is accreditation and why should I select an accredited
laboratory for my source of calibrations?
“Accreditation” is a formal
recognition by an authoritative entity that an organization
or individual is competent to carry out specific tasks.
In the field of metrology, a third party such as NVLAP
or A2LA, totally unaffiliated with the organization
in question, ensures the calibration is being performed
correctly by technically competent individuals. Accreditation
translates to a cost savings in minimizing or eliminating
the need for onsite visits by the customer. If your
calibration service provider is not accredited, you
are taking a risk with your quality control program.
What is the difference
between NVLAP (the National Voluntary Laboratory Accreditation
Program) and A2LA?
The only real difference between these
two organizations is that NVLAP is affiliated with the
US National Institute of Standards and Technology (NIST)
while A2LA resides the private sector, however, both
agencies audit to ISO/IEC IS 17025 (formally ISO Guide
25). Specific criteria that is covered by both organizations
during an assessment include:
Methods and procedures
system and reviews
UKAS is the United Kingdom Accreditation Service
formerly NAMAS. UKAS was formed from the merger of NAMAS
and NACCB in 1995 as a national accreditation body (NAB).
It is part of a network of NABs that assesses technical
competencies of laboratories and grant accreditation
to those meeting its requirements.
Why is Troemner Double
Troemner pursued these accreditations to obtain the
domestic and international recognition that NVLAP and
UKAS carry. By having this double accreditation, Troemner
can provide certification all over the world.
Guide 25 (proposed revision: ISO/IEC IS 17025)
This guideline sets out the general provisions which
a laboratory must address to carry out specific calibrations
or tests. ISO Guide 25 provides laboratories direction
for the development and implementation of a fundamental
quality management system.
Good Laboratory Practices (GLP’s) not only address
the organizational aspects of operation and the conditions
under which laboratory tests are scheduled, conducted,
and monitored; but, also the aspect of recording and
reporting test results. GLP principles are basically
used for testing substances and acquiring data on their
properties and/or their harmlessness to human health
and the environment.
The National Voluntary
Laboratory Accreditation Program (NIST/NVLAP)
Sponsored by the National Institute of Standards and
Technology (NIST), assesses and accredits organizations
based on their technical ability and quality system,
following ISO/IEC IS 17025 (formerly ISO Guide 25).
The National Committee
for Clinical Laboratory Standards (NCCLS)
A globally recognized, voluntary consensus, standards-developing
organization that enhances the value of medical testing
within the healthcare community through the development
and dissemination of standards, guidelines, and best
College of American Pathologists (CAP)
An organization that accredits in order to improve the
quality of clinical laboratory services throughout the
United States, through voluntary participation, professional
peer review, education, and compliance with established
The Clinical Laboratories
Improvement Act (CLIA)
A document that outlines minimum standards for personnel,
testing, and quality control for clinical laboratories.
A standard, generally of the highest metrological quality
available at a given location, from which measurements
made at that location are derived.
In order to establish traceability, a standard must
have an unbroken and valid relationship to some nationally
or internationally recognized standard. A standard itself
can not really be traceable, but the value assigned
to it can as long as there is an unbroken chain of comparisons
all having stated uncertainties.
A measure of the inherent variability of repeated
measurements of a quantity. Uncertainty characterizes
the dispersion of the values that could reasonably be
attributed to the measurement. This parameter is typically
reported at a 95% confidence level.
The closeness of agreement between the nominal or accepted
value and the measured value.
The reproducibility of multiple measurements and is
usually described by the standard deviation, standard
error, or confidence interval.
A statistical measure of the degree of variation of
a set of quantitative data around its mean.
Why do mass standards take
more time calibrate than most laboratory equipment?
For a proper calibration, weights need adequate
time to establish and maintain a thermal equilibrium
with the laboratory before any measurement. One-piece
weights take the longest time to calibrate - adjustments
are made on a polishing wheel that generates heat and
requires 4–8 hours to reestablish thermal equilibrium
before any calibration/tolerance checks on a balance.
A quality calibration just can not be performed in a
matter of days and if someone tells you otherwise then
they simply are not following good measurement practices
and the quality of the calibration will be drastically
inferior. In fact, it is our supposition that if this
occurs you are not getting the service that you paid
for and you are simply paying for a calibration sticker.
think my weight may be out of calibration, what should
Check for contaminants, scratches, and discolorations.
