Environmental Requirements for the Use of Balances

It is not sufficient to purchase high quality weighing equipment in order to obtain good measurement results. Many other factors will have an influence on how well a mass balance or comparator will perform. One key factor is the location and environment in which the balance is located.

General Laboratory Design

It is inappropriate for a mass calibration laboratory to have external windows as this will lead to large fluctuations in the temperature through the day. In non-laboratory situations it may not always be possible to have a windowless room for the mass balances. In this case care must be taken to ensure that the balance is not situated near to a window or in direct sunlight.

It is recommended that mass laboratories have a small antechamber to prevent the ingress of dust from the rest of the building and to shield it from rapid temperature fluctuations as doors to corridors are opened and closed. Such a space may also be used to house specialist laboratory clothing and dust mats.

Temperature Control

Balances of all types are prone to drift with temperature change, and most have a recommended operating temperature range. In addition to this operators become uncomfortable at higher or lower temperatures so it is often necessary to control the temperature of the room containing the balance. If unacceptable drift in weighing results is to be avoided the control system should not induce rapid fluctuations in temperature. In some cases it is better to allow the temperature to slowly drift rather than introduce a rapidly cycling temperature profile. The International Organization for Legal Metrology (OIML) has a recommendation (R111) covering the calibration of weights that sets out recommendations for the environmental conditions under which various Classes of weights should be calibrated.

Balance draught exclusion.

One of the greatest problems with air conditioning is the air turbulence that it can introduce into a room. Mass balances and comparators are extremely sensitive to this and they will not achieve stable readings if there are any draughts. If the user is able to feel any air movement it is probable that the most sensitive balances will not function at all. In the case of automatic comparators it may be advantageous to place the instrument inside a supplementary draught shield (see image).

It is worth noting that balances should be switched on for several hours before use (ideally they should be left on at all times) to prevent drift as they 'warm-up'. If the most demanding measurements are to be made it is best to leave all the electrical equipment in the room (including lights) switched on at all times.

Balance vibration exclusion.

Vibration

It is essential that a mass balance should be mounted on a rigid support that is not susceptible to vibration. Granite is a popular material for use in the construction of such tables, or plinths as they are often called. A photograph of such a plinth is shown below. In this example the granite top is supported on two brick piers, but it would be equally satisfactory for it to be supported on two upright granite piers or reinforced concrete legs. If brickwork or concrete is used in the construction it is important that it should be sealed to prevent dust forming on the rough surfaces and moisture being absorbed into the mortar.

It is essential that no part of the table should be made from magnetic materials (including any reinforcement in concrete legs) as this may lead to erroneous balance readings.

Care should be taken to ensure that plinths are at a convenient height for the operator. Ideally there should be a separate surface for the operator to put books or other ancillary items on during the calibration process. This prevents any vibration being imparted to the plinth by the operator.

Other Influences

Most electronic balances are influenced by external magnetic fields, and some older instruments are influenced by radio interference. It is important to minimise these effects where weighing is being carried out.