- the essential facts about test equipment calibration to enable the
best to be gained from the test equipment.
In order that any item of electronics test equipment
performs to its specification, it is necessary to ensure that it receives
its periodic calibration. Like any other item, electronics test equipment
tolerances change slightly with time. There are a variety of reasons for
this, and while it is possible to minimise the levels of drift, they cannot
be totally eliminated. As a result, it is necessary to ensure that the test
equipment receives its calibration. Only when an item of test equipment is
within its calibration period, will it be guaranteed to produce results that
are within its stated specification. Once the calibration period has been
exceeded, then it is not possible to guarantee the results.
When using an item of test equipment there are some
circumstances when the calibration of the test equipment is required, under
other circumstances it is not. It may be that a measurement is only required
to provide a rough order of magnitude reading and for this a fully
calibrated test instrument may not be needed. However for development test,
production test and other areas such as qualification test, it is crucial
that the results from the test equipment can be guaranteed. Under these
circumstances it is necessary to ensure that the test instrument is within
its calibration period.
In view of the fact that most test instruments and other
pieces of test equipment are used both for ad hoc measurements as well as
those for more formal test applications, it means that all test equipment
available for use should be calibrated. While calibration adds an additional
expense to be covered by the company, the consequences of not having all
equipment calibrated is far more costly.
Test equipment calibration
Test equipment, or test instrument calibration is more than just comparing
an instrument with a similar one to check whether the two readings are the
same. It requires the full calibration procedure for the instrument to be
followed. Normally a calibration procedure will be available for
sophisticated instruments. Specific adjusters will be provided to enable the
instrument to be calibrated. Additionally the required test set-up and
suitable test equipment for performing the calibration procedure will be
defined.
Calibration traceability
When any test equipment calibration is performed, it is obviously necessary
to ensure that the equipment against which the item is being calibrated is
itself accurate. In order for this to the calibration of the standard
equipment must be performed against a high quality standard. To ensure this
is of a high standard, many test houses offering test equipment calibration
are able to prove or trace back the measurements to primary measurement
standards held at national institutes such as NPL (National Physical
Laboratory) in the UK, and NIST (National Institute of Standards and
Technology) in the USA. Here highly accurate measurements of fundamental
quantities of time, current, voltage and the like can be made against known
standards.
In view of the fact that the measurements can be traced
back to basic standards, the calibration of the final test equipment is know
to be accurate to the basic known or absolute figures, rather than relative
measurements that may not be accurate to the absolute standards.
Calibration policy
In any company or organisation using test equipment, it is necessary to set
up a properly controlled calibration procedure. In this way, all the
required equipment can be maintained in a calibrated state and it will be
available for use. It is also necessary for the system to visibly flag or
note when any equipment is due for calibration.
The most obvious method of controlling test equipment is
to use adhesive labels attached to each piece of equipment. This provides
information of the date when the equipment was calibrated and the date when
the test equipment is next due for calibration. This shows any users
instantly, the state of the equipment.
Visible calibration stickers should not be the only
method of tracking the equipment state. It is also necessary to ensure that
a central record of the test equipment is maintained. In this way it is
possible to view the status of all the test equipment that is on site and
plan the overall calibration strategy. Trying to achieve this by locating
the equipment and noting its status is not viable. It is always difficult
and time consuming to locate all the test equipment on any site or within a
laboratory. Locating the exact location of the test equipment is often left
to the time when it is needed for calibration.
It is also necessary to record each piece of test
equipment that is used when making measurements. For example all test
results sheets for a production test will record the serial numbers of the
test equipment used. In this way if any problems or discrepancies are
noticed later, these can be investigated and all the relevant test equipment
identified in case the discrepancy has been caused by a test equipment
failure.
Summary
For any organisation using electronics test equipment, it is necessary to
ensure that the equipment is operating to its specification. This is
achieved by rigorously maintaining a process whereby all the equipment in
use is calibrated. This will ensure that all the measurements are accurately
made and with sufficient records, any test equipment problems can be traced
and rectified.
