Any "toxic" breathing tube materials to avoid please?

Mdemon

I'm playing around with my unit and want to use rigid pipes to extend the convoluted hose. (If I like the change, I'll splash out on longer hoses)

Are there any pipe materials (metals, plastics) which shouldn't be used please? (For toxic reasons)

This is for a BOB, so exposure time should be minimal...

TIA

Genesis

I like HDPE if it works; you can't glue it easily (or at all) but if its a piece with two hose barbs to be turned on it, then it works well. HDPE is damn near inert and doesn't offgas. HDPE is the current "gold standard" for municipal main water piping, in no small part because of its ease of heat welding and flexability (e.g. it tends not to break as easily as other solutions), but its inert qualities are not to be ignored.

For that reason I used it for my scrubber can and am making "T" pieces out of it now as well (the latter is a bit tricky because it requires three pieces, but it does machine and take threads well.)

There are people who will argue you should not use PVC. That's too general - you should not use PVC that is not potable water rated. Some is, some is not. Most these days is BUT DWW pipe is NOT - do not use that.

Metals are potential trouble with possible caustics in contact with them. Brass is probably ok, stainless is probably ok. But, I'd prefer a highly-inert plastic.

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Mdemon

Excellent! Many thanks.

marekm

Hi Mdemon,

Unplasticised PVC is also a good option for making all sorts of rebreather parts if your plastic welding technique is not up to scratch. It does not offgas and is very easy to work with by glueing it with PVC cement. The resulting bond is very strong and the solvent in the glue takes about a week to offgas. If you need to reposition or disassemble parts they can be detached with MEK solvent, so it makes it good for prototyping.

Cheers.
-Marek

AD_ward9

If you got a full MSDS on the PVC you suggest, you would be surprised. PVC does not belong in rebreathers. Also, VOCs in solvent glue stay a long time: Helium is very good at flushing toxins out. Note that toxic effects increase linearly with depth (SINTEF data and VOC dive guidelines).

Flexible breathing tubes should be EPDM that is free of plasticers and softeners: most EPDM is not but Crushproof have a nice new mix specially for diving that is just that. If the hoses are fixed, then stainless steel, or use the CL as the routing hose to eliminate the need for that hose.

Alex

marekm

Hi Alex,

Can you please send me a link to that full uPVC MSDS?

Cheers.
-Marek

AD_ward9

The MSDS varies greatly from supplier to supplier. A majority are superficial but delving deeper, the manufacturer should disclose what softeners or flame retardents are used.

As an example of the hazards, we use EPDM. Touted widely as safe. Lots of glowing MSDS available, but digging deeper, we found that the EPDM used for breathing hoses contained Thiram. Once the manufacturer was alerted, he went back and came up with Thiram and softener free EPDM that still performed (albeit at 4x the cost). With PVC there is usually multiple chemicals involved.

You need to identify exactly which uPVC you plan to use, then contact the manufacturer for a full MSDS on every single compound in its manufacture.

Just as a general pointer, there are two distinct problems with PVC, that causes it to be identified as hazardous in dive applications, for example see Page 3 of NOAA Draft CCR Standard HERE. These are:

1. Volatile Organic Compounds involved with the manufacture, as plasticisers, softeners or flame retardants. Often PVC is advertised as plasticiser free but contains the same chemicals as softeners, such as one of four phthalates used as a plasticiser. The adhesives are also potentially toxic: in one reference I checked the adhesives contain naphthalene, formaldehyde, acetaldehyde, phenol, 1-methyl-2-pyrrolindinone or 2-ethyl hexanoic acid. For normal household applications, 10 days and the VOCs are considered to have fallen to a safe level. For diving applications, the material is subject to helium infusing the plastic and drawing out VOCs and the gas has a distinct smell for many months after it has been stored. These can have short term or long term effects.

2. Breakdown, whether from age (10 years +), heat or presence of high PPO2. The effect of high temperatures (or oxidation, which occurs more easily in a high PPO2 environment) is summarised HERE.

Finding plastics that break down safety is a problem. All give off something nasty - often a formaldehyde. We like Kynar because it is pure, breaks down at twice the temperature for a PVC or ABS and the breakdown product dissolves readily in water.

Cheers,

Alex

Mdemon

Can I pop down to B&Q and get a metre of kynar tube then? Needs to be just slightly wider than the internal diameter of the inspo hose so I get a nice snug fit...



It's an eye-opener really. Are all these toxins present at dangerous levels, or is it just that the toxins are nasty and zero risk is prefered?

Duncan Price

I would avoid PVC (I've even not bought houses with uPVC windows). Polyolefins are good as is Delrin (POM). Polypropylene is better than HDPE. I'm not too concerned about thermal degradation as I try to keep my Rebreather cool but generation of nasties from machining is an issue e.g. perspex.

marekm

Having gone through the MSDS's of the specific materials used in the construction of my scrubber, it appears its made of only three chemicals:
1. PVC (biochemically inert polymer)
2. VCM (toxic, carcinogenic, continually forming)
3. MEK (relatively not toxic and offgassing with time)

The only potential concern here is the VCM that is continually forming by breaking off from the chains of PVC. However, having done some calculations, the resulting VCM concentration in the loop is below TCLO for humans at the nitrox depths for which my rebreather was designed. Even though there is exposure to the VCM, the potential liver damage caused by contact with such tiny concentrations during diving are insignificant compared to the oxgen exposure involved at the same time.

-Marek