Code of Practice for the area measurement of leather by
the pinwheel measuring machine
1. Calibration and storage of templates. New templates
should be verified by an impartial scientific body which can
accurately determine the size of the template. Used templates
should be re-submitted for testing every 12 months.
Each machine should be provided with templates appropriate to
the size of the leather measured. Templates should be kept flat
and at room temperature away from light.
2. Calibration of machine with template. Regular calibration
of the machine with templates is a necessary and convenient
safeguard of accuracy and in the best interests of all parties.
The machine must be calibrated with the template of approximately
the same area of the leather to be measured. The templates should
be passed through the machine a sufficient number of times and
in such a way that all pinwheels are tested. It is good practice
to calibrate the machine at least the start of each work period,
i.e. twice a day. Additional random tests by supervisory staff
are also recommended.
3. Conditioning of leather. Leather for check measurement
should be exposed to a standard atmosphere of 65% (+/-5%) relative
humidity at 20°C +/-2 °C in such a way that each skin
has free access to the surrounding air for at least 48 hours.
If the foregoing is found difficult to achieve in practice then
the conditioning regime should be stated in the test report
as a deviation from the method. The relative humidity is more
important than atmospheric temperature. Some leathers, eg. chamois,
exhibit considerable hysteresis in their regain of moisture
from standard atmosphere. For this reason, and to minimise disputes,
it is recommended that in arbitration such leather is conditioned
on the descending side of the hysteresis loop i.e. from as moisture
content corresponding to a higher relative humidity down to
4. Operation of the machine with leather
a. It is self evident that the dial-pointer must be allowed
to return to zero before each measurement. This can be ensured
either by careful work and supervision of operators, or by installation
of the damper which slows up the return of the pedal to its
starting position after the pointer has returned to zero.
b. Machines should not be run at any speed different
from that recommended by the manufacturer.
c. It is good practice for the area of a piece of leather
to be marked or recorded by the operator who reads the dial
measurement. Where "straight through" operation is
used, careful supervision is needed to avoid "calling out"
errors, or preferably, the machine should be fitted with two
d. To avoid errors due to parallax, the operator should
face the dial squarely and the distance between the pointer
and the dial face should be the minimum.
e. It is good practice for supervisory staff to carry
out or supervise re-measurements at random on samples of measured
5. Method of measurement. The leather should be fed into
the machine with the higher friction surface coming into contact
with the pinwheels. It should be absolutely flat and without
creases at the moment when it passes between the pinwheels and
the top of the transport roller. In the case of soft leathers
this may involve pulling the leather from belly to belly, or
from edge to edge, with sufficient force to prevent the pins
pushing the leather into the transport feed slots, the leather
being held in such a manner that it remains flat as it passes
through the machine. To ensure this, more than one operator
may be needed to feed the specimen into the machine. Any area
gained in pulling the leather in this way will be lost in the
direction at right angles to it, as, provided the pinwheel machine
is zeroed correctly, it cannot over-measure
6. Tolerances. A tolerance of +/-2% (on the whole
parcel and not on individual hides or skins) is accepted.
Also accepted is a tolerance of +/-3% for softer, stretchier
leathers, such as gloving and clothing leathers, light-weight
suedes, chamois leathers, bellies, and upholstery leather.