Code of Practice for the area measurement of leather
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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 65%R.H.
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 dials.
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 leather.
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
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.