BS EN ISO 9913-2:2000 pdf – Optics and optical instruments — Contact lenses — Part 2: Determination of oxygen permeability and transmissibility by the coulometric method.
1? Scope This part of ISO 9913 describes a coulometric method for the determination of oxygen permeability of both rigid and non-hydrogel flexible contact lens materials and oxygen transmissibility of rigid and non-hydrogel flexible contact lenses. It specifies the procedures for the measurements and establishes the conditions under which measurements are made. This part of ISO 9913 is applicable to the determination of oxygen transmissibility of rigid and non-hydrogel flexible contact lenses, incorporating various refractive powers and radially symmetric contact lens geometries, and the oxygen permeability (Dk) of rigid and non-hydrogel flexible contact lens materials in the form of standardized test samples. This part of ISO 9913 is especially useful for the determination of permeability values above 75 ? 10 –1 1 (cm 2 /s) [ml O 2 /(ml?hPa)], which fall above the usual range of the standard polarographic method of measurement (FATT method, see ISO 9913-1). This part of ISO 9913 is not applicable to hydrogel materials or hydrogel contact lenses. 2? Normative references The following normative documents contain provisions which, through reference in this text, constitute provisions of this part of ISO 991 3. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this part of ISO 991 3 are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referred to applies. Members of ISO and IEC maintain registers of currently valid International Standards. ISO 5725 (all parts), Accuracy (trueness and precision) of measurement methods and results. ISO 8320:1986, Optics and optical instruments — Contact lenses — Vocabulary and symbols. ISO 9339-1:1996, Optics and optical instruments — Contact lenses — Determination of the thickness — Part 1 : Rigid contact lenses.
4 Principle 4.1 Overview A rigid or non-hydrogel flexible contact lens is placed in the apparatus described in 6.1 , with exposed front and rear contact lens surface areas in contact with gas mixtures at eye temperature (35 °C). The gaseous environments at the anterior and posterior contact lens surfaces are separated by the contact lens, which acts as a barrier to the net flow of oxygen from the anterior environment to the posterior environment. The two environments and the contact lens can be purged of all detectable oxygen gas. Once purged, an oxygen-containing gas is allowed to fill the anterior environmental chamber and to diffuse through the contact lens. An inert carrier gas, initially oxygen-free, is allowed to flow across the posterior environmental chamber at the posterior contact lens surface and to remove oxygen molecules that have crossed the contact lens barrier. The carrier gas, now containing a small concentration of oxygen, is directed to a coulometric sensor which creates a current proportional to the concentration of oxygen flowing past the detector. With appropriate instrument calibration, such that the concentration of oxygen at the detector is precisely known, the rate of oxygen flow q V in microlitres per second, past the detector can be determined and recorded.
4.3? Precision of measurement A single determination of oxygen transmissiblity (Dk/t) and/or oxygen permeability (Dk) shall have a reproducibility value (R) of 1 0 % of the corrected value, in accordance with ISO 5725. 5? Reagents and materials 5.1 Oxygen-free carrier gas, consisting of nitrogen gas (volume fraction 99,9 % or more) or a mixture of nitrogen gas (volume fraction 97 % to 99,5 %) and hydrogen gas (volume fraction 0,5 % to 3 %). The carrier gas shall be dry and contain not more than a volume fraction of 0,01 % of oxygen. An oxygen trap and a moisture trap shall ensure that the carrier gas is essentially oxygen-free and dry prior to reaching the diffusion cell (6.1 .1 ) and that vapour pressure P P is zero. 5.2 Oxygenated test gas, comprising either a mixture of oxygen gas (volume fraction 20,9 %) and nitrogen gas (volume fraction 79,1 %), or compressed air, or oxygen gas (volume fraction 99,9 % or greater). A moisture trap dries the test gas prior to introduction into the anterior environmental chamber of the diffusion cell (6.1 .1 ). 5.3 Sealing grease, either a high-viscosity non-silicone stopcock grease or a high-vacuum grease which is nearly impermeable to oxygen. A “sealing grease” is required for sealing the contact lens test sample to the two halves of the diffusion cell, as noted in 6.1 .1 . 5.4 Standard reference material (SRM), comprising a non-hydrogel flat plastic film, typically used for preliminary calibration of the apparatus, having parallel surfaces and a certified oxygen transmissibility (Dk/t) 2? . 6? Apparatus 6.1 Oxygen gas transmission apparatus, depicted in Figure 1 , consisting of a diffusion cell (6.1 .1 ), O-ring, heating unit (6.1 .2), flowmeter, coulometric oxygen sensor (6.1 .3), load resistor and recorder (6.1 .4).