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

3. General laboratory procedures 69Fig. 3.32 Components of an analytical balanceAS1, AS2: adjusting screws; B: beamrelease screw; CB: cross-beam;KE1, KE2, KE3: knife edges; P: pans;Pt: pointer; S1, S2: stirrups.Fig. 3.33 Set of weights for use with ananalytical balanceSingle pieces: 1 g, 2g, 5 g, 10g, 20g,50g, 100 g, 200g and 500g.Single fractional pieces: 2 mg, 5mg,10mg, 20 mg, 50mg, 100 mg, 200mgand 500mg.●●●●●Check that the pans are balanced (after closing the glass case) by unscrewing thebeam release screw.Always place the substance to be weighed on a piece of paper folded in four, orin a watch glass or porcelain dish.Use the adjusting screws (AS1 and AS2) to obtain a perfect balance when compensatingfor the weight of the receptacle in which the substance will be weighed.Always use forceps to pick up the weights.Always put the cross-beam back at rest before removing the weights andthe substance that has been weighed from the pans.3.2.4 Dispensary balance (Fig. 3.34)This balance also has two suspended pans, but it has no glass case and nosupports.Sensitivity: 5–10mg.The dispensary balance is more accurate than the open two-pan balance,but weighs only up to 50g.After using the dispensary balance, put it away in a closed cupboard.3.3 Centrifugation3.3.1 PrincipleA body is rotated in a circular movement at speed. This creates a force thatdrives the body away from the centre of the circular movement (Fig. 3.35).To calculate the relative centrifugal force (rcf) for an individual centrifuge,measure the radius (r) of the rotor arm (in cm) and the number of revolutionsper minute (rpm) and use the formula below:rcf = 1.118 ¥ 10 -6 ¥ r ¥ (rpm) 2For example, if the radius is 25cm and the rpm is 1300rev/min, the rcf isabout 50g.Fig. 3.34 A dispensary balanceFig. 3.35 Principle of centrifugation