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68 Manual of basic techniques for a health laboratory
Fig. 3.30 Measuring out the substance to be
weighed
Fig. 3.31 Keep weighed and unweighed substances
apart to avoid confusion
Thus place:
— the weighed substances on the right
— the unweighed substances on the left.
This avoids confusion.
Read the label three times:
— before taking the bottle off the shelf;
— while weighing the substances (label facing upwards);
— after weighing, when you move the bottle to the right of the balance.
3.2.3 Analytical balance
This balance has two pans suspended from a cross-beam inside a glass case.
Use the balance:
— to weigh small quantities (up to 20 or 200g, depending on the model);
— when great accuracy is required: e.g. 3.85g, 0.220g, 6.740g.
Sensitivity: 0.1–0.5mg, depending on the model.
Components (Fig. 3.32)
●
Cross-beam (CB). This is the structure from which the pans are suspended.
● Knife edges (KE1, KE2, KE3). These support the beam at the fulcrum during
the weighing and give sensitivity to the balance. Those on the beam support the
suspended pans.
● Stirrups (S1, S2).
● Pointer (Pt).
● Pans (P).
● Beam release screw (or pan lock control) (B). Locks the pan so that the sudden
addition of weights or chemicals will not damage the delicate knife edges.
● Adjusting screws (AS1, AS2). Used only for initial adjustment of the unloaded
balance to a reading of zero.
Figure 3.33 shows a set of weights for use with an analytical balance.
Instructions for use
● Always ensure that the cross-beam is at rest (beam release screw tightened) before
placing the weights and the substance to be weighed on the pans.