The Criteria for Selecting a Weighing Instrument

The following data is usually specified in the product folders of laboratory balances from the majority of manufacturers:

  1. Weighing range /maximal capacity Max/
  2. Readability /reading unit/
  3. Repeatability
  4. Linearity
  5. Stabilization time
  6. Pan size
  7. Working temperature
  8. Class of a balance (i.e. If it meets the requirements of 90/384/EEC Directive on verification in specific circumstances as defined in this directive).

The manufacturer of a balance specifies the weighing range and this has to be rigorously followed by the operator. Permanent damage to a balance can be caused if weighing loads above the maximum are used.

Readability is a parameter referring to the maximal capacity of a balance. The mass of a measured sample along with the accuracy in which the sample will be measured needs to be paid attention to by the operator. A situation may be caused where none of the manufacturers can meet an operator’s expectation if an analysis is performed imprecisely.

Most controversies and questions from operators are in reference to parameters described in commercial materials as “repeatable” in relation to analytical balances.

“Repeatability” is expressed using standard deviation of a specific type of balance in most commercial and informative materials from competitive manufacturers. Standard deviation is a numerical value that is much lower than the difference between the maximal and minimal indication of a balance from a series of measurements (dispersion of indications as described in PN-EN 45501). For example, a balance with a reading unit d = 0.01 mg and a difference between the maximal and minimal indication has 76 units dispersion (0.07 mg) and standard deviation of 0.02 mg. The lower the difference between the maximal and minimal indication, the lower the standard deviation.

An error of a balance in its full weighing range is known as linearity. Calibration should be carried out at the point of weighing if a balance is to be utilized in a very limited weighing range. This is a crucial step as the error is set for the whole weighing range and rounded to the maximal one. In practice, the maximal error most likely occurs with the maximal load, but this error is smaller with lower loads.

The time taken for a balance between reaching a stable result of a measurement is known as the stabilization time.

The range of temperatures for which a balance is guaranteed by the manufacturer is known as the working temperature. A balance should be calibrated according to ambient conditions on the workstation and the possible error of indications considered if the workstation temperature conditions are different to the working temperature.

The maximal permissible errors with reference to a verifying unit is known as accuracy class.

Supervision from legal metrology, also known as verification may be required in some situations. Balances currently undergo conformity evaluation and EC verification processes. A notified body (for example, Glowny Urzad Miar in Poland) is able to perform both of these processes based on type approval documents or the manufacturer confirming and certifying quality management system compatible with Directive 90/384/EEC.

Comparison of Balances Due to Differences in Construction

As presented on schema of laboratory balances, the components of electronic balances are usually present in all options available to the market. As such, the influence of any unique solutions on a balance operation and price should be considered before deciding on which balance to purchase.

Weighing Pan

The dimensions of the weighing pan should be no bigger than what is required. The bigger the weighing pan, the higher the price of a balance. As standard, Weighing pan are offered in stainless steel (non-magnetic) technology as standard. Anti-draft shields are typically made of plastics, which are usually resistant to electrostatic charges.

Casing

Aluminum cast is used to make the majority of casings for laboratory balances (verified in class II). This is then powder coated with a lacquer resistant to external factors. The casing can also be made with plastic to offer a cheaper solution. However, this is not resistant to chemical substances that are usually weighed in laboratories.

Anti-Draft Shield

Analytical balances (verified in class I) are equipped with an anti-draft shield. This protects the sample being weighed from ambient conditions, like the movement of air around the balance. The size of an anti-draft shield will differ depending on balance application. It is not always easy to predict the size of samples that a balance will be required to weigh. As such, it is worth considering purchasing a balance with a bigger anti-draft shield.

The location of the weighing mechanism (either under the weighing pan or behind it) will differ between analytical balances. It is usually cheaper to purchase a balance with its weighing mechanism located under the weighing pan. In the case of resolution, d = 0.1 mg such balances as just as accurate as balances with their mechanisms located at the back part of balance casing.

If magnetic elements are weighed, then an error may occur. In this scenario, it would be better to select a balance with its weighing mechanism located further away from the weighing pan, i.e. at the back part of the balance casing.

Force-Motor

The main measuring part of a balance is known as a force-motor. This is responsible for the very precise conversion of force into a proportional electric signal as well as for stability during the measuring process. The instrument must have a type approval document so an operator must check for this when selecting a type of balance. During the verification process, certifying units confirm whether a balance has proper metrological characteristics and temperature errors. There may be some scenarios where a temperature error is specified in the commercial materials of a balance.

The Mechanisms of a Monoblock

Manufacturers of laboratory balances have been divided into two groups. Each of these manufacturers praise the advantages of their solutions but omit any drawbacks. An operator should take into consideration the following aspects when comparing designs:

Weighing accuracy - the spring elements in the lever bearing of a straight-line mechanism are usually better quality in a traditional mechanism (RR factor of aluminum is lower than RR factor of bronze or steel). This provides better resistance to overloading and lower hysteresis errors. Proper quality aluminum elements of monoblocks were designed for average resolutions. When resolutions are very high, traditional mechanisms are applicable.

Durability to defects – aluminum monoblock is less durable than a traditional mechanism due to the material used in its construction (lower RR factor). As such, additional construction elements are required to protect it from damage. These elements are not always effective in operation. A steel monoblock is more durable than an aluminum one, but this is a much more expensive alternative. Furthermore, it is difficult to design a high-resolution balance with steel monoblock.

Weighing speed – as a rule of operation of a compensatory set, elements of a mechanism do not relocate and, as such, their mass does not have much of an influence on the weighing speed. It is the proper operation of electronic controlling elements of an important balance.

Resistance to ground vibrations during the weighing process – due to the possibility of obtaining lower mechanical masses of moveable elements, a monoblock is more resistant. Manufacturers currently use a set of digital filters and minimize the traditional design of mechanisms to eliminate this drawback.

Service and repairs – a monoblock-based balance is irreparable if damaged. It would cost just as much to repair it as it would to buy a new balance. It is possible to repair a traditional mechanism several times by an authorized service point of the manufacturer.

A straightforward answer as to which balance has a better design, a monoblock or a mechanism, is not provided with the above. Errors in the weighing process, the weighing speed, resistance to defects are all much worse with a monoblock.

However, this technology is less labor-intensive and, in Europe, labor costs are very high. Around twenty years of continuous research and gigantic capital investments have yielded satisfactory results. However, over the last 20 years, things have changed. Globalization has made it possible for countries with low production costs (for example, Eastern Europe and Asia) to compete on the same rights. These countries have started to manufacture laboratory balances with a traditional mechanism at a much lower labor cost. New technologies for manufacturing traditional mechanism materials have been developed.

These days, an operator may either be swayed by a commercial campaign that highlights the superiority of new monoblock technologies or they may purchase the same quality balance equipped with a traditional mechanism due to price.

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Last updated: Nov 28, 2019 at 6:32 AM

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