The most important benefit of Metrology is that it allows give you confidence in the data upon which you will make quality decisions.
It also gives you confidence that your product truly does meet your customers requirements and that your process equipment is operating under a state of control. All of these items add up to one important business conclusion, utilizing Metrology greatly reduces the overall cost of quality.
Important Concepts in Metrology
Metrology is considered the science of measurement. As a CQE you will at some point be involved in taking measurements (collecting data), and to properly utilize the tools and concepts of Metrology is it important to first understand what they are:
- Calibration Standards & Traceability.
- Measurement Error
- Maintenance of Measurement & Test Equipment (M&TE)
The first major idea of metrology is Calibration which is simply defined as a comparison between measurements. In this comparison of measurements, one of the items being measured is of know correctness or magnitude, this item is known as the Calibration Standard.
What do CQE’s Calibrate?
Measurement & Test Equipment (M&TE)
Calibration of M&TE ensures that your data that is accurate. Without accurate data, trust is lost and the measurement exercise loses all value.
The act of inspection or measurement ensures that your product truly meets the customers expectations. Without it, you could potentially ship non-conforming product.
Calibration Standards & Traceability
How can you truly know if your M&TE is “true”. One way is to create a National Standard to which you can show Traceability.
By consistently verifying your M&TE results against those from a National Standard you obtain confidence in your data and can thus make high quality decisions.
Put differently, by performing Traceability, you can show that your M&TE uncertainty is stable when compared to the National Standard.
This in turn shows your M&TE data is consistent throughout time and no additional uncertainty or error has been introduced into the measurement system since equipment inception.
Here in the U.S. The National Institute of Standards and Technology (NIST) is the keeper of the standards of measurement.
ISO 10012:2003 is the ISO Standard that mandates Traceability to National Standards as a basic function of the Metrologic Function.
As we discussed in the section on Measurement & Test Equipment, all measurement systems have error.
Measurement Error, otherwise known as variation, can be considered either normal (or random) or special cause. Normal variation is the natural variation associated with the measurement process.
This variation is inherent to your process and probably cannot be reduced or eliminated. Special Cause Variation is any variation that is not common to the process that add unwanted uncertainty.
Special Cause Variation can include things like incorrect calibration of measurement equipment, or damage to measurement tooling.
Some of the different sources of normal variation includes the equipment, environment, the DUT (Device Under Test), operator, time, etc.
Both of these types of measurement error adds uncertainty to your measurement values, reducing the precision of your equipment and ultimately reducing your trust in the measurements themselves.
For example, let’s say you take a final measurement on your product before it goes out the door, but your measurement system has a large amount of variation. It is possible that you have accepted product that is non-conforming, and rejected product that is conforming.
In this situation, a good CQE could utilize a control chart to visually depict the measurements from the system to make sure the system is still operating in a state of control. This tool can help you detect and eliminate sources of special cause variation in your measurement system.
Control of Measurement Equipment
One key concept in the Calibration/Metrology world is the idea of controlling measurement equipment.
How do we control measurement equipment?
By performing Calibration at regular and appropriate intervals using well maintained standards.
This last part is key, have a calibration “standard” or a “gold Standard” that is maintained and accurate is key to ensuring your measurement equipment is functioning properly.
This it ensures the data collected from those instruments and the resulting decisions made from that data are accurate.
This control is achieved through the maintenance of accurate “Gold Standards” used for Calibration.
This is sort of the linchpin in process control, good quality data. ANSI/ISO/ASQC Q9001:2000 Control of Monitoring and Measuring Devices provide further guidance on this topic.
What is a regular and appropriate interval for Calibration?
Well, it depends! It depends on a number of variables including historical information on the equipment, the equipment’s stability (resistance to change), the purpose of the measurement system (is it a critical measurement or fairly minor) and the cost ($$) of an inaccurate measurement (another form of criticality).
Calibration should be performed before any measurements are taken and if production is sporadic calibration may need to be performed on a “prior to use” basis.
The Calibration interval you pick should be based on data (duh?).
The equipment manufacturer will most likely make a recommendation for this calibration interval and a conservative approach is to release the equipment based on a shorter than the recommended one on a trial bases with a plan to lengthen the interval with subsequent re-calibrations or re-evaluations of the equipment showing it to still be within calibration.
You can continue to lengthen the calibration interval until the equipment fails a re-calibration. The maximum Calibration interval is typically 12 months and like I said above, is production is sporadic (months apart) perhaps a “prior to use” interval is appropriate.