Hello, I'm Ray Sheen.
Now key elements of the management and maintenance of any measurement system is
the use of standards and the calibration of those.
Let's explore both of those and see how a measurement system error can be managed.
I'll start with an explanation of a measurement standard.
In fact, let me back up for a moment and introduce the term metrology.
Metrology is the study of measurement systems.
Many organizations have a metrology department.
It may be just a one man show but that department is responsible for
the management of the measurement systems in the organization.
Now, I don't mean the operational management of those measurement systems,
but rather for managing the process of calibration and maintaining,
certifying, and sometimes even designing the measurement systems.
This organization will also normally guide the conduct of Gage R&R studies, but
more about that in other lessons.
The metrology department will determine the standards that will be used to
calibrate all measurements system.
A measure standard is an object system where experiment that
bears a defined relationship to the unit of measurement of the physical entity.
The standard defines a unit of measure, but just to be clear
this is a different definition of standard than what is used to set policies and
procedures, such as an ISO standard or IEC standard.
This is an actual physical item that is measurable.
These measurement standards are used as references for
determining the efficacy of a measurement system.
The precise value of a standard is known.
The measurement system measures the standard and
will ideally register that value.
If not, there is a problem with the measurement system that must be corrected.
The use of a measurement standard builds confidence in the measurement system.
The operators and
managers trust that the values registered in the measurement system are correct.
It also enables an organization to use different technologies for
measuring the same parameters and have confident that these
measurement values can be combined or compared for analytical purposes.
And with the use of this standards, a common language for
describing characteristics and performance can be used universally.
An aeroplane's air speed value is understood the same way
no matter what country the aeroplane is flying over.
Standards are often used to perform calibration.
Let's look at this process.
The purpose of a calibration is to ensure the measurement system
is registering accurate values,
that means that it is used to remove bias in the measurement system.
Calibration relies upon a standard.
The standard item is a known value.
The measurement system measures the standard item.
If the value registered by the measurement system does not match the known value,
the system is adjusted until it does match.
If a measurement system has relatively high precision errors, the calibration
checks should be accomplished several times to create a distribution, and
the adjustment is then made to shift the average or mean of that distribution.
Let's focus on the standard for a minute.
An obvious question is how do we know the true value of the standard.
What if the measurement system used to give the value of the standard
had errors in it?
That is why a metrology department will always use standards that are traceable to
national or international standards.
Many countries have national standards labs, which will calibrate measurement
equipment for organizations or provide a very precise and accurate measurement
of a standard that the company will use to perform their own calibrations.
If not,
the company will send the equipment off to these labs to be calibrated by them.
Then of course these natural labs will trace their standards back to
international standards of measure.
Measurement systems used in most companies especially those that have certified audited
quality systems will have a calibration system in place for
all measurement systems in use.
Most items have a duration period for that calibration.
At the end of that period,
the item is recalibrated in order to remove any stability concerns.
Measurement systems should not be used if the calibration time and
period has expired.
The organization's metrology department will usually have a list of all
calibrated measurement systems and track when calibration time periods expire.
They're responsible for
scheduling a recalibration of the system when appropriate.
So let's sum up the last few lessons concerning the measurement system errors
and what we can do about them.
First, the good news is that measurement system errors can be identified and
either controlled or even eliminated.
But that doesn't occur by accident, it requires actions by the metrology
department, the process management, and the operator.
Accuracy or bias errors can be eliminated and controlled by regular calibrations
with known traceable standards that are appropriate for the type of measurement.
Precision errors that are due to common cause variation in the use of
the measurement equipment can be identified and
quantified through the use of a Gage Repeatability and Reproducibility Study.
If this common cause errors cannot be controlled,
a different measurement system should be used.
Our next lessons will discuss how to do a Gage R&R study.
Stability or
drift errors can be identified by the regular use of Golden Standards.
These problems can be controlled by addressing the underlying cause of
the drift.
It might be the use of the measurement system procedures or
the monitoring management and maintenance of the measurement system itself.
Ultimately, if the stability cannot be controlled,
a new different system should be used.
Linearity or bias shift problems are based upon the ideal range of the measurement
system being used.
If there is a problem, the only option is to select a different measurement system.
And finally, discrimination is a resolution designed into the system.
Again, if it's a problem, you must pick a different system.
Standards and calibration are two of the important tools that we use to manage and
maintain our measurement system.