Newsflash! Low Pressure Hose Deterioration
By Francois Burman, Pr. Eng, MSc
Crystallized nylon-hose interiors can cause regulator failures. Polymorphic crystallization inside hoses has recently emerged as a hazard divers should be aware of.
Scuba equipment failure or malfunction is a relatively rare factor in diving-related accidents and fatalities. When it does occur, the most common and hazardous malfunctions involve regulators and buoyancy compensator (BC) power inflators.1 Thus, a recent report to DAN® of a regulator failure was not necessarily surprising, but the cause in this particular case turned out to be quite unusual.
Although the diver involved in the incident managed the situation very well, and no one was harmed, an inexperienced or nervous diver might not have been so fortunate. It was particularly strange that although the diver's cylinder was not empty, the gas flow had slowed and then ceased in a way that resembled an out-of-air situation. Closer examination of the equipment led to a puzzling discovery: A large amount of yellow crystallized material was blocking the inside of the braided second-stage regulator hose. The hose had been in use for a few years but showed no external abnormalities or signs of deterioration.
While trying to solve the mystery, we discovered this was not an isolated case. The same situation occurred July 22, 2015, and was reported subsequently in a technical diving blog.2 Further reports from a hose manufacturer and at least one equipment-servicing workshop in a popular diving region revealed that these were not isolated events. Although no injuries have been reported, the discovery prompted a wider, global investigation, which is ongoing.
Analysis revealed the culprit to be a form of polymorphic crystallization — a phenomenon associated with cyclical heating and cooling at oil-water interfaces.3 In both incidents, the crystallization seemed to be related to the molecular structure of the internal tube. The current theory is that repeated cyclical heating and cooling of the hose lining promotes this form of crystallization in materials either unsuitable for this application or affected by certain chemicals or bacteria. The sun heats the hose, then the flow of breathing gas cools down the internal surface of the hose again. This process recurs with each dive, and the crystals grow and accumulate over time. Enough crystals eventually form to encroach on the gas flow, or they migrate toward the second-stage regulator, resulting in significant failure of the breathing device.
So far the reported incidents have been in tropical climates with gear that is several years old. We have not received reports of this problem with vinyl hoses, and given the number of braided hoses out there (both as original components and replacement parts) the incidence is likely quite small. But because this hazard threatens divers' air supplies, it is of great potential interest to the dive community. The challenge is determining how best to respond to this discovery. It's important to identify what divers need to know right now as well as what precautionary maintenance guidelines and early detection strategies they should implement at this stage.
The standard safety recommendation regarding regulator hoses has been that they should be inspected regularly for signs of external deterioration. Disintegration or abrasion of the outer rubber coating eventually predisposes hoses to rupture during pressurization or even when in use. Rubber hoses are quite prone to this condition, which is why polymer-braided hoses were developed. But herein lies the problem: The outside of braided hoses can appear normal, flexible and free from obvious abnormalities, while a deteriorated internal surface would be completely invisible to a cursory external inspection.
It would be premature to speculate that this deterioration is limited to braided hoses. Previously, hoses comprised a rubber inner hose, a single braid layer for reinforcement and finally a rubber outer sealing layer. Today the rubber inner hose is sometimes replaced with a polyurethane or nylon hose (referred to as thermoplastic). The middle or reinforcing layer is a polymer-filament braid, and the outer layer is either a second braid — which has the advantage of indicating when the inner hose has a leak — or a polyurethane or synthetic-rubber sealing layer.
Newer second-stage hoses with a thermoplastic inner layer may be vulnerable to degradation. This risk does not apply to internal layers of synthetic rubber, which appear to be impervious to this phenomenon. DAN Research would normally wish to conduct more in-depth analysis with a wider sample of defective hoses, but because this is a potential hazard to regulator function we prefer to err on the side of caution and report this phenomenon now.
What Does DAN Recommend?
DAN has contacted manufacturers of outer braided hoses to assess possible causes and precautionary measures, and we feel it is important to advise divers as follows:
- All regulator hoses, including braided hoses, have a limited service-life – irrespective of the external appearance, or the reinforcement and protection provided by hose protectors or the braiding itself. Prior to 2010, the stated service life of 5 years or 500 dives for braided hoses appears to be inaccurate; failures have occurred in hoses with less than 4 years of in-service life. After this date, the manufacturers appear to have made satisfactory changes.
- The inside lining of all hoses appears to be uniquely prone to degradation in the form of 'crystallisation’, especially in hot, tropical locations. This is a gradual process, reportedly especially active in the range of 27° - 32°C, but the disruption of gas-flow and regulator function is unpredictable and invisible to external inspection.
- Any sign of gas-flow restriction in the regulator assembly should prompt careful inspection of the regulator and the hose. If the regulator is not the cause, suspect the hose.
- A physical examination including squeezing the hose every couple of inches to assess whether the hoses exhibits the same degree of flex should indicate if all is well. Any indication of a change in the resistance.
- Make all divers aware of this problem and the need for regular equipment servicing.
- Practice emergency alternative gas source sharing procedures to ensure preparedness and appropriate action in the event of regulator failure as well as out-of-gas situations.
- Be aware of what you buy - ensure that any hose purchased clearly shows information on the manufacturer, the production date and the standard used on the ends, and check that this information is consistent with the packaging.
Originally brought to light in 2015 (see link)... this issue has been reported by many scuba divers. http://scubatechphilippines.com/scuba_blog/regulator-hose-diving-emergency/
Can anyone comment on the brand of hose, number of dives, diving air or EANx? Were these rental gear used every day or did they sit in someone's closet most of the time?
I had a complete shutdown of airflow in January 2015 at 140 ft with a braided hose. The hose was stamped 2008, used daily for about 4 years in Cozumel.
The DAN Alert Diver article titled 'Air Hoses: A Closer Look' has triggered some inquiries regarding material used in the construction of double braided flex hoses. We supply Miflex hoses and they ARE NOT constructed with an inner lining of Polyester-TPU. That material can undergo changes, called hydrolysis, which cause the rapid failure of the hose especially in hot and humid conditions.
Miflex branded hoses are constructed with an inner lining of Polyether-TPU. Some other braided flex hoses are constructed with an inner lining of High Syntactic Polyvinyl Chloride. Regardless, neither Polyether-TPU or HS-PVC is Polyester-TPU. In our long 10 year history with all the double braided Miflex hoses we've sold, we've never seen anything we think could be the hydrolysis problem described in the DAN Alert Diver article.
Miflex hoses are the real deal and we believe safer with longer life than rubber hoses. They are manufactured in Italy with more than thirty individual performance and safety automated tests. All Miflex hoses are designed to meet CE EN250 standards and are Nitrox Ready. But make sure you buy high quality, Miflex hoses, not cheap brand flexible braided hoses made from inferior materials.
DAN does not promote or demote specific manufacturers' products. Our intention is to point out the fundamental reasons for failure and then to advise the diver as to what measures to take to remain safe.
Miflex themselves state that a 5-year or 500 dive count is what the diver should understand as the life span of the hose; of course assuming that there is no abrasion or other mechanical damage. This is their recommendation.
Miflex do indeed use poly-ether PU: this we determined when we found early onset degradation in hoses containing a poly-ester PU liner. They confirmed this although they also informed us that up to 2008, they had some doubts about the quality of the liner materials they used. After this date, they enhanced their quality control to make sure that the material used was indeed the poly-ether version.
Lastly, Miflex cooperated fully with our investigation and in fact both approved of the articles as well as being grateful for our publishing them.