What is the difference between measuring system calibration and verification?
This is a question that I get asked a lot, and I’m not surprised. These terms can be used interchangeably by some service providers, which has understandably led to confusion. But it’s important that customers are certain what they are getting when they buy verification and/or calibration services.
So let’s look at the terminology as it is widely accepted and understood in the world of metrology equipment manufacturers.
A verification (or certification) is the process of confirming whether a coordinate measuring machine (CMM) performs within its stated specifications. A verification indicates the probability that the measurement error is smaller than the specified maximum permissible error.
In a verification, the user can define the maximum permissible error as the largest error that they are prepared to accept. But the verification process does not compensate for any lack of trueness or apply a correction. That’s where the calibration comes in.
A calibration is the process of restoring a machine back to its stated specifications. This is a separate service from a verification and there are a number of factors to bear in mind when considering a calibration for your measuring machine.
Firstly, how accurate do you need your metrology equipment to be, i.e.,what is the largest permissible error you would accept? It should be noted that widening tolerance acceptances for the sake of certification can reduce control over measurement uncertainty and quality.
What does your service provider actually provide? Will they service your equipment, changing filters, fine-tuning the machine as part of annual preventative maintenance, or can they only provide a certificate verifying the machine’s permissible error?
Crucially, how long will it take your service provider to perform verification and calibration? How soon will it take for them to get round to your machine, and how long do the processes take to complete? A major consideration are calibrated gauge blocks.
Gauge blocks, and any other calibration reference artefact, need to undergo a soak time to acclimatise to the machine’s environment. This takes a minimum of 24 hours, and it’s why the very first thing our engineers do when they start work on a customer’s machine is unpack the gauge block in order for it to start to soak during the first 24 hours whilst they carry on with other service-related work.
For the same reason, our portable equipment is usually calibrated at a Hexagon Manufacturing Intelligence service centre within temperature-controlled environments and anti static floors that aren’t cut off from the additional effects of light and vibration. Likewise, it’s why the customer’s portable equipment is left for 24 hours or more before service and calibration work begins.
Perhaps the most fundamental consideration is the certification of the service provider. Is the provider a recognised calibration laboratory accredited in accordance with ISO 17025, certifying equipment to the ISO 10360 standard? These levels of accreditation are essential for ensuring competence of testing and calibration services; it’s the minimum standard you should expect from your service provider. This is standard practice for any OEM engineer that is looking to perform a full calibration, or even verification.
Read the next blog of this series as we look at what Hexagon offers in a service and calibration.
