<———16.5 cm——>

[ 4
Cooling Coil
q v4 Stainless Steel
bd q B Jacket
o
zs
Stainless Steel
Basket
€ NY
°o
w
N
| a Solvent
t =

Hot Plate

Figure 1. Vapor Degreaser

GMP 5-5


weights. The particles are blown off the weights. Therefore, neither the
nozzle nor any other part of the syringe need touch the weights, and care
must be taken that they do not.

F. Brushes

All of the equipment used in cleaning and handling weights should, of
course, be clean. But, brushes require special attention because they are
easily contaminated and often are the last cleaning device used before the
weights are calibrated. Only soft brushes, such as camel hair brushes,
should be used on weights.

The brushes are cleaned by washing with soap and water, then rinsing in
ethyl alcohol and allowed to dry in air. When drying the brushes, place
them so that the bristles do not touch anything. New brushes are cleaned
before using to remove any oil or other matter that might contaminate the
weights. Used brushes are cleaned as often as necessary to be sure that
the brushes themselves do not contaminate the weights. Store cleaned
brushes in containers that will keep them clean until needed. When
handling the brushes, do not touch the bristles, as oil from the skin will
contaminate them. When the brushes are laid down, place them so that the
bristles do not touch anything.

G. Steam Generator

A steam generator may be purchased from a laboratory supply house or one
can be cons: -ucted.

A simple steam generator may be constructed from a 500 mL wash-bottle, a
two hole stopper, some glass tubing, a hot plate for the heat source, a
large graduate, and some flexible tubing.

H. Vapor Degreaser

Weights may be cleaned by vapor degreasing, using apparatus such as shown
in Figure 1.

The apparatus consists of a stainless steel cylinder placed on a hot plate.
The cylinder has several loops of copper tubing coiled around the outside
center of the cylinder as cooling coils. The cylinder also has a cover to
minimize the escape of vapors of the solvents used in the cleaning process. A
basket is suspended from the cover inside the cylinder. A single weight or
several small weights are Placed in the basket. Solvent (e.g. ACS grade
trichloro-ethylene) is placed in the cylinder to a depth of about 2.5 cm. The
solvent is heated to boiling. Cold water is circulated through the cooling
coils. The solvent condenses on the weight, thereby cleaning the weight, and
drops off into the liquid at the bottom of the cylinder. The cleaning process
is run for 15-20 minutes. The standards are then air dried and allowed to
return to room temperature. The weights, when removed from the degreaser, are
hot so that residual solvent evaporates in a few seconds. (Caution: Weights
should be removed from the degreaser immediately after cleaning to prevent
possible electrolytic corrosion. )

GMP 5-4




GMP No. 5

WEIGHT CLEANING PROCEDURES*

It is essential that weights being calibrated as well as the standards used
be clean if the calibration is to be accurate and meaningful. Therefore, a
cleaning procedure should be considered as a part of every calibration.

A. Categories of Weights

For cleaning purposes, weights may be divided into four categories:

1.

4.

One-piece weights.

This category will include all one-piece weights except lacquered
weights, sheet metal weights, and small wire weights.

Screw-knob weights.

This category will include all weights with adjusting cavities except
lacquered weights.

Lacquered weights.
This category includes all lacquered or painted weights.

Sheet metal weights and wire weights.

B. Gleaning Procedures

1.

One-piece weights.

One-piece weights, one gram and larger, are steam cleaned. The weights
are either held or placed in a jet of steam and manipulated so that the
entire surface of the weight is subjected to the cleaning action of the
steam long enough to clean it. A superficial steaming is not enough.
The weight is then dried, either by evaporation or careful wiping with
a soft non-abrasive material such as high grade cheese cloth, free from
oil and other substances that could leave a residue on the weights.
Care must be exercised that no water spots are left on the weights as
they dry. Visible particles on the weights should be brushed or wiped
off before steam cleaning them. If a steam generator is not available,
one-piece weights may be cleaned either by immersing them in a hot or
boiling distilled water bath in a non-metallic container, or according
to the procedures for screw-knob weights. Occasionally, a weight will
have foreign material adhering to it that requires the use of solvents.
Ethyl alcohol is a good general solvent. If alcohol does not remove
the material, other solvents may be used. Alcohol is then used to
remove any film left by the other solvents. The weights are then steam
cleaned as outlined above.

