| 1.
Calibration
Calibration - set of operations that establishes,
under specified conditions, the relationship between values
of quantities indicated by a measuring instrument or measuring
system, or values represented by a material measure or reference
material, and the corresponding values realized by standards.
( International Vocabulary of Basic and General Terms
in Metrology (VIM; 1993) definition)
In other words, the calibration determines the values of the
errors of a measuring instrument (and if necessary determine
other metrological properties as well).
As a result of the calibration, a Calibration Certificate is
issued accompanied by a Test Report with the test results. Scantek,
Inc. provides all test results at no extra cost.
Calibration and measurement certificates -
the result of a calibration may be recorded in a document,
called a calibration certificate or a calibration report.
(VIM and ISO Guide 30: Terms and definitions used in connection
with reference materials. See also ISO Guide 31: Contents
of certificates of reference materials)
According to these definitions, the calibration is not required
to provide a statement about compliance with accepted specifications.
Nevertheless, in order to help the users, Scantek, Inc. provides
"passes" or "fails", when appropriate, in
the terms defined by ISO 17025. If reliable specifications are
not available, then no statement of compliance is made.
Calibrations may include adjustments,
always reported, to correct any deviation from the value of
the standard, but this is not covered by the definition of the
service.
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2. Traceability
The definition of traceability is contained in the International
Vocabulary of Basic and General Terms in Metrology (VIM; 1993):
"the property of the result of a measurement or the value
of a standard whereby it can be related to stated references,
usually national or international standards, through an unbroken
chain of comparisons, all having stated uncertainties."
Unbroken chain of comparisons - complete,
explicitly described, and documented series of comparisons
that successively link the value and uncertainty of the result
of a measurement with the values and uncertainties of each
of the intermediate reference standards and the highest reference
standard to which traceability for the result of measurement
is claimed
In other words, from the best (national laboratory) to Scantek,
Inc. Calibration Laboratory, we account for all uncertainties
to assure what we report is accurate.
It is important to note that traceability is the property of
the result of a measurement, not of an instrument or calibration
report or laboratory. Following any one particular procedure
or using special equipment does not achieve it. Merely having
an instrument calibrated, even by NIST, is not enough to make
the measurement result obtained from that instrument traceable.
The measurement system by which values are transferred must
be clearly understood and under control.
To achieve and maintain traceability to the International System
of Units (SI), Scantek, Inc. implemented a quality system, environment
controls and increased competence so it complies with every
one of the NVLAP requirements listed in the IEC 17025 - 2005
standard, as the accreditation audit recently confirmed.
Following are the key elements of the implemented traceability:
- reference standards calibrated directly by NIST, NRC-Canada,
NPL England or PTB Germany
- use of validated procedures and test methods for all tested
parameters
- documented measurement conditions and uncertainties, which
are reported with each measurements
- internal measurement assurance program to insure maintenance
of the quality of the standards and of the services provided
- competent, factory trained personnel to perform service
and calibrations.
Internal measurement assurance program -
program of sufficient complexity, within an organization,
to provide credibility to the measurement uncertainty and
measurement result for which traceability is to be established.
An internal measurement assurance program usually involves
monitoring the performance (e.g., stability, reproducibility)
of the instrument, standard, or measurement system, both before
and after it is characterized calibrated, or used to obtain
the traceable measurement result
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3. Accreditation
Laboratory accreditation - procedure by
which an authoritative body gives formal recognition that
a laboratory is competent to carry out specific tasks. Accreditation
does not itself qualify the laboratory to approve any particular
product. However, accreditation may be relevant to approval
and certification authorities when they decide whether or
not to accept data produced by a given laboratory in connection
with their own activities. (ISO Guide 58: Calibration and
testing laboratory accreditation systems - General requirements
for operation and recognition, 1993)
The laboratory accreditation, whether conducted by NIST/NVLAP
or any other recognized accreditation body, is a finding of
a laboratory's competence and capability to provide scientifically
sound and appropriate measurement services within their scope
of accreditation. Embedded in the process is an evaluation of
the lab's ability to achieve and maintain traceability for the
accredited services. Accreditation to ISO/IEC Guide 25, now
replaced with international standard ISO/IEC 17025 : General
requirements for the competence of testing and calibration laboratories
, determines that a laboratory has all of the necessary
facilities, equipment, standards, procedures, uncertainty analyses,
personnel, etc., which make it capable of providing traceable
measurement results. Laboratory accreditation speaks to the
overall capability of a lab to provide the service. NIST experts
often participate in the accreditation process, but the end
result is a finding of competence and capability only it does
not validate each particular result.
