Dive tables for accelerated decompression

[Storker]

British police search and rescue divers spend extended periods at over 40m using just air, with just a single 12L of air on their back in case the umbilical supply fails. This fact seems to contradict the requirement for helium in the mix or having a large amount of back up gas for decompression.

The ignorance is strong in this one.

If the umbilical fails, that diver will most probably switch to their bailout and then start their ascent immediately. Now, even owing a noticeable amount of deco, it's unlikely that they'll need to spend a significant amount of time at extended depth. How long would a full 12L (I'm assuming 232 bar here) last for ascent and deco? Hint: for quite some time. Also, since they're diving with an umbilical and a surface tender, it's unlikely that they have to include their non-existent buddy in their gas planning. Also, since these blokes are rather well trained and likely spend more time under water during a month than you or I do in a couple of years, they'll probably use less than the 2x gas consumption normal sports divers ought to reckon with if something untoward happens.

I don't know the standard ascent speed for British tables, but I wouldn't be surprised if it's rather close to the 10 m/min they use in our tables. So even if they don't have He in their bailout tank, they'll probably be at depths where narcosis isn't particularly noticeable in a rather short time.

How much experience do you have with diving deep-ish on air or lean nitrox? I know that people are very different when it comes to how much they notice narcosis, but based on personal experience I'm not very comfortable with going much deeper that some 30-ish meters on air or nitrox. Both because of narcosis and because the gas density is getting uncomfortably close to what is now considered problematic somewhere between 30 and 40m. Gas density affects CO2 ventilation, and CO2 is considered extremely narcotic in addition to its other rather unsavory properties.

I have a strong suspicion that I won't get through to you either, so I'm writing this more for the benefit of others who might stumble over this thread.

 

[rjack321]

The ignorance is strong in this one.

If the umbilical fails, that diver will most probably switch to their bailout and then start their ascent immediately. Now, even owing a noticeable amount of deco, it's unlikely that they'll need to spend a significant amount of time at extended depth. How long would a full 12L (I'm assuming 232 bar here) last for ascent and deco? Hint: for quite some time. Also, since they're diving with an umbilical and a surface tender, it's unlikely that they have to include their non-existent buddy in their gas planning. Also, since these blokes are rather well trained and likely spend more time under water during a month than you or I do in a couple of years, they'll probably use less than the 2x gas consumption normal sports divers ought to reckon with if something untoward happens.

I don't know the standard ascent speed for British tables, but I wouldn't be surprised if it's rather close to the 10 m/min they use in our tables. So even if they don't have He in their bailout tank, they'll probably be at depths where narcosis isn't particularly noticeable in a rather short time.

How much experience do you have with diving deep-ish on air or lean nitrox? I know that people are very different when it comes to how much they notice narcosis, but based on personal experience I'm not very comfortable with going much deeper that some 30-ish meters on air or nitrox. Both because of narcosis and because the gas density is getting uncomfortably close to what is now considered problematic somewhere between 30 and 40m. Gas density affects CO2 ventilation, and CO2 is considered extremely narcotic in addition to its other rather unsavory properties.

I have a strong suspicion that I won't get through to you either, so I'm writing this more for the benefit of others who might stumble over this thread.

At least on this side of the pond...

police and firefighters do not dive deeper than 40m
they do not use helium, at least in this state they don't use nitrox either
they are almost always tended with a line and FFM communications
they rarely if ever use surface supplied gas
they usually but not always use pony bottles
they never plan to get into deco (their dives are short when its deep water)

I would be shocked if H&S in the UK allowed police or firefighters to do any deco at all. And at 45m on air you would very quickly have a deco obligation.

 

[Bob DBF]

I would be shocked if H&S in the UK allowed police or firefighters to do any deco at all. And at 45m on air you would very quickly have a deco obligation.

It depends on the mission of the team. The guys around here are usually shallow, mostly S&R in the local rivers, sloughs, and lakes. When one forensic recovery was below 70' and would take forever or need deco to complete, they turned it over to one of the FBI rebreather teams that do recovery and forensics when locals do not have the expertise.

I am sure the NYC has a team that has much more capabilities than our guys. It's a matter of what, and how often a skill is needed, whether a community can afford to keep men with that skill set on the payroll. Quite a while back, several local counties had to make a regional team to justify having one available.


