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To select a weight from the wide range of classes, tolerances and materials available, the most important aspect to consider is the accuracy required by your application. This will usually determine the tolerance and material required. If you are not sure of the accuracy required, then consider the readability of the balance or scale you will be using the weight with, and take this as the required accuracy. Now using the chart below, choose an appropriate weight class with a tolerance about one half of your required accuracy. If the weight is to be used to calibrate a balance, then choose a weight with a tolerance approximately one half of the balance readability. For some high resolution analytical balances, it may not be possible to choose a weight with a small enough tolerance. In this case choose the best tolerance available (E2) and order a NATA calibration certificate which will state the actual value of the calibration mass.
Cast iron weights will vary in weight value due to moisture absorption (even from the atmosphere), corrosion (rust) and wear. Even small daily/weekly and seasonal variations can be expected. They are highly magnetic so will be effected by magnetic fields from electric motors and electronic balances etc. With typical light use, cast iron weights will retain their weight value to within ½ to ¼ of the original tolerance over a one year period. Good clean dry storage will improve this. With regular use, the weight variation may fall below the original tolerance, due to wear, within less than one year. Cast iron weights are adjustable, so have your weights checked and readjusted regularly to maintain the tolerance. Cast iron weights should be re-calibrated annually.
Brass weights have better short term (daily to monthly) weight stability than cast iron, but will vary with time due to wear and corrosion of the brass and lead adjusting slug. With light use and good storage and handling, they will retain their value to within ½ to ¼ of the original tolerance over many years. As brass is a soft material, regular use will cause the weight value to reduce more quickly through wear. Brass weights are essentially non magnetic, but can be effected slightly by magnetic fields. Brass weights should not be used as precision reference standards due to daily weight variations with atmospheric air density (air buoyancy), wear and contamination. They should not be used to calibrate precision balances. Brass weights should be re-calibrated annually.
Precision stainless steel weights are manufactured to a correct nominal density of 8000 kg/m³ to minimise daily weight variations due to air buoyancy effects. The weight is highly polished to minimise the surface area for long term stability and to provide an easily cleaned surface. With light use, careful handling and good storage, these weights will retain their value to within a small fraction of the tolerance over many years. Daily use may cause larger weights to fall below the tolerance within a few years. Polished stainless steel reference weights are manufactured from selected grade stainless steel for it's low magnetic properties, and are essentially non magnetic, but may be effected slightly by magnetic fields.