*From:

H. E. Almer, "Weight Cleaning Procedures" NBSIR-74-443 (1974).

GMP 5-1


Screw-knob weights.

Weights in this category ire usually cleaned by wiping with a soft
non-abrasive material suc: .s high grade cheesecloth, free from oils or
other substances that wou. leave a residue of any kind on the surface.
Occasionally, a weight will have foreign matter adhering to it that
requires cleaning by using solvents, applied with a cloth. Ethyl
alcohol is a good general solvent. If alcohol does not remove the
foreign material, other solvents may be used. Alcohol:is then used to
remove any film left by the other solvents.

Lacquered weights.

The cleaning of lacquered weights requires special care because their
Protective lacquer coating is soluble in most solvents. Lacquered
weights are cleaned by wiping with a soft non-abrasive material, free
from oils and other substances that would contaminate the weights, or
by brushing with a soft brush such as a camel hair brush, or both. A
rubber bulb type syringe may be used to blow off lint or other small
particles, however, be careful not to touch the weights with the
nozzle. An electrostatic charge may be placed on the surfaces of the
weights during the cleaning process or while handling them. This could
be especially troublesome in a very dry atmosphere. If reliable mass
values are to be obtained, the charge must be bled off of the weights
before calibration.

Sheet metal weights.

Either of the following two procedures may be used in cleaning sheet
metal and other small one-piece weights.

a. Two-step method.

First, the weights are placed in an acetone bath agitated to help
loosen any foreign material. A soft brush, such as a camel hair
brush, may be used to agitate the weights. The weights are removed
from the acetone, allowed to dry and then steam cleaned. For steam
cleaning, the weights are held in front of a jet of steam with
forceps until the entire surface has been covered with steam. (See
Note on next page). In order that the portion of the surface under
the forceps may be steamed, the weight is set down and picked up
again with the forceps holding the weight at a different spot than
the first time; the weight is again steamed. Do not allow the
weights to touch the steam nozzle. A low ash filter paper should
be used for drying the sheet metal weights. A circular disk is
folded unsymmetrically. The main body of the weight is placed
between the folds of the Paper with the turned up edge of the
weight. protruding. The main body of the weight is dried by
pressing lightly on the top of the paper. The turned up edge is
brushed lightly with a piece of filter paper. In some cases, it
may be necessary to brush the body of the weight with filter paper
to remove drops of water. Care must be exercised that no water
spots are left on the weights as they dry.

GMP 5-2


b. Three-step method.

First, the weights are placed in an acetone bath and agitated with
a soft brush to help loosen any foreign material adhering to the
weights. The weights are removed from the acetone, allowed to dry,
then placed in an ethyl alcohol bath and agitated. The weights are
removed from the alcohol bath, allowed to dry, and then steam
cleaned and dried as outlined in the two step procedure.

Note: The smaller fractional weights, say smaller than 1g, may be placed in a

hot or boiling distilled water bath for the final cleaning instead of
steam cleaning them. A hot or boiling distilled water bath also may be
used for the final cleaning of all sheet metal weights when a steam
generator is not available.

Cleaning Interval
1. Weights under test.

The weights under test are cleaned before calibration.
2. Standards.

Standards need not be cleaned every time they are used. If the
standards are handled carefully, and kept under a reasonably dust tight
cover, when not in use, in a clean atmosphere, the interval between
cleanings may be several months, or until the calibration procedure
checks indicate that the standards are changing and may need cleaning.
Under less favorable conditions the interval may be only a few weeks.

Temperature Equilibrium

Newly cleaned weights should be allowed to come to temperature equilibrium
before they are calibrated. This may take several hours for the larger
weights that have been steam cleaned.

Generally, laboratory weights will come to temperature equilibrium over
night.

Storage

Usually, weights are not placed in the balance immediately after cleaning,
but are stored for varying periods. The weights should be stored under
cover so that they will stay clean. Weights, one gram and larger, may
be stored on a tray lined with filter paper and covered with an inverted
glass dish. The smaller weights may be stored in a small glass dish
covered with a watch glass. In both cases, the container should be labeled
with the weight identification. When the weights are to be moved, carry
the tray or dish in a ievel position so that the weights do not slide
around,

When the weights are placed in the balance, they should be carefully

brushed to remove any particles that may be on them. A small bulb type
rubber syringe is useful in removing lint and other small particles from

GMP 5-3