Be sure to look on the bottom of the weight, since occasionally
matter attaches itself to the bottom and it may go unnoticed.
can I determine balance error from weight error?
error is the measured value correction plus its associated
uncertainty (please refer to a Troemner NIST/NVLAP Weight
Calibration Certificate for an example). You can use
the weight to determine the balance error, but you can
never use the balance to determine the weight error.
This means that if you want to determine the weight
error, you must have a mass standard with a certificate
that enumerates the value correction plus its associated
uncertainty or you must have another certified weight
to make a comparison. If you want to determine the balance
error, it is relatively easy if you have a certified
mass standard with the aforementioned data. With this
information you should know how the weight performs
plus or minus its associated uncertainty value. Therefore,
the balance error can be deduced by performing multiple
measurements to see how well the balance executes a
measurement. This, of course, can be recorded and tracked
over time, so that a balance uncertainty can be ascertained
How Often Should I have
my weights calibrated? Why? Are there any regulation
and/or standard pertaining to calibration intervals?
It is recommended that if you use your
weights infrequently to moderately, you should have
them recalibrated once a year. However, if your weights
are used very frequently or several times a day, then
you should have them calibrated once every six months.
Although there are no standards or regulations that
specifically address recalibration intervals, this is
the industry norm in order to ensure that these standards
are still within their specified tolerances. It is not
unusual to see significant “as found” failure
rates on those weights that are not recalibrated on
an annual basis.
A solid solution in iron of carbon and sometimes
other solutes that occurs as a component of steel under
certain conditions. Austenitic stainless steels usually
offer many advantages such as increased strength, corrosion
resistance, lower magnetic susceptibility, and desirable
mechanical and magnetic properties.
The act of determining the mass difference between a
standard of a known mass value and an “unknown”
test weight or set of weights. The process uses a comparison
method and a series of calculations to establish the
mass value and conventional mass value of the “unknown”
and determines a quantitative estimate of the uncertainty
to be assigned to the measurement process as well as
the mass or conventional mass value for the “unknown”.
(Adjustment Tolerance or Maximum Permissible Error)
The maximum amount by which the conventional mass of
the weight is allowed to deviate from the assigned nominal
Mass values are traditionally expressed by two numbers,
one being the nominal mass of the weight and the second
being a correction. The mass of the weight is the assigned
nominal value plus the assigned correction. Positive
corrections indicate that the weight embodies more mass
than is indicated by the assigned nominal value.
platinum-iridium cylinder maintained at the International
Bureau of Weights and Measures (BIPM), at Sevres, Frances
with an internationally accepted defined mass of 1kg
with zero uncertainty.
National Prototype Standard
Platinum-iridium kilogram identified as K20, maintained
at the National Institute of Standards and Technology
(NIST), with a value assigned relative to the International
The mass value as marked on a weight.
An object representing a specific mass, regulated in
regard to its physical and metrological characteristics:
shape, dimension, material, surface quality, nominal
value, and maximum permissible error.
Mass (Mass in Air)
The conventional value of the result of weighing in
air, in accordance to International Recommendation OIML
R33. For a weight taken at 20 degrees C, the conventional
mass is the mass of a reference weight of a density
of 8000 kg/m3, which it balances in air density of 1.2
Found Conventional Mass Correction
conventional correction of the result, as it was received
by Troemner, of weighing in air accordance to International
Recommendation R 33. For a weight taken at 20o C, the
conventional mass is the mass of a reference weight
of density 8000 kg·m-3 which it balances in air
density of 1.2 kg·m-3. If the customer requires
cleaning prior to calibration, the after cleaning correction
will be reported.
Left Conventional Mass Correction
The conventional correction of the result, after adjustment
repair, or replacement of weighing in air in accordance
to International Recommendation R 33. For a weight taken
at 20o C, the conventional mass is the mass of a reference
weight density 8000 km·m-3 which it balances
in air density of 1.2 km·m-3. The As Found will
equal the As Left Conventional Mass Correction if the
mass(es) did not require adjustment, repair or replacement.
Air buoyancy is a net upward force due to atmospheric
pressure. In a gravimetric analysis, you must have an
air buoyancy correction to get to the true mass.
in a Vacuum (True Mass)
|The mass of a weight
as if it were measured in a vacuum.
Found Mass in a Vacuum
The measured value of the mass(es) as they were received
by Troemner. If the customer requires a cleaning prior
to calibration, the after cleaning value would be reported.
Left Mass in a Vacuum
The measured value of the mass(es) after they were adjusted,
repaired or replaced when necessary. The As Found Mass
in a Vacuum will equal the As Left Mass in a Vacuum
if the mass(es) did not require adjustment, repair or
What is a pipette?
