Balances play an integral role in a materials testing laboratory. This article will introduce you to some of the basic terminology associated with balances as well as explain the different variations that exist.
Laboratory Balance Terms
Accuracy. The ability of a scale to provide a result that is as close as possible to the actual value.
Calibration. The comparison between the output of a balance against a standard value.
Capacity. The heaviest load that can be measured on the instrument.
Precision. Also known as repeatability, this is the amount of agreement between repeated measurements of the same quantity.
Readability. This is the smallest division at which the scale can be read. It can vary as much as 0.1g to 0.0000001g.
Tare. The act of removing the weight of the weighing container to zero. This means the final measurement will be from the material to be weighed only. Most balances allow taring to 100% of capacity.
Types of Laboratory Balance
Proper Care for Your Balance
A balance that is used frequently needs to be efficient, durable and reliable, so it is important that your equipment is properly maintained. Remember that the items to be measured should be at room temperature before weighing; a hot item will give a reading less than the actual weight due to convection currents that make the item more buoyant. If your balance is enclosed, the warm air in the case will weigh less than the air of the same volume at room temperature.
Always clean the balance as it is exposed to many chemicals that can react with the metal in the pan and destroy the surface. Also, keep in mind that a potentially dangerous situation could occur if chemicals are left on the balance pan. In many lab and classroom situations, more than one person uses a single balance for weighing. It would be impossible for each person to know what everyone else has been measuring and there is a chance that incompatible chemicals could be brought into contact if left uncleaned. A camel’s hair brush can be used to remove any dust that can spill over during weighing.
Another way of taking care of the balance is by doing a regular calibration. There are weight sets available that allow users to calibrate the scale themselves or the scales can be calibrated by hiring a professional, like GlobalTroxler, to calibrate them on site. The correct weight set needs to be chosen when calibrating a scale. The classes of weight sets start at Class One, which provides the greatest precision, then moves to Class Two, Three, Four and F, and finally down to a Class M, which is for weights of average precision. Weight sets have class tolerance factors, and as a general rule, the tolerance factor should be greater than the readability of the scale.
a.) Class 1 provides the greatest precision and is used for calibrating high precision analytical balances.
b.) Class 2 is used for calibrating high precision top-loading balances. The remaining classes utilize weights of decreasing precision. In fact, calibration weights themselves often need to be re-certified depending on the degree of accuracy required from the weighing instrument. Many governments and industry specifications require proof of accuracy. The weights can change due to scratches, wear, the accumulation of dirt and atmospheric corrosion, thus reducing the accuracy of the set.
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