Proper Weighing with Laboratory Balances

Introduction

Weighing is one of the most common tasks in the laboratory.Advanced micro, semi-micro, analytical and precision balances have now been perfected to such a degree that, in general, no special weighing rooms are needed.

Technological advances in electronics have considerably simplified the operation of balances, drastically cut weighing times, and made the balances so adaptable that they can now be integrated directly into a production process.

The risk inherent in this progress, however, is that not enough care will be taken to prevent disturbing influences from the surrounding area.

These usually involve physical effects which are measurable for micro, semi-micro and analytical balances, and which analytical balances cannot suppress because they result in measurable weight changes (e.g. through slow evaporation, moisture uptake), or forces which act on the weighing pan and weighing sample (e.g. magnetism, electrostatics) and which are interpreted by the balance as weight changes.

The purpose of these instructions is to explain the most important points to be noted when working with micro, semi-micro and analytical balances if high-quality results are required.

After some brief pointers concerning the location and proper operation of the balances, the disturbing influences on weighing will be discussed in detail. Most of these influences are recognizable by a slow change in the weight display (drift).

Location of the Balance

The precision and reproducibility of weighing results is closely associated with the location of the balance. To ensure that your balance can work under the best conditions, please observe the following guidelines:

Weighing bench

 Stable (lab bench, lab table, stone bench). Your weighing bench should not sag when work is carried out on it and should transfer as few vibrations as possible.

 Antimagnetic (no steel plate).

 Protected against electrostatic charges(no plastic or glass).

 Wall or floor installation-The weighing bench should be fixed either to the floor or on the  wall. Mounting the bench on both places at once transfers vibrations from wall and floor.

 Reserved for the balance.

The place of installation and the weighing bench must be stable enough that the balance display does not change when someone leans on the table or steps up to the weighing station. Do not use soft pads underneath, such as writing mats. It is better to position the balance directly over the legs of the bench, since the area is subject to the fewest vibrations.

Work room

 Vibration-free

 Free from drafts

Place the weighing bench in a corner of a room. These are the most vibration-free areas of a building. Ideally, the room should be accessed through a sliding door to reduce the influence of door movements.

Temperature

 Keep the temperature of the room as constant as possible. Weighing results are influenced by temperature! (Typical drift: 1-2 ppm/°C).

 Do not weigh near radiators or windows.

Atmospheric humidity

 Ideally, the relative humidity (% RH) should be between 45 and 60 %. Balances should never be operated above or below the measuring range of 20 to 80 % RH.

Constant monitoring is advisable with micro balances. Changes should be corrected whenever possible.

Light

 If possible, place the balance on a window-free wall. Direct sunlight (heat) will influence the weighing result.

 Place the balance a significant distance from lighting fixtures to avoid heat radiation. This especially applies to light bulbs. Use fluorescent tubes.

Air

 Do not place the balance in the air flow of air conditioners or devices with ventilators, such as computers or large laboratory devices.

 Place the balance a sufficient distance from radiators. In addition to the potential temperature drift, strong currents of air could interfere with operation of the balance.

 Do not place the balance next to a door.

 Avoid places with high traffic.

Passers-by will usually create a draft at the weighing location.

Next

1. Operation of the Balance
2. Physical Influences
3. Technical Terms