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4. Periodic Calibrations
According to OIML (Organisation Internationale de Métrologie
Légale) the tests are divided in two categories:
- pattern evaluation (type testing) and
- verifications (periodic calibrations)
The pattern evaluation contains the appropriate tests necessary
to ascertain that an instrument entirely satisfies the requirements
of the applicable standards.
The purpose of the periodic calibrations is to assure the user
that the performance of the instrument has not changed significantly
from that determined in the initial tests. Scantek, Inc. provides
periodic tests for sound and vibration measuring instrumentation.
For these units, some of the tests necessary for the pattern
evaluation can also be provided, at reference conditions.
In order to ascertain that a meter is still within the requirements
of the applicable standards or specifications, one can perform
all the tests possible or only test the main parameters. The
consequence of one of the other of these choices is illustrated
in a simplified intuitive manner in the next picture:
The illustration shows that reduced tests may not catch the
eventual out of tolerance conditions, reducing the confidence
in the measurement results. Normally, measurements performed
in this manner should be reported with higher uncertainty.
More and more the new standards include a list of the tests
that should be performed for the periodic calibrations in order
to claim that the tested unit complies with the standard.
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5. Calibration intervals
Based on the ISO 17025 requirements, Scantek, Inc. does not
provide a calibration interval on calibration sticker or certificate
unless specified in writing by the customer and agreed by
both parties . Nevertheless, based on our experience and
calibration history of similar instruments, we can assist the
customers in establishing their own calibration intervals for
the units they possess.
Besides the manufacturer recommendations, which only rarely
can provide proof of studies the recommendation is based upon,
the unit calibration history is a reliable source of sufficient
information to make a decision on the suited calibration interval.
When a drift or an increase of accuracy is not observed (which
is often the case for the electronic equipment), then the
decision about the length of the calibration interval must
be based upon other aspects:
- age of the unit compared to the estimated lifetime of the
class of instruments
- calibration results for instruments grouped by model and
manufacturer
- conditions of use of the instrument (risk of mishandling,
overload, hostile environment, maintenance/cleaning)
- sensitivity of the instrument parts to hostile environment
(the microphone is far more sensitive and fragile than the
instrument itself)
- costs of damages if measurements are performed with an out-of-tolerance
unit (fees, damage repair or cost of repeating the tests if
the instrument is found out-of-tolerance
- requested accuracy
- calibration costs and frequency of use of the unit. The
lack of use does not give insurance that the unit is within
tolerances. Nevertheless, if the unit is rarely used one should
be concerned about the efficiency of calibrating the unit
too often
Finally, for reliable instruments, the frequency
of the periodic tests is only determined by the need to obtain
the proof or confidence that the instrument is within its known
specifications.
Example:
For its own sound and vibration measuring instrumentation
Scantek, Inc. established differentiated calibration intervals,
in parallel with checks of functionality after each measurement
in the field. The more sensitive instruments like calibrators,
microphones and accelerometers are more frequently calibrated
(9-12 months) than the electronic instruments (1-1.5 years).
Also very new and very old units are checked every year.These
intervals are updated any time a calibration that revealed
an out-of -tolerance condition
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6. Calibration services
provided by Scantek, Inc. Calibration Laboratory
Scantek Inc provides various calibrations, some under the scope
of accreditation, other are not, as shown below.
Under this service the measurements are performed using methods
and procedures that have been assessed by NVLAP. The uncertainties
reported for these measurements were also audited and certified.
