Technical consultation on specifications and quality control of netting materials and mosquito nets
tested by the buyer for compliance with the specification must be decided by the buyer, according to the required balance between cost and risk. Risk depends on the probability (or frequency) of non-compliant nets occurring within a batch/consignment. A method for determining sampling frequency based on probability is given in Tables 1a and 1b, page 19.
Samples for testing physical properties should be taken from nets according to figure 1, page 20. Samples must be sufficiently large to conduct all tests required and be representative of the net. Except where seams are to be tested, do not test material within 10 cm of seams or selvedges. Where a net is made from more than one type of netting, each type of netting should be sampled and tested separately.
Use sharp scissors, or equivalent, to minimize damage to the fibres and fabric. Roll up the strips or squares and place them in labelled, new, clean aluminium foil prior to testing. Samples should be kept cool, avoiding heat sources (including sun heat) or freezing, and tested with the minimum delay. Representative portions (sub-samples) for testing should be taken as described in each test method.
In the absence of a simple or standard method to determine the size of holes, which may have complex shapes, in highly flexible fabrics, mesh size is determined by counting the number of holes in a square of the fabric. Counting may be done directly on the fabric or indirectly by scanning/photocopying the fabric. Indirect methods may ease counting and provide a permanent record. Before scanning/photocopying or counting directly, the fabric should be conditioned according to ISO 139 (2005) (4 h, 20 ºC, 65% relative humidity).
Use a template to define the square of netting, taking care not to stretch or distort the fabric. The template should be a rigid sheet, 1–2 mm thick, in/on which a calibrated (±1% in each dimension) square (e.g. 1 x 1 in or 2 x 2 cm) has been accurately cut/marked. Alternatively, a Pick glass may be used. If a template is not available and a ruler must be used, great care is required to ensure that the area counted is square. Where practicable, at least one edge of the square to be counted should be aligned with a row of complete holes in the fabric. Incomplete holes ≥½ are counted as complete holes, whereas those <½ are not counted. Count 5 replicate squares in pieces taken according to Note 3, calculate the average and note the lowest value.
Test method: ISO 13938-1 (1999) or ISO 13938-2 (1999), with conditioning of the fabric according to ISO 139 (2005) (4 h, 20 ºC, 65% relative humidity). The declared minimum bursting strength, and testing for compliance with it, should be based on tests of 7.3 cm2 areas of fabric. Proposed specifications based on tests of 50 cm2 area must be supported by data showing the suitability of the proposed value and its relationship to the minimum of 250 kPa (which is based on 7.3 cm2 area). Five replicate tests should be conducted on samples taken as described in Note 3. The average of the 5 measurements is calculated.