Weighing accuracy is a primary criterion for precision scales, analytical balances and moisture analyzers where readout (the smallest weight difference displayed) and linearity (the ± deviation from the actual weight) are governed by GLP, ISO and other standards. Test weights have a critical role to play when scales with capacities from milligrams to tons are not automatically internally (see below) calibrated . The type of calibration, whether internal or external, is indicated on scales’ spec sheets.
Today’s precision test weights differ mightily from weights found on olde tyme grocer’s and druggist’s balances where the desired weight is placed on one platform and the product being weighed on the other. When the balance’s indicator points to the middle the desired weight is achieved. Today, scales for products sold by weight must be certified as legal for trade. Examples are found on supermarket deli counters. These are periodically checked for accuracy using test weights.
Test Weight Classifications
Because a scale will never be more accurate than the test weight used to calibrate it, test weights are manufactured to assure extreme accuracy. The selection of the test weight is generally specified in an externally calibrated scale’s operations manual. For example, the Tovatech-offered Kern ABS 220-4N with a maximum range of 220 g requires a 200 gram calibration weight. The 320 g capacity model calls for a 300 g calibration weight. As a general rule of thumb the test weight should equal or approximate the scale’s maximum weighing capacity.
Test weights are classified by the internationally recognized Paris-based OIML: The International Organization of Legal Metrology (Organisation Internationale de Métrologie Légale ). This was created in 1955 to promote the global harmonization of the legal metrology procedures that underpin and facilitate international trade. OIML norm R111-2004 classifies test weights in seven accuracy classes E1, E2, F1 , F2, M1, M2 and M3.
What does this mean? For example:
- E1 weights are specified for customers requiring a high degree of accuracy for the most demanding applications. These weights are used for balances with readouts greater than 1,000,000 (1 microgram)
- E2 test weights are the most accurate for high-resolution analytical balances and are recommended for balances such as the ABS models noted above, both with readouts of 0.01 mg.
Progressing through the classifications the permissible ± tolerances increase. As examples here are the nominal ± value in milligrams for 1g: E1 = 0.01, E2=0.03, F1= 0.1, F2= 0.3, M1=1.0, M2=3, M3=10mg. As a scale’s weighing capacity increases so do the permissible tolerances. A 50 kg scale is the highest capacity unit offering all 7 OIML accuracy classes ranging from ±25 mg for E1 to ±25g for M3.
Test weights for calibrating precision scales, analytical balances and moisture analyzers are manufactured to precise standards, as you might well imagine. Materials include brass, polished stainless steel, nickel-plated polished brass and brass. Not all material options meet all OIML conformance classifications. Configurations include shaped wire and polygonal sheets typical for milligram calibration weights to formed, machined and/or polished cylinders with lifting knobs. Block weights in several configurations are produced for calibrating high-capacity scales.
Calibration Procedures using Test Weights
Calibrating or recalibrating procedures used for precision scales, analytical balances and moisture analyzers should be spelled out in a company’s operation manual. Professional association, government regulations and other standards may apply.
Calibration should be performed when units are first installed. Recalibration should be performed when scales or balances are relocated within the facility. Highly sensitive weighing instrument mechanisms are influenced by changes in gravity, changes in temperature, vibrations and humidity among other variables. These sensitivities should be kept in mind at all times.
The topic of this post is the use of test weights for calibration. Earlier we mentioned internal vs. external calibration. Internally calibrated scales and balances handle this automatically with no user-required intervention except taking notice of the process. For example the Kern ABT series automatically recalibrate using internal weights under four scenarios: 1) Four hours after the previous calibration; 2) When there is a fluctuation in temperature of 0.5oC (0.9oF); 3) When the balance is switched from standby to weighing mode and condition (1) or (2) has been met or 4) If the balance was disconnected from its power source.
Externally calibrated balances require the use of test weights as described. The process, however, is painless. These units have a calibration mode that when activated simply requires users to place the proper calibration weight on the pan and activate the calibration program. The unit will compare the calibration weight with its reading and internally make corrections if needed.
The Care and Calibration of Test Weights
Calibration test weights, as indicated earlier in this post, are manufactured to exacting conditions because, as noted above, the equipment will never be more accurate than the test weight used to adjust it. Test weights can represent a substantial investment to be protected by proper handling, care and periodic recalibration.
Surprised? Read on.
As with the equipment they test, test weights should also be recalibrated as part of ongoing GLP. The frequency of recalibrating test weights depends on the frequency of use, the conditions of use and the security needs of your organization in terms of standards compliance. Tovatech suggests that test weights used intensively should be recalibrated at least every six months, otherwise at least once a year. Recalibration is done under strict laboratory conditions. Weights that pass are issued a recalibration certificate detailing the exact conditions under which the recalibration was made.
Sometimes test weights that fail recalibration can be brought back into tolerance limits by repair or remanufacture. Otherwise they must be replaced.
Usage procedures for test weights should be clearly specified relating to handling and storage. For example, acids on your hand could cause etching that could invalidate the accuracy of a 1 mg test weight – especially one with a tolerance of ± 0.003 mg. For this reason gloves should always be used when handling test weights. Gloves, along with tweezers and dust brushes are supplied by companies such as Kern as a further means to promote safe handling. The weights themselves are stored in fully lined wooden boxes and milligram weights in removable plastic boxes.
Precision scale test weight can be obtained individually or in multiple-unit sets. Tweezers, gloves, dusting brushes and individual or multiple-unit storage container and carrying cases are also available to maintain and protect these precision test weights. These help organization to meet internal or external guidance procedures supporting precision weighing and measuring requirements.
Contact the weighing professionals at Tovatech for help in selecting test weights for calibrating scales, balances and moisture analyzers used in your organization.