DIY Compressor Filter and Dryer designs

[iain/hsm]

So Iain, this is all great info, especially since you are a well known authority on the Rix - unfortunately its late to the game for me. C'est la vie.

As such, I'd like to hear more about "Don't mix Hopkalite with AC and MS in the same tower, its a cheap trick" since this statement makes you the only outlier on the bell curve so far. Can you further explain your position please? I want to learn, and I feel there's learning to be had here! Thanks Mani

Mani. Never too late really, no problem.

As a cheap trick in a nut shell its about making space in a fixed length cartridge for a very expensive chemical (hopkalite) at the expense of the drying time media molecular sieve.

1. Replacing active water removing chemical MS content in the cartridge reduces filter life. But as its a throw away it make the supplier more money.

2. Removing the cheap carbon part however, would risk oil break through and so is not practiced.

3. Hopkalite if kept dry will last 2 years or 100 hours running not much profit at that rate for the throw away cartridge lads.

4. No real requirement to supply the very best chemical, when an average life is more profitable.

5. In the main the hopkalite thrown away is perfectly good its just the reduce amount of MS Moleclular sieve that expended, and sooner
than normal because 20% of it was removed to make room for the Hopkalite.

6. Your 18 inch filter tower if using a 2.5" diameter cartridge is 10 inches long. By the time you subtract an inch each end for the
cap and plug another inch for the pads and spacers you have very little space left for any active chemical, hence my suggestion
not to bother with anything other than good quality MS

7. Also be aware the quality of each of the chemicals used can vary enormously.

Iain Middlebrook

 

[Wookie]

So, I've been reading this thread with interest, not because I have any intention of building my own filter canisters, but I'm trying to follow the logic.

You built a Rix, which arguably doesn't need filtration at the outlet, it just needs the water removed if you put it in steel tanks. I would remove the water anyway, as everything I make goes in banks, but that's another story. Water can be removed with a surplus chiller, or a number of ways, but you chose to go with a self-packable filter tower.

Rather than buying a used tower and pressure testing (hydroing) it, you chose a different path that I don't understand. I think you bought a tower and a packable cartridge. Now, you will buy chemicals with a definite shelf life to pack your cartridge with. You may or may not use the chemicals before they expire, and you may or may not use them after.

Why didn't you buy a used tower and a 1501 or 1801 type Lawrance Factor cartridge and be done with it? I can understand why I might repack cartridges, after all, I go through 2 a week. I don't because I sell my gas, and I want someone else to pass the liability on to if a filter cartridge channels or otherwise goes south. You are pumping air for yourself, so there is no reason to buy a cartridge except for ease and less waste.

Is it the engineer aspect? You know what I mean....

 

[iain/hsm]

So, I've been reading this thread with interest, not because I have any intention of building my own filter canisters, but I'm trying to follow the logic.

You built a Rix, which arguably doesn't need filtration at the outlet, it just needs the water removed if you put it in steel tanks. I would remove the water anyway, as everything I make goes in banks, but that's another story. Water can be removed with a surplus chiller, or a number of ways, but you chose to go with a self-packable filter tower.

Rather than buying a used tower and pressure testing (hydroing) it, you chose a different path that I don't understand. I think you bought a tower and a packable cartridge. Now, you will buy chemicals with a definite shelf life to pack your cartridge with. You may or may not use the chemicals before they expire, and you may or may not use them after.

Why didn't you buy a used tower and a 1501 or 1801 type Lawrance Factor cartridge and be done with it? I can understand why I might repack cartridges, after all, I go through 2 a week. I don't because I sell my gas, and I want someone else to pass the liability on to if a filter cartridge channels or otherwise goes south. You are pumping air for yourself, so there is no reason to buy a cartridge except for ease and less waste.

Is it the engineer aspect? You know what I mean....

Frank.

A couple of points just by means of explanation, for general compression and specific oil free Rix compression.

Granted you can remove liquid water a number of ways but:

1. Only water condensate ie collected liquid water can be removed by a water separator or mechanical filter, cost free.

2. To "knock out" the water as condensate from a hot HP gas discharge requires cooling and a back pressure or both ideally.
The cooler the gas the more water condensate and conversely the less the filter chemical has to work so the longer the filter life.

3. Water being pretty much incompressible has no where to go but condense out under increase pressure hence the BPR and the oil less system cools this pure water out from vapour into condensate very much quicker than a hot oil and water condensate conventional oil lubricated compressor.

4. Try heating a pan of oil, a pan of water and a pan of air, to see when left which cools quicker

5. However the kicker is the HP gas/air is still a saturated at water vapour and to lower the water vapour you need to lower the pressure dewpoint.

6. So having pressurised up the gas using a BPR and collected the water condensate in the towers a chemical dehydrator filter is used simply to reduce the dewpoint such as molecular sieve.

7. This is in order to ensure no water condenses out when you jump from a say a 30C gas discharge temp into a bottle that due to adiabatic compression increases to say 40C Then you jump into a 6 C sea water temp or indeed inside the storage cylinders when they cool out. Pressure vapour density is the key.

8. As for the filter I agree never buy a used tower of some old scrapper hydraulic unit, hydro is a of little use with the unknown pressure cycling of the shell.

