Laboratory balances: choosing the right precision balance or analytical balance

Laboratory balances: choosing the right precision balance or analytical balance

A laboratory balance provides the quantitative basis for numerous testing and analysis methods. Particularly in sieve analysis, reliable weighing is crucial: the masses of the individual sieve fractions are compared with the original sample weight. Even small weighing deviations can therefore affect the calculated particle size distribution.

But which laboratory balance is the right one? Is a precision balance with a readability of 0.01 g sufficient, or is an analytical balance with a readability of 0.0001 g required? And what role do weighing capacity, repeatability, adjustment, calibration, and legal-for-trade capability play?

The right balance is not automatically the model with the highest resolution. What matters is that the weighing capacity and measurement performance are appropriate for the sample, the test method, and the environmental conditions.

What is a laboratory balance?

The term laboratory balance refers to balances designed for weighing, testing, and analysis tasks in laboratories and quality control. These include precision balances and analytical balances.

When selecting a balance, the application should therefore be considered first:

• What is the combined weight of the sample and weighing container?
• What is the smallest difference in mass that must be reliably detected?
• What level of repeatability does the test method require?
• Under what environmental conditions will weighing take place?
• Do measurement results need to be transferred and documented digitally?
• Is a legal-for-trade or verified balance required for the specific application?

For sieve analysis in particular, the balance is part of a larger testing system comprising sampling, sample division, test sieves, a test sieve machine, weighing, and evaluation. NEXOPART therefore offers laboratory balances as part of its accessories for sieve analysis.

Precision balances for laboratories and quality control

Precision balances combine relatively high resolution with practical weighing capacities. For many laboratory and sieving applications, readabilities of 0.01 g or 0.001 g are suitable. Typical applications include:

• Sand, stone, and soil
• Construction materials and aggregates
• Granules and pellets
• Recycled materials
• Fertilizers
• Raw materials and bulk solids

In these applications, the balance often needs to accommodate not only individual small fractions, but also larger samples and weighing containers.

In industrial quality testing, it is not enough to determine a measurement correctly just once. Results must remain comparable, reproducible, and traceable over extended periods. This applies, for example, to:

Construction materials and aggregates:

• Weighing the sieve fractions provides a basis for determining the particle size distribution.

Chemical products and powders:

• The particle size distribution can influence mixing and flow behavior, among other properties.

Pharmaceutical products:

• Accuracy as well as traceable testing and documentation processes play a particularly important role.

Recycling and raw materials:

• By determining defined particle sizes, material properties can be controlled and processes monitored.

Analytical balances for the smallest differences in mass

Analytical balances are used when very small differences in mass need to be reliably determined. Typical models achieve a readability of up to 0.0001 g, or 0.1 mg. Possible applications include:

• Fine powders
• Pharmaceutical raw materials
• Pigments
• Fine chemicals
• Small sample weights

At this level of resolution, however, air movement, temperature fluctuations, vibrations, and other environmental influences have a greater impact. Analytical balances therefore typically feature a draft shield.

An analytical balance is therefore not inherently more accurate or better suited to every laboratory application. The higher resolution must be necessary for the specific process and must be practical to use under the existing conditions.

Readability is not the same as accuracy

A common misconception when selecting a laboratory balance is that readability and accuracy are the same. Readability describes the smallest increment shown on the display. With a readability of 0.01 g, for example, the displayed value changes in increments of 0.01 g. However, other characteristics must also be considered when assessing the actual quality of a weighing result, particularly repeatability and measurement uncertainty.

A display with 0.001 g increments therefore does not automatically mean that every measurement result is accurate to 0.001 g. When comparing different laboratory balances, the number of decimal places should therefore not be the only factor considered.

Why is a laboratory balance important for sieve analysis?

In sieve analysis, a defined sample is placed on a Test Sieve or a sieve stack. The individual particle fractions are then weighed and compared with the initial sample weight.

Weighing therefore has a direct influence on the calculated particle size distribution. A simplified example illustrates the relationship: For a fraction mass of 20 g, 0.01 g corresponds to 0.05%. For a fraction mass of only 1 g, the same 0.01 g already corresponds to 1%. The smaller the fraction masses to be determined, the more important an appropriate weighing resolution becomes.

For sieve analysis, NEXOPART offers not only laboratory balances, but also Test Sieves, Test Sieve Machines, Sample Division equipment, and Evaluation Software. This allows weighing to be integrated into the entire Particle Analysis process chain.

Buying a laboratory balance: 7 important selection criteria

Anyone looking to buy a laboratory balance should not compare models solely on the basis of readability. The following criteria are essential for making a reliable selection.

1. Determine the weighing range

First, determine the largest load that will regularly occur:

Sample + weighing container = required capacity

The maximum weighing range must be able to accommodate this total mass with sufficient reserve capacity. Overloading can damage the balance.

2. Determine the required readability

Next, determine the smallest difference in mass that is actually relevant to the test result. For many conventional sieve analyses, a precision balance with a readability of 0.01 or 0.001 g may be suitable. For very small sample weights or high-resolution analytical applications, an analytical balance with a readability of 0.0001 g may be required.

3. Consider repeatability

A laboratory balance should provide repeated measurements of the same sample under comparable conditions with as little variation as possible. This is particularly relevant for:

• routine testing,
• batch comparisons,
• trend analyses, and
• documented quality control.

4. Distinguish between adjustment, calibration, and legal verification

These three terms refer to different procedures:

Adjustment: The balance is adjusted to align the displayed value with a reference value.

Calibration: The measurement deviation from a traceable reference is determined and documented under defined conditions. Calibration does not automatically mean that the balance is subsequently adjusted.

Legal verification: Legal verification applies to measuring instruments used for legally regulated measurement applications. Whether a laboratory balance that is eligible for legal verification or has already been legally verified is required must therefore be determined based on the specific intended use and applicable legal requirements.

Important: Eligibility for legal verification should be checked before purchasing. A standard calibration does not replace legally required verification.

5. Choose between internal and external adjustment

With balances that feature internal adjustment, the required adjustment can be performed using an integrated system. This can be useful for frequent measurements and changing environmental conditions. With external adjustment, however, a suitable external test weight is required. Which solution is more appropriate depends primarily on the laboratory's testing workflow and quality management system.

6. Consider the installation location

The higher the resolution of a balance, the more important suitable measurement conditions become. The following factors in particular should be considered:

• air currents
• vibrations
• temperature fluctuations
• a stable installation surface
• correct leveling

For high-resolution analytical balances, a draft shield also helps reduce disruptive environmental influences.

7. Check interfaces and data transfer

For routine analyses, digital interfaces can reduce the need to manually transfer weighing results.

Depending on the laboratory process, interfaces such as RS-232 or USB may be relevant. Especially with high sample throughput, it is important to check before purchasing how the balance can be connected to the evaluation or documentation software being used.

FAQ on laboratory balances for sieve analysis