General - Calibration

What 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:

What is UKAS?
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 Accredited?
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.

General - Terms

ISO 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.

GLP Standards
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 practices.

The 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 performance standards.

The Clinical Laboratories Improvement Act (CLIA)
A document that outlines minimum standards for personnel, testing, and quality control for clinical laboratories.

Reference Standard
A standard, generally of the highest metrological quality available at a given location, from which measurements made at that location are derived.

Traceability
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.

Uncertainty
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.

Accuracy
The closeness of agreement between the nominal or accepted value and the measured value.

Precision

The reproducibility of multiple measurements and is usually described by the standard deviation, standard error, or confidence interval.

Standard Deviation
A statistical measure of the degree of variation of a set of quantitative data around its mean.

Weight - Calibration

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.

I think my weight may be out of calibration, what should I do?
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.

How can I determine balance error from weight error?

The weight 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 as well.

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.

Weight - Terms

Austenitic
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.

Calibration
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”.

Tolerance (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 value.

Correction
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.

International Prototype Kilogram

The 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.

U.S. 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 Prototype Kilogram.

Nominal Mass
The mass value as marked on a weight.

Weight (Mass Standard)
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.

Conventional 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 kg/m3.

As Found Conventional Mass Correction

The 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.

As 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
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.

Mass in a Vacuum (True Mass)
The mass of a weight as if it were measured in a vacuum.

As 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.

As 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 replacement.

Pipette - Calibration

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:

I just had my pipette calibrated but it seems to be dispensing inaccurately. What can I do to resolve this problem?

-	Check to see if the tips are securely attached
-	Was the pipette dropped, or has the pipette been subject to over-winding / under-winding the micrometer?
-	Was the operator using good techniques? Based on our experience, as many as 25% of all pipetting errors are directly related to the operator.

How 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?
1. Gravimetric: 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).

2. 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.

Pipette - Terms

Glass Pipettes
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 and “verified”.

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.

Positive Displacement Pipettes
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.

Autoclaving
The act of placing an instrument inside a machine specifically designed to sterilize an instrument by reaching very high temperature and pressures.

Air Buoyancy
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.

Dead Volume

The part of the total liquid volume that is held in the operational part of the device and not delivered.

Dilutor
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.

Dispenser
A measuring device designed to deliver predetermined volumes of liquid from a reservoir. The reservoir may be integrated with the instrument or connected externally.

Expansion Factor
The quantification of expansion due to thermal conductance.
K = 1-µ (T-20)

Where:
K = Expansion Factor
µ = Cubic expansion coefficient
T = Temperature, degrees Centigrade

Isothermal Condition
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.

Nominal Volume
The stated volume for which performance is specified.

POVA
A piston or plunger operated volumetric apparatus.

Pre-rinsing/Pre-wetting
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
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.