Page 102 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES
support, noting the reading of weight dials and of an optical scale, returning
the beam to rest, and replacing weights which have been removed, are eliminated.
With an electronic balance, operation of a single on-off control permits the
operator to read the weight of an object on the balance pan immediately from
a digital display: most balances of this type can be coupled to a printer which
gives a printed record of the weight. The majority of balances incorporate a
tare facility which permits the weight of a container to be balanced out, so that
when material is added to the container, the weight recorded is simply that of
the material used. Many balances of this type incorporate a self-testing system
which indicates that the balance is functioning correctly each time it is switched
on, and also include a built-in weight calibration system. Operation of the
calibration control leads to display of the weight of the standard incorporated
within the balance, and thus indicates whether any correction is necessary. A
more satisfactory calibration procedure is to check the balance readings against
a series of calibrated analytical weights.
Electronic balances operate by applying an electromagnetic restoring force
to the support to which the balance pan is attached, so that when an object is
added to the balance pan, the resultant displacement of the support is cancelled
out. The magnitude of the restoring force is governed by the value of the current
flowing in the coils of the electromagnetic compensation system, and this is
proportional to the weight placed on the balance pan: a microprocessor converts
the value of the current into the digital display in grams.
The balance must of course be protected from draughts and from dust, and
the balance pan is situated within an enclosure provided with glass doors which
can be opened to provide access to the pan. The remainder of the balance,
including the electrical components, is contained in a closed compartment
attached to the rear of the pan compartment.
Electronic balances are available to cover weight ranges of
1. up to about 150 g and reading to 0.1 mg (macrobalance),
2. up to about 30 g and reading to 0.01 mg (semimicro balance),
3. up to about 20 g and reading to 1 pg (microbalance),
4. up to 5 g and reading to 0.1 pg (ultramicro balance).
Thus a wide variety of analytical balances is available.

3.3 OTHER BALANCES
For many laboratory operations it is necessary to weigh objects or materials
which are far heavier than the upper weight limit of a macro analytical balance,
or small amounts of material for which it is not necessary to weigh to the limit
of sensitivity of such a balance: this type of weighing is often referred to as a
'rough weighing'. A wide range of electronic balances is available for such
purposes with characteristics such as, for example,

Maximum capacity Reading to