INTRODUCTION: Diving rebreathers use canisters containing soda lime to remove carbon dioxide (CO2) from expired gas. Soda lime has a finite ability to absorb CO₂. Temperature sticks monitor the exothermic reaction between CO₂ and soda lime to predict remaining absorptive capacity. The accuracy of these predictions was investigated in two rebreathers that utilise temperature sticks. METHODS: Inspiration and rEvo rebreathers filled with new soda lime were immersed in water at 19°C and operated on mechanical circuits whose ventilation and CO₂-addition parameters simulated dives involving either moderate exercise (6 MET) throughout (mod-ex), or 90 minutes of 6 MET exercise followed by 2 MET exercise (low-ex) until breakthrough (inspired PCO₂ [PiCO₂] = 1 kPa). Simulated dives were conducted at surface pressure (sea-level) (low-ex: Inspiration, n = 5; rEvo, n = 5; mod-ex: Inspiration, n = 7, rEvo, n = 5) and at 3-6 metres' sea water (msw) depth (mod-ex protocol only: Inspiration, n = 8; rEvo, n = 5). RESULTS: Operated at surface pressure, both rebreathers warned appropriately in four of five low-ex tests but failed to do so in the 12 mod-ex tests. At 3-6 msw depth, warnings preceded breakthrough in 11 of 13 mod-ex tests. The rEvo warned conservatively in all five tests (approximately 60 minutes prior). Inspiration warnings immediately preceded breakthrough in six of eight tests, but were marginally late in one test and 13 minutes late in another. CONCLUSION: When operated at even shallow depth, temperature sticks provided timely warning of significant CO₂ breakthrough in the scenarios examined. They are much less accurate during simulated exercise at surface pressure. Copyright: This article is the copyright of the authors who grant Diving and Hyperbaric Medicine a non-exclusive licence to publish the article in electronic and other forms.

The permeability properties of composite gas cylinders for breathing gas with polymer inner-liner are investigated. The cylinder wall can be described as a composite membrane consisting of two layers. The permeability properties of the cylinder are presented as permeability coefficient and permselectivity. Deviation from the expected gas components might lead to incidents and potentially harmful situations when breathing gas from a compressed gas cylinder. Hence, gas permeability and potential changes in gas composition, must be considered when choosing cylinder materials. Cases of decompression sickness initiated this study. Experimental data show that pressure and oxygen fraction in the gas cylinder drops and that the permeability coefficient varies depending on the inner pressure. Permeability coefficients of 0.62–0.90 Barrer for oxygen and 0.44–0.56 Barrer for nitrogen are measured. Cracks in the inner-liner have caused an accentuated drop in of oxygen fraction and pressure. © 2020 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals LLC.