Calibration

The purpose of this section is to outline

the procedures for calibrating an
instruments while guaranteeing the
'goodness' of the calibration results.
Calibration is a measurement process that
assigns values to the property of an
instrument or to the response of an
instrument relative to reference standards
or to a designated measurement process.
Electronic Insrtrumentation

Purpose of calibration

The purpose of calibration is to

eliminate or reduce bias in the user's
measurement system relative to the
reference base. The calibration
procedure compares an "unknown"
or test item's or instrument with
reference standards according to a
specific algorithm.
Electronic Insrtrumentation

Issues in calibration

Calibration ensures that that the measurement

accuracy of all instrument used in a measurement
system is known over the whole range.
Environmental conditions must be the same as
those under which they were calibrated.
Under different environmental conditions,
appropriate correction has to be made.
Instrument calibration has to be repeated at
prescribed intervals.
Magnitude of the drift in characteristics depends
on the amount of uses, even ageing effect while
storage.
It is difficult or even impossible to determine the
required frequency of instrument recalibration.
Electronic Insrtrumentation

Basic Instrument
Calibration: The User
Perspective

Users of electronic measurement instruments

have several key objectives in mind when
obtaining calibration/adjustment services, such
as:

Ensuring the validity (within specification) of

recent measurements.
Having a high confidence level that after a
calibration/adjustment process is completed, the
instrument will operate within specifications until
the next scheduled calibration/adjustment event.
Obtaining records/reports that satisfy the needs
of the user's company, including:
A calibration due date sticker on the product.
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A calibration certificate showing a record of the

calibration type (usually a calibration laboratory
"standard" calibration, or a calibration that complies
with an industry standard).

Any out-of-tolerance points for the product that are

received prior to adjustments.

Calibration due reports that provide sufficient time

warning to allow for appropriate resource planning.

Inventory tracking and status reporting of

instruments that are "out-of-service," denoting
where they are in the calibration/adjustment
process.
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Controlling the cost of all the above services

to match the user's budget needs.
Keeping the "out-of-service" time period to a
minimum.
Making the calibration/adjustment processes
very reliable (that is, free of errors).
Records that allow the user to evaluate an
instrument's performance and adjust the
time interval between calibration/adjustment
events based on the instrument's
performance with time (drift rate).

Electronic Insrtrumentation

The Manufacturer's
Perspective

Instrument manufacturers have operational

objectives that are very much in synch with those
of the users:
Products should easily meet their specifications for
the recommended period between
calibration/adjustment events, thereby reducing
the questions and concerns that instrument users
may have, and generating a very positive impact on
support costs and warranty claims/costs.
Calibration/adjustment service providers should be
able to perform the calibration/adjustment process
easily, efficiently and effectively. Manufacturers
know that keeping all costs associated with
calibration/adjustment to a minimum will have a
strong influence on users' cost of ownership, and
improve their instruments' competitive position.
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Minimize the equipment set needed to

perform calibration and adjustment, and
use equipment that is commonly available
in smaller calibration facilities to increase
the number of suppliers that can provide
"quality" calibration/adjustment services.
Competition makes the cost of "quality"
services less expensive.
It is clear that the "quality" levels of the
calibration/adjustment services will
strongly dictate how well the issues and
objectives for both instrument users and
manufacturers will be met.
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Process Instrument
Calibration
Calibration consists of comparing the output of the
process instrument being calibrated against the output
of a standard instrument of known accuracy, when the
same input is applied to both instruments.

Standard calibration instruments must be totally

separate.

The calibration function must be managed and

executed in a professional manner.

Separate room should be used for calibration purposes.

Better environmental control to be applied in the

calibration area
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Level of environmental control should be

considered carefully.
Appropriate corrections must be made for
the deviation in the calibration
environmental conditions away from those
specified.
As far as management of calibration
function is concerned, it should be the
responsibility of only one person.
Calibration procedure which relate in any
way to measurement used for quality
control functions are controlled by British
Standard BS 5750.
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Training must be adequate and targeted at the

particular needs of the calibration systems involved

People must understand what they need to know

and specially why they must have this information.

Determination of the frequency at which instrument

should be calibrated is dependent upon several
factors which require specialist knowledge.

The quantities which effect the performance of

instrument over a period of time are mechanical
wear, dirt, ambient temperature and frequency of
usage.

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A proper course of action must be defined

which describes the procedures to be
followed when an instrument is found to be
out of calibration.
Whatever system and frequency of
calibration is established, it is important to
review this from time to time to ensure that
the system remains effective and efficient.
A separate proper maintenance record
should be kept for every instrument in the
factory, whether it is in use or kept as a
spare.
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Standard laboratories

The instrument used for calibration

purpose is known as secondary
instrument. This must obviously be a very
well-engineered instrument which gives
high accuracy and is stabilized against
drift in its performance with time.
When the working standard instrument
has been calibrated by an authorized
standard laboratory, a calibration
certificate will be issued, this will contain
following information's,
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The identification of the equipment

calibrated.
The calibration results obtained.
The measurement uncertainty.
Any use limitations on the equipment
calibrated.
The date of calibration.
The authority under which the
certificate is issued.
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Important

The establishment of a company standards

laboratory to provide a calibration facility of
the required quality is economically
possible only in the case of very large
companies with large number of
instruments to be calibrated.
In small and medium size companies, they
would normally use the calibration service
provided by various companies, which
specialize in offering a standard laboratory.
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Validation of standards
laboratories
In the United Kingdom, the appropriate
national standards organization for
validating standard laboratories is the
National physical Laboratory. This has
established a National Measurement
Accreditation Service (NAMAS) which
monitors both instrument calibration
and mechanical testing laboratories.
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Conditions for standard

laboratories

The head of laboratory must be suitably

qualified.
The management structure should be
such that Rush or skip calibration
procedures for production reasons can be
resisted.
Proper temperature and humidity control
must be provided.
High standard of cleanliness and
housekeeping must be maintained.
Full documentation must be maintained.
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Primary reference
standards

Primary reference standards, describes

the highest level of accuracy that is
achievable in the measurement of any
physical quantity.
All equipment used in standard
laboratories have themselves to be
calibrated against primary standards.
National standards organization maintain
suitable facilities for this calibration.
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Traceability

Calibration has a chain-like structure

in which every instrument in the chain
is calibrated against a more accurate
instrument immediately above it in
the chain.
The knowledge of full chain of
instruments involved in the calibration
procedure is known as Traceability
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Instrument Calibration
chain
National standard organization

Standards laboratory
Company instrument laboratory

Process instruments
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Documentation in the
workplace

An essential element in the maintenance

of the measurement system and the
operation of calibration procedures is the
provision of full documentation.
This must give a full description of the
measurement requirements throughout
the workplace, the instrument used, and
the calibration system and procedures
operated.
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Important point to be
noted

Documentation must include with a

statement of what measurement limits
have been defined.
The instruments specified for each
measurement must be listed.
The subject of calibration must be defined.
Documentation must specify what
standard instruments are to be used for
the purpose.
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Define a formal procedure of calibration.

A standard format for recording
Calibration result must be defined in the
documentation.
The documentation must specify
procedures which are to be followed if
an instrument is found to be outside the
calibration limits.
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