Bob

 

[rjack321]

It depends on the mission of the team. The guys around here are usually shallow, mostly S&R in the local rivers, sloughs, and lakes. When one forensic recovery was below 70' and would take forever or need deco to complete, they turned it over to one of the FBI rebreather teams that do recovery and forensics when locals do not have the expertise.

I am sure the NYC has a team that has much more capabilities than our guys. It's a matter of what, and how often a skill is needed, whether a community can afford to keep men with that skill set on the payroll. Quite a while back, several local counties had to make a regional team to justify having one available.


Bob

In Washington State our "OSHA" prohibits deco unless you have a chamber on-site.
So no public safety divers do any deco. They have used ROVs for deeper recoveries mostly. Not sure about calling in a team not subject to our health and safety regs.

 

[Bob DBF]

In Washington State our "OSHA" prohibits deco unless you have a chamber on-site.
So no public safety divers do any deco. They have used ROVs for deeper recoveries mostly. Not sure about calling in a team not subject to our health and safety regs.

I imagine CA has the same rules down here. The local dive team did bring in an ROV from an outside department and made a survey. I think they brought in the FBI because of their expertise, since they thought, and it was, remains from a ten year old case and they were looking for a cause of death, not just a recovery.


Bob

 

[happy-diver]

More generally if people only did what they had learned from others we would still be living in the stone age or possibly not that advanced.

I watched a couple of dudes tapping away at some skinny trees looking to make a didjeridoo

I wonder how many trees Stradivarius went tapping in order to find good stock for his violins

 

[Graeme Fraser]

At least on this side of the pond...

police and firefighters do not dive deeper than 40m
they do not use helium, at least in this state they don't use nitrox either
they are almost always tended with a line and FFM communications
they rarely if ever use surface supplied gas
they usually but not always use pony bottles
they never plan to get into deco (their dives are short when its deep water)

I would be shocked if H&S in the UK allowed police or firefighters to do any deco at all. And at 45m on air you would very quickly have a deco obligation.

The system you describe in the US is very similar to police diver training in the UK. Although trained in air surface supply for very short dives to 40m, practical application is usually much shallower depths on Comms and FFM with independent backgas, very similar to HSE commercial scuba part 4. ROV's are the norm for deeper / longer stuff.

 

[Storker]

At least on this side of the pond...

police and firefighters do not dive deeper than 40m
they do not use helium, at least in this state they don't use nitrox either
they are almost always tended with a line and FFM communications
they rarely if ever use surface supplied gas
they usually but not always use pony bottles
they never plan to get into deco (their dives are short when its deep water)

I would be shocked if H&S in the UK allowed police or firefighters to do any deco at all. And at 45m on air you would very quickly have a deco obligation.

The system you describe in the US is very similar to police diver training in the UK. Although trained in air surface supply for very short dives to 40m, practical application is usually much shallower depths on Comms and FFM with independent backgas, very similar to HSE commercial scuba part 4. ROV's are the norm for deeper / longer stuff.

Of course, thank you for pointing that out. I completely overlooked H&S regulations and limits and only thought about the technical aspects. On afterthought, I'm pretty certain it's about like this also up here.

So even disregarding the technical aspects I pointed out, the comparison that 60plus made is totally irrelevant.

 

[lermontov]

Im all for innovation - im also one who will resist the status quo - I don't like blindly following without questioning the way things are. However Ive spent the money and done the training.
I have sense your challenging the way things can be done by taking a bit of this and a bit of that -copying what more experienced divers are doing mixing it all up and applying it to your diving.
There are a lot of experienced divers on this forum all trying to caution you -do a tech course and get a solid foundation before deviating from the norm ( please)

 

[TrimixToo]

At least on this side of the pond...

police and firefighters do not dive deeper than 40m
they do not use helium, at least in this state they don't use nitrox either
they are almost always tended with a line and FFM communications
they rarely if ever use surface supplied gas
they usually but not always use pony bottles
they never plan to get into deco (their dives are short when its deep water)

I would be shocked if H&S in the UK allowed police or firefighters to do any deco at all. And at 45m on air you would very quickly have a deco obligation.

I learned about a year ago, to my surprise, that the NYSP dive team is Trimix-capable, though the local and county teams are not. As I understand it there's only one contracted supplier; but, as one of the team pointed out to me, it does not take long to get from mid-state to anywhere else by helicopter.