Analytical Balances

An analytical or semi analytical balance is a form of scale that measures mass to a high degree of precision.
It has a weighing capacity in the range of 100-500 g (typically 200 g) and a readability of 0.1 mg – 0.001 mg. Analytical balance sensitivity requires it to be protected by a draft shield or an enclosure.

METTLER TOLEDO XPE, XSE and XS analytical balances are equipped with the SmartGrid weighing pan minimising air turbulence and therefore shorten stabilization times and allowing faster weighing results.

An analytical balance scale, often called a "lab balance", permits quantitative analysis of a sample. Typical applications of an electronic analytical balance include: Sample/standard preparation, formulation, differential weighing, density determination, interval weighing and pipette routine testing.

To start a weighing operation, open the door and place a container on the pan using gloves or tweezers.Close the door and wait until the value stabilises, noting the weight. Clear the display by pressing the tare button until it reads 0.0000 g. Add the sample until the desired weight is reached. Close the door and again wait for stability - record the net weight. If the instrument is unplugged for cleaning, allow readjustment time before reuse.

METTLER TOLEDO’s analytical balances are designed for easy cleaning with draft shield disassembly in seconds, and dishwasher-safe panels. Cleaning Recommendations and Regulations for Balances

Electrostatic charges can cause unstable, non-repeatable weighing results. Static electricity exerts a force on the weighing pan, which directly affects analytical balance results. Precautions to reduce these effects include:

• Ensure adequate air humidity (≥ 45%...50%)
• If possible, use antistatic weighing containers (metal is ideal)
• Avoid rubbing containers
• If the sample is small enough, use a metal dish with an edge to help shield the electric fields

To ensure analytical balance static elimination, discharge the sample and container with an ionizer before weighing.

Read additional information on electrostatic charges and the physics behind it.  

When making measurements there is always an element of uncertainty. An estimate of doubt that surrounds it (the uncertainty) is key to any measurement. This is determined by measuring the performance of the balance, properties of the weighing object and environmental surroundings. Determination of measurement uncertainty is an integral part of a balance calibration.

Analytical balance calibration ensures that weighing equipment is accurate and satisfies standards such as ISO, GLP/GMP, IFS, and BRC. Documented calibration procedures indicate the quality of measurements, and should be performed regularly throughout the balance’s lifetime.

Calibration should not be confused with adjustment of a balance, and must be repeated after adjustment/repairs. Read more on Calibration.

The calibration of an analytical balance is performed by an authorized service technician following a standard procedure, using calibration software:

1. Establish how well the instrument performs by testing the relationship between measurement standards and indications through calibration.
2. Verify whether the instrument meets specific requirements with a Pass/Fail statement.
3. If requirements are not met, adjust the instrument so that any deviation does not exceed tolerance limits.
4. Re-calibrate the instrument to confirm compliance with specified tolerances.

In between calibration intervals, routine testing by the user can detect potential issues.

METTLER TOLEDO has developed a global scientific standard for the secure selection, calibration and operation of weighing equipment, GWP®, or Good Weighing Practice™.

Tolerances determine whether a balance or scale behaves "well enough" to meet a particular set of process requirements. Tolerances set the criteria to issue a Pass/Fail statement. Tolerances come from a variety of sources, including:

• Legal tolerances:
  Legal tolerances stipulated by OIML R76 or NIST Handbook 44 (US only) assess legal for trade requirements. These tolerances are quite large and easily met with laboratory balances, or when weighing at the lower end of the measurement range.
• Manufacturer tolerances:
  Manufacturer tolerances ensure that equipment meets manufacturer specifications. Manufacturer tolerances do not consider user specific process requirements, and are unsuitable for improving the weighing process.
• Process tolerances:
  Specific process tolerances defined by the user, support process improvements and savings on material, waste and rework. For scales in a legal for trade application, process tolerances should therefore be applied in addition to legal tolerances.

Legal tolerances protect consumers but do not consider specific producer requirements. Optimising process tolerances that are applied to measuring instruments can have a large impact on process profitability.

Every analytical balance minimum weight is different, depending on the performance of the load cell, its location, and ambient conditions. The minimum weight is the accuracy limit of the instrument, below this minimum weight, the relative measurement uncertainty (absolute measurement uncertainty divided by the load, usually indicated in %) is larger than the required weighing accuracy, and the weighing result cannot be trusted.

In order to determine the minimum weight for a balance, measurement uncertainty needs to be assessed in the working environment. Alternatively repeatability, as the dominant source of error in the lower balance range, can be assessed to determine the minimum weight, carried out using a small weight, below 5% of balance capacity.

The MinWeigh function of METTLER TOLEDO analytical balances, installed by a certified technician, monitors the minimum sample weight of the balance. If a weighing is below the determined acceptable minimum value, the balance display turns red.

Precision refers to the closeness of two or more measurements to each other. Accuracy refers to the closeness of a measured value to a known value, and is based on readability, repeatability, non-linearity and eccentricity. Accuracy can be tested using

• An external reference weight with a known mass (traceable calibrated weight)
• Automatic internal adjustment using a reference mass built into the balance
• Accuracy can only be determined by testing of the instrument at its location of use.

View Laboratory Balances Tutorial - Basics and Weighing Influences course on good weighing practice.