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How to select the most suitable calibration weight
What class of calibration weight do I need?
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 above, 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, brass or Stainless Steel?
Some times we are asked if a lower class weight (cast iron or brass) can be adjusted to a better tolerance rather than purchase a more expensive weight. As the material the weight is manufactured from determines the daily and long term stability, adjusting to a tighter tolerance is not recommended as the weight will soon drift out of tolerance.
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 E2 tolerance over many years. Daily use may cause larger weights to fall below the E2 tolerance within a few years. Masscal 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. They are not stamped with their nominal value to aid in cleaning the highly polished surface. Nominal value stamping is available if required.
Stainless steel cylindrical balance calibration weights are essentially non magnetic and corrosion resistant, as per the polished stainless weights, but the plugged cavity which alters the density, and the unpolished surface, will not provide the same weight stability of a solid polished weight. These weights should not be used as precision primary reference standards. Stainless steel weights should be calibrated every three years. Weights used on a more regular basis may need more regular attention.
What is Air Buoyancy?
The internationally accepted method for measuring weight is to compare an unknown weight with a standard weight of a known density by weighing in air. Now weighing in air creates a problem, as objects of differing density are buoyed up by the surrounding air, much the same as heavy objects become lighter in water. For example, if a one litre bottle of water is lowered by a string into water, it will feel lighter as it is submerged. In this case the bottle is almost fully buoyed up by the water. As we are all fully submerged in air, what affect can the air have? Well if a 1kg brass weight (density 8400 kg/m³) is made to exactly balance a 1kg stainless steel standard weight (8000 kg/m³) today, then tomorrow when the atmospheric air pressure and temperature (density) have changed, the weights will no longer balance. This is because the brass and stainless steel weights are buoyed up in the air by differing amounts, as the buoyancy is proportional to air density. By convention, it is taken that the standard density for standard weights shall be 8000 kg/m³, and all other weights are compared to this. This is called the conventional weight and is different to the true weight by a small amount. Unfortunately this means that the conventional weight of weights with a density other than 8000 kg/m³, will vary daily with air density changes. Buoyancy corrections can be made if the air density is measured each time the weight is used if required, but this can be a lengthy task. Fortunately the buoyancy effect is small and in the case of cast iron and brass, is very small compared to the tolerance. Provided these weights are used with consideration of their tolerance, buoyancy will never be a problem. However, these weights are not recommended as precision reference standards or for the calibration of precision analytical balances. For Masscal stainless steel weights, the density is nominally 8000 kg/m³, so buoyancy can be neglected in all but the most precise measurements.