Afraid of the cold?
Diving a rebreather in cold water where temps are below 45°F/7°C to that of freezing can impose a very significant affect to the overall functionality of the unit’s scrubber.
Looking back of rebreather basics, CCR Divers know that carbon dioxide (CO2) is removed through a chemical reaction as it passes through the scrubber absorbent. This chemical reaction is exothermic in nature, meaning heat is generated as part of the chemical process. For unit’s scrubber to work at best (let along work at all) the core temperature of the sorb must remain relatively warm. Allowed to cool too much, the exothermic reaction-taking place in the absorbent is diminished, eventually rendered the absorbent’s ability to extract CO2 in affective.
Necessity is the Mother of Invention
For upcoming a scientific project in the Himalayas funded by Red Bull, deemed the “Steep N' Deep Project”, a light weight closed-circuit rebreather like the KISS Spirit LTE was the most desired as it was the most transportable and easy to handle. The one concern was how well would it handle in a high attitude lake environment with water temps close to freezing.
From prior performance testing of the Spirit CCR at Micropore’s testing facility, the Spirit’s dual scrubbers (packed with 797 grade Sofnolime) came through the endurance tests delivering a full 2 hours in 45°F/7°C water.
In case you are not familiar with what that means, when rebreathers are being evaluated, they are subjected to a workload test which a breathable gas is pushed through the scrubber at a rate the of 75 liter/minute RMV, while at the same time immersed in water as cold as 39°F/4°C. The idea is to simulate a diver breathing on the system under a high workload, producing higher than normal amounts of CO2 during respiration. Realistically, a stress level that even an incredibly fit diver with the conditioning of a Navy SEAL would be hard pressed to handle for more than 2 minutes straight, let alone 2 hours.
Coming back to the topic of heat generation during the exothermic reaction in absorbent material, what happens when the test criteria is changed that the water temp the rebreather is immerse in is raised from 45°F/7°C to 75°F/24°C?
When the same KISS Spirit was run through the same endurance test, the scrubbers this time lasted an additional 45 minutes bringing the time to 3.5 hours before signs of breakthrough occurred.
The stock scrubbers on the Spirit LTE, are made of high impact resistant PVC material featuring a wall thickness of 5mm. On its own, composite plastic materials have a greater insulation capacity than aluminum. No surprise there as aluminum is better suited as a heat exchanger to disperse heat rather than retain it. A classic example is found in cooling system of your car where engine coolant flows through the radiator’s aluminum coils so that air flowing over it cools the metal thereby the fluid inside. Not what you would want happening with absorbent in a rebreather’s scrubber.
Composites with a lower density like foam usually have even higher thermal insulation properties. The down side, most closed cell type foam like neoprene and polyethylene foam compress under pressure, thereby losing most of their insulative properties. The one type of high-density foam that does not is Syntactic Foam.
Syntactic foam has been around since 1960s. It’s most common application is for buoyancy modules on highly specialized remotely operated underwater vehicles (ROVs) and autonomous underwater vehicles (AUVs), used in deep-sea exploration with working depths up to 20,000 meters.
Appearance wise, Syntactic foam looks a lot like normal, high-density foam, but without any form of springiness. Instead, the foam’s tensile strength is in creditably hard and stiff making it extremely resistant to compression under extreme pressure.
This is because Syntactic foam ("syntactic" means "put together") is a composite material synthesized by filling a metal, polymer, or ceramic matrix with hollow particles called microballoons. These microballoons, also referred to as hollow phenolic microspheres, are then bonded to a matrix of phenolic, epoxy, or polyester, thereby resulting in material with a very low density to high specific strength ratio, with also a lower coefficient of thermal expansion.
Back when the Orca Spirit design was being worked, some of the units tested in Northern California and Vancouver were fitted with Syntactic foam sleeves. The Syntactic foam sleeves in their original form worked well in keeping the working temperature of the sorb up in the cooler water, but they also had a tendency slip loose, even crack due to them only being fitted over the scrubber tubes.
Faced with short deadline of the Steep N' Deep Project, Mike Young, CEO of KISS Rebreathers and designer of the Spirit looked back on those tests, to come up with a rather simple solution – why not bond a thin layer of Syntactic foam to the outside surface of the Spirit’s dual scrubbers.
Turning Theory into Application
Before the expedition would take place, Mike Young wanted to see how the Spirit LTE with the Syntactic foam performed against a Spirit LTE with just bare PVC scrubber tubes in the real world environment instead of indoor a testing facility. For this, Mike choose Crater Lake in the Elk Mountains near his family’s farm in Colorado which is situated at 11,640-feet above sea level. Between the high altitude and wintertime temps where water within lake often drops to the freezing point of 32°F/0°C, made for a reasonable simulation to the Steep N' Deep planed project in the Himalayas. To establish a baseline, Mike started with bare PVC scrubber tubes in the near freezing water temps.
At the start of the dive, working temperature inside the scrubbers averaged 77°F/25°C, dropping to 62°F/17°C by the end of the first 15 minutes. As time when on, Mike continued to watch the incremental drops in temperature till a cut off point where exothermic reaction was no longer likely reached with the scrubber temp below at 45°F/7°C in the span of 45 minutes span.
Playing around with various thicknesses of Syntactic foam bonded to the scrubber tube’s outer surface, found instead of experiencing a drop from a starting working temp of 77°F/25°C like it did with the bare scrubber tubes, the scrubbers with a 3mm thick layer of the Syntactic foam never drop below 70°F/21°C, but instead actually returned to the same working temp of 77°F/25°C during the span of a 2-hour dive. Even more remarkable, according to Mike, the working temp measured inside the scrubbers actually rose to 79°F/26°C towards the end of that dive.
A cool feature about Syntactic foam is that hollow phenolic microspheres (microballoons) which are comprised of a matrix of phenolic, epoxy, or polyester, material, is that it can be bonded molecularly to materials like PVC.
The process for applying the Syntactic foam insulation is very labor intensive, as the material has to be properly bonded in a series thin layers to the scrubber tube, and then spun on a lath so the layer sets evenly. Once harden, a second layer is applied where it undergoes the same process as the first layer. After the material is allowed an ample time to cure the scrubber tubes are then given their rhino guard coating. When complete, the outward appearance between it and a stock standard coated tube is nearly indistinguishable. Only when closely examined, the Syntactic lined scrubber will be 6-7mm fatter in diameter than that of the standard stock Spirit LTE scrubbers.
The Syntactic foam insulation is available as an option to both KISS Spirit and Classic Explorer.