This gives the highest degree of confidence that the measurements
are accurate and traceable. These reports contain the NVALP
logo
It is possible that a Calibration certificate and Test report
having the NVLAP logo contain tests that are not covered by
the scope of accreditation. These tests are individually identified
with the text: " not covered by the current NVLAP accreditation.
"
These comply with the requirements of the standard and were
audited by an internal audit only. These are also traceable
services that either were validated only after the NIST assessment
or were not to be accredited. Calibration certificates for these
services do not contain the NVLAP logo.
These are regular services in various development stages, mostly
with the measurement uncertainty budget not fully developed.
Upon request, special tests can be developed and provided.
These can be selected from the existing tests without modifications,
by customizing tests or by developing new ones. If agreed upon,
the adapted or new procedures can be developed according to
the requirements of Scantek, Inc. Quality system and be submitted
to an internal audit. Only then the traceability of the test
results can be claimed.
The price list for the calibration services shows the type
of the available services for each particular unit.
Scantek, Inc. offers a variety of calibration services to meet
various needs of the customers.
For the case when multiple standards (international and national)
are applicable to sound level meters and sound measuring systems
to be tested, we ask the customer to choose the standard to
use for testing.
The following standards are current and may be applicable to
the sound level meters or sound level systems presented for
calibration:
US national standards:
- ANSI S1.4: 1983 Specification for Sound Level Meters and
- ANSI S1.43: 1997 Specifications for Integrating-Averaging
Sound Level Meters
International standards:
- IEC 61672-1: 2003 Electroacoustics – Sound level meters.
Part 1: Specifications
- IEC 61672-2: 2003 Electroacoustics – Sound level meters.
Part 2: Pattern evaluation tests
- IEC 61672-2: 2006 Electroacoustics – Sound level meters.
Part 3: Periodic tests (which we use for periodic calibration)
The series of three standards IEC61672 replaced the old standards
IEC 651: 1979 "Sound level meters" and IEC 804: 1985
"Integrating-averaging sound level meters" (which
were very similar in requirements with ANSI S1.4 and ANSI S1.43
respectively).
In selecting the standard to test to, one needs to consider:
- the specifications of the instrument/system to be calibrated
(the manufacturer's claims of compliance with the standards)
- the requirements for the later use of the instruments to
be calibrated, specifically from the standards governing the
test methods that one uses
- the requirements of the customers or auditors of the instrument
owner
For the test performed according to the IEC 651/8904 or ANSI
S1.4/S1.43, we are following the guidelines given in OIML R
58 and OIML R 88. The test content for each service is detailed
in the following.
For sound level meters, dosimeters, analyzers the customer
may choose between the following accredited calibration services:
- Standard calibration: a series of accurate
acoustical and electrical tests are performed in order to
thoroughly test all the functions and features present in
the device, accomplishing all the requirements of the standards;
these tests provide the highest degree of confidence that
the unit complies with the necessary specifications, therefore
the recommended ones; (What is tested: sound
level meters or dosimeters?
)
- Basic calibration: a reduced number of
acoustical tests are performed to test only the main functions
of the unit; the measurement uncertainty of the acoustical
methods is larger than the ones of the previous electrical
tests. Additionally, as shown in the periodic calibration
section, there is a higher risk that the eventual out-of-tolerance
condition is not discovered (What is tested: sound
level meters or dosimeters?
)
- Dedicated only to the pattern evaluations, a special Extended
Calibration Service is available upon request. The
content of this service has to be established for each particular
case.
As required by the applicable standards, these complex units
are tested as systems, including the microphones and preamplifiers;
all component parts are listed on the Calibration Certificate.
In order to diminish the calibration cost of multi-channel
systems, we adopted a test structure that is testing both the
software functions and the hardware parameters while avoiding
redundant tests. Consequently, for multi-channel systems, we
fully test one channel according to the selected standard, and
for the remaining channels we only perform the tests that focus
on the hardware parameters.
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