Manual of basic techniques for a health laboratory - libdoc.who.int

3. General laboratory procedures 67Important: If water has been used to clean the balance, make sure that it is thoroughlydry before weighing. Always set the balance to zero before weighing. Checkthe accuracy of the balance regularly according to the manufacturer’s recommendations.Handle loose weights with forceps.3.2.1 Sensitivity of a balanceThe sensitivity corresponds to the smallest mass that makes the pointer move overone division on the scale. For example, if the sensitivity of a balance is 1mg, thismeans that a mass of at least 1mg is needed to move the pointer.For routine laboratory purposes, the sensitivity of a balance can be considered tobe the smallest mass that it will measure accurately.3.2.2 Open two-pan balance (Fig. 3.28)The two-pan balance has two pans supported by shafts. It may be designed for usewith separate weights, as illustrated in Fig. 3.29, or may incorporate a graduatedarm with a sliding weight. It is used to weigh large amounts (up to several kilograms)when a high degree of accuracy is not required, e.g. 22.5g, 38g, 8.5g,380g.Sensitivity: 0.5g.If the pans are made of easily scratched or corroded material, protect them withcircles cut out of strong plastic or old X-ray films; the two circles should be of equalweight.Instructions for use1. Place the bottle containing the substance to be weighed to the left of thebalance.2. Place on the left-hand pan the receptacle (folded paper or dish) in which thesubstance will be weighed.3. Place on the right-hand pan the weights equivalent to the weight of the receptacleplus the amount of the substance to be weighed.4. To measure out the substance to be weighed, hold the bottle tilted in your lefthand (label upwards) and tap the neck of the bottle gently with your right hand,so that the powder or crystals to be weighed fall little by little into the receptacle(Fig. 3.30). (Use a clean spatula to dispense small amounts of substances forweighing.)When the substance has been weighed, move the bottle to the right-hand side ofthe balance (Fig. 3.31).Fig. 3.28 An open two-pan balanceFig. 3.29 Set of weights (in grams) for usewith an open two-pan balance