece of equipment is used during a certain time period. This gives you a very significant advantage in being proactive when it comes to scheduling your recalibration. Since nothing magical happens on that 365th day, causing a piece of equipment to fall out of tolerance, this allows you to allocate your calibration dollars as efficiently as possible.
Over time, you can track how many uses it takes, on average, for your different types of equipment to fall out of tolerance. Once a good baseline is established, it is as simple as recalibrating each piece of equipment before it hits this usage limit. This system is very easy to follow for commonly used gages, such as hand tools (calipers, micrometers, etc.) and hard gaging (plugs, rings, etc.).
If you look at implementing this system, you will want to make sure that you still have a periodic cycle listed as well. An example of this would be having all calipers set to be recalibrated at 150 uses, OR 18 months, whichever comes first.
Prior to Each Use
The final method of scheduling your recalibration only comes into play in unique scenarios. Although it is rare, it does happen that you will need to purchase very specific types of measuring equipment. This measuring equipment may only need to be used once every handful of years. Obviously, calibrating this equipment each year in between is overkill. In this scenario it is much more cost effective to measure the equipment prior to each use.
Extreme caution must be taken, however, to ensure that this equipment is not being used in between (sometimes to the extent of putting this equipment behind lock and key).
Which Is Best For You?
As you can imagine, there is no one-size-fits-all approach to determining your calibration intervals. Every company is unique and the equipment you have is even more unique. What works best for you does not mean it will work best for others. To even further complicate matters, you will see that even within your own facility, there are many different types of equipment requiring different levels of care.
However, worry not. The process to determine which interval is best is not difficult. It is simply a matter of analyzing your calibration results to see where small improvements can be made. If all of your equipment is passing, then great, you have an adequate program set up. If a large percentage of your equipment is failing, you may want to shorten your interval or switch to a usage based cycle. If most everything passes, but certain pieces of equipment always fail, shorten the interval on those pieces of equipment. If certain pieces are always passing, but never seem to be used in between, maybe switch those to prior to each use.
At the end of the day, only you can determine what interval is best for your needs and risk management practices. But don't worry, once you start diving in, you will see that the process is quite easy.