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Leather area measurement
How SATRA’s SURESIZE machine can provide reliable and accurate measurement of hides or skins.
It is always important to source raw materials and components from trustworthy suppliers. Companies within supply chains develop product specifications to ensure such things as consistency of manufacture, quality of production, environmental and social compliance, all of which should contribute to every party being in agreement.
Unfortunately, disputes do still occur, and this can certainly be the case when purchasing leather, which is sold by its area. A small discrepancy in area, due to a fault in measuring equipment or operator error, can be costly if repeated over an entire batch. There are also a number of types of measuring equipment available, so different methods of measurement combined with varying regularity and reliability of calibration can lead to problems in area measurement.
A number of factors relating to the leather itself can also contribute:
- it has a non-uniform shape
- it does not always lie perfectly flat due to the softness or firmness of the leather. Soft leathers are particularly prone to curling if not laid flat on conveyor-type machines, as they often ‘dip’ between the series of parallel strings or cords used to transport the leather. They can actually be under-measured which, in turn, may lead to the tanners trying to ‘compensate’ when setting up the machines
- the hide or skin may have holes in it which need to be taken into account
- the area of leather can change when subject to different atmospheric temperature and moisture conditions (humidity), as leather is a hygroscopic material that adjusts its moisture content until it reaches equilibrium with the surrounding environment.
In an attempt to remove some of these anomalies, the International Union of Leather Technologists and Chemists (IULTCS) produced an official method of area measurement in 1989 (IUP 32). This was later adopted as an International Standard ISO 11646 in 1993 and was replaced by ISO 11646:2014 – 'Leather – Measurement of area'.
SATRA is United Kingdom Accreditation Service (UKAS)-accredited to carry out this test as part of our SURESIZE leather measurement service.
ISO 11646 requires the use of a mechanical pin-wheel area measurement machine. This consists of a driven grooved roller on which rests a row of wheels, with the distance between the centres of two adjacent wheels being 25mm. These wheels contain blunt pins on their rims which can drop into the grooves of the driving roller. The leather is fed over the driving roller which, in turn, prevents the pin dropping into the groove. This action, through a complex series of wheels and steel tapes attached to levers, transmits the movement to a dial gauge which records the area of the leather. An important factor is that any holes in the leather will not register as the pins drop through. SATRA’s SURESIZE machine has a full working width of 3.2 metres, and is capable of measuring up to 5.48 square metres (59 square feet), both values being displayed on the dial. ISO 11646 discourages the use of imperial units (square feet) but recognises that commercially this system is still commonly used, and includes the conversion 1 sq ft = 0.0929m2. The test method also requires that the leather is conditioned to ISO 2419:2012 (20°C and 65 per cent RH) for at least 48 hours, as this will minimise any variability due to the hygroscopic nature of the leather.
To ensure that every measurement is correct, an independently-calibrated template made from a flexible reinforced material is used. This is of known area which is not less than 5 per cent of the pin-wheel machine’s capacity and the width of the template is not greater than 50 per cent of the machine’s working width. Ideally, the template should be within +/-50 per cent of the anticipated area of the leather to be tested.
To make sure that the pin-wheel machine is calibrated correctly, a template (generally an older or uncertified example) is initially passed through the machine in a random fashion approximately 25 times to ensure that all the pins are engaged and that the machine is running freely. Once running satisfactorily, the machine is ‘zeroed’ and the process repeated using the certified template. This is passed through the machine a number of times, after which a cumulative measurement is taken. The measurement must record a total area of within +/-0.01m2 of the theoretical total before testing commences. Only when the machine is within tolerance is measurement for the customer carried out.
The leather is fed into the machine with the higher-friction surface in contact with the pin-wheels, and often requires two operators to ensure that the leather is held flat as it passes through the machine. For soft leathers, this may involve pulling the leather with sufficient force to prevent the pins pushing the leather into the transport roller slots. It should not involve so great a force that the leather becomes over-stretched, resulting in too high an area measurement.
If the leather has a linear edge (such as found on side leather), it is fed into the machine so that the straight edge forms an approximate 30 degree angle to the direction of movement. In the case of full hides or complete skins, the line of the backbone is perpendicular to the axis of the feed roller.
If the area of the leather to be tested is more than 5 per cent of the pin-wheel’s measurement capacity, the area is measured twice. If the readings differ, this process is repeated until two equal readings are obtained. The area is typically reported to the nearest 0.01m2, rounded to two decimal places. In cases where the leather to be tested is less than 5 per cent of the machine’s capacity, a slightly different procedure is followed, which involves more measurements being taken.
SATRA’s SURESIZE service is invaluable in the case of dispute, and is also an essential verification service for tanners who want to have an independent check on their internal leather measurement, thus avoiding disputes before they arise.
How can we help?
Please email email@example.com for further information on leather area measurement.