A pipette is a hand held measuring instrument
designed to deliver a pre-determined volume of liquid
from one vessel to another.
My pipette leaks and can you assist me with a solution?
Pipettes can leak for the following reasons:
tip is incorrectly attached
is a foreign object (i.e. dirt, grit, etc) between
the piston, o-ring, and nose cone.
||The o-ring has been
damaged or warped
to the aforementioned problems:
the tip to the nose cone
||Clean and grease
tip cone module and o-ring; attach new tip.
||Change the o-ring
just had my pipette calibrated but it seems to be dispensing
What can I do to resolve this problem?
to see if the tips are securely attached
the pipette dropped, or has the pipette been subject
to over-winding / under-winding the micrometer?
the operator using good techniques? Based on our
experience, as many as 25% of all pipetting errors
are directly related to the operator.
many times a year should a pipette be calibrated?
It depends on the number of times the pipette is used
and the quality standards of the laboratory. ASTM E:
1154-89 11.23 recommends that pipettes receive an evaluation
at least on a quarterly basis.
Can my pipette be autoclaved?
Some pipettes can be autoclaved while others can not.
It is best to check with the manufacturer if there is
ever any doubt.
What are the common methods
of pipette calibration?
requires a controlled environment, a high precision
balance, a skilled pipette technician and the use of
statistics. Principle: given a certain mass of water
with a known specific gravity, its volume can then be
calculated. The gravimetric method of calibration entails
weighing deonized (i.e. distilled) water on a balance
and converting from a mass unit to a volumetric unit
by utilizing the Z-factor. (Z-factor converts from a
mass unit to a volumetric unit and includes calculations
for air buoyancy, temperature, humidity, barometric
pressure and the cubic expansion coefficient).
Colorimetric or Photometric involves
the analysis of volumes of diluted dye in a cell of
a known path length. If a beam of monochromatic light
passes through homogeneous solutions of equal path-lengths,
the absorbance measured is proportional to the dye concentration.
Principle: An unknown volume of dye can be pipetted
into a known volume of diluent, the resulting dye concentration
can be measured photometrically and the volume can be
calculated. This method employs the use of a spectrophotometer
and reagent dyes. As more and more of the dye is dispensed,
the greater the amount of energy that is being absorbed
and if course, the less that is being detected. The
instrument uses the information to calculate the volume.
volumetric pieces of glass calibrated to deliver a specific
volume of liquid. Glass pipettes cannot be “calibrated”
in the true sense of the word since the graduations
cannot be adjusted. The glass pipette can be tested
Air Displacement Pipettes
Pipettes that have a piston in a cylinder or capillary
tube that moves to the appropriate position once the
volume is set. The key feature of an air displacement
pipette is the fact that a specified volume of air always
remains between the piston and the liquid.
Pipettes that also have a piston in a cylinder or capillary
tube that moves to the appropriate position once the
volume is set, however, this type of pipette always
has the piston in direct contact with the liquid.
The act of placing an instrument inside a machine specifically
designed to sterilize an instrument by reaching very
high temperature and pressures.
Air buoyancy is a net upward force due to higher pressures
at lower altitudes. Since in a gravimetric analysis,
you are essentially converting the “weight”
of a liquid to a volume, you must have an air buoyancy
correction to get to the true mass.
The part of the total liquid volume that is held in
the operational part of the device and not delivered.
A measuring instrument designed to take up different
liquids and deliver them in combination so that they
comprise a predetermined ratio, a predetermined volume,
or both. The reservoir of diluent may be integrated
with the instrument or connected externally.
A measuring device designed to deliver predetermined
volumes of liquid from a reservoir. The reservoir may
be integrated with the instrument or connected externally.
The quantification of expansion due to thermal conductance.
K = 1-µ (T-20)
K = Expansion Factor
µ = Cubic expansion coefficient
T = Temperature, degrees Centigrade
This means that the pipette and the environmental temperature
are equal. This is accomplished by allowing the pipette
to equilibrate to the temperature of the laboratory
for a certain period of time.
|The stated volume
for which performance is specified.
A piston or plunger operated volumetric apparatus.
The action of pre-coating the inside of the liquid contacting
parts with a thin film of the same liquid to be pipetted.
Z-factor converts from a mass unit to a volumetric unit
and includes calculations for air buoyancy, temperature,
humidity, barometric pressure and the cubic expansion