9. With a Rix you are dealing with a compressor that can neither add or subtract from the original purity of the incoming gas/air.
It cannot contaminate the gas, under any circumstance unless the contamination is external to the compressor.
It has no oil so cooling is more efficient,

10. The only question of air quality is now that of the chemical itself, most including those you have described contain an inert binder that from some manufactures is even carcinogenic

11. This inert binder gives the pure white zeolite as molecular sieve its tan or brown colour. Once again in EU we supply only a non bound MS pure white medical grade and non carcinogenic.

12. Consider also than in some scuba filter brands this inert binder can be up to 30% of the chemical weight. Money for nothing, and a further reduction in chemical life.

13. Shelf life of the non bound medical grade chemical is 5 years provided it is stored in UN certified steel drums (even if small amounts in new fresh empty paint tins and the like)

14. Shelf life for a plastic throw away cartridge in a foil bag around 3 months before it begins to decay and around the same for raw chemical in a plastic bottle you see common to sports dive shops.

Iain Middlebrook

 

[maniago]

You built a Rix, which arguably doesn't need filtration at the outlet, it just needs the water removed if you put it in steel tanks. I would remove the water anyway, as everything I make goes in banks, but that's another story. Water can be removed with a surplus chiller, or a number of ways, but you chose to go with a self-packable filter tower.

Rather than buying a used tower and pressure testing (hydroing) it, you chose a different path that I don't understand. I think you bought a tower and a packable cartridge.

Is it the engineer aspect? You know what I mean....

Wookie, I assume youre question is to me since I started this thread, but Iain really answered it correctly (while I've been out at the stores working up a re-purposing solution for my input filter - I'll post that on the rebuild thread).

But to answer the question, yes I bought a new 16" tower from Jim (used I was afraid of and I don't have access to any buddies in the hydro business...as of yet) AND I bought two LF 65240s to get me going (same as 1801s). The repack i also bought is for later adjustments in the chem pack based on more reading&research, which Iain was kind to provide above before I even asked (thanks Iain!).

So I'm confused why youre confused...which is thus making me double check my math...again!

Yes, I am a tinkering engineer as you know, but I do have my limits....

 

[swamp diver]

Activated charcoal, or activated carbon on a Rix is a waste of time and frankly a risk and it reduces performance duration.

If a catalytic converter CO removing chemical is needed its when using a petrol or gas engine drive and for that we
recommend an additional 10" tower of the same 3.75" diameter filled purely with Hopkalite downstream at the
dry end, with a filter change every two years or 1000 hours.

IMHO Don't mix Hopkalite with AC and MS in the same tower, its a cheap trick, it is a waste of money and
adds risk even more so on a conventional oil lubricated compressor.

Iain I hesitated in recommending an AC bed on a Rix however one really needs to look at what is in the proximity of the compressor intake and even what neighbours and roadways are near the intake. Recall that for any VOC in the air the surface effective concentration is multiplied by the absolute pressure at depth.

I have a friend who is one of your friendly tech divers and who purchased a Rix in order to reduce the risk of VOC exposure coming off the oil-lubed compressor. One day he told me a story that changed my opinion on not using an AC bed on a Rix for diving applications. As you suggested he only had a 28 inch column of MS 13x and kept his air bone dry given he often dives when there is snow and ice on the shore. During this one summer he called me up to come and smell his tank air which he thought smelled like tar. The compressor was at his workplace in an industrial strip mall and the intake drew from an outdoor vertical wall at the back of the wood working shop where normally he had no odoriferous neighbours or so he thought .

I arrived the next day and instead of going to the front door I went to the back door where the compressor inlet was located and about 200 metres upwind quite out of view was a roofing company's black tar heater spewing fumes into the local ambient air. When the compressor was started the day before these guys were not there but clearly they arrived and the diver had not noticed their presence during the fill.

We also know that if one's intake is located within a few hundred metres of a freeway or certain industrial plants that VOC concentrations (i.e. BTEX) can be quite high and while not a problem at 1 atmosphere if diving trimix at 12 ATM they could very well become an issue.

Even in the rural areas with farms and manure spreading there are lots of VOCs that you don't want in your tank. And when I hear the word garage I expect the worst as most people's garages are full of VOCs from paints, glues, engine cleaner, etc which can get accidentally sucked into the intake.

So yes it is a bit of a judgement call and if one was very sure that the neighbours were not going to contaminate the intake air I'd could see forgoing the AC bed. A small AC bed is inexpensive and if it is kept dry with a visual humidity intake there is no risk of water displacing the stored VOCs especially if it is just ambient VOCs (oil lubed compressors is different story) being stored between cartridge changes.

There is a way to check one's breathing gas prior to jumping in for that 12 ATM dive and that is to use a portable photoionization detector (PID) as seen below. They are finicky to use and not cheap but it can be done if all you have is a oil-lubed compressor but you want to make sure your breathing gas is VOC-free air. I know the military has experimented with using such devices in the field as well.

As for whether or not to add a Hopcalite bed (the name Hopcalite comes from the inventors who were at John's Hopkins university and University of California) again it really depends on the risk of an unexpected external CO contamination incident. I do think your idea of having a separate CO tower and bed is a good one but clearly adds to the upfront expense. For most it is just easier to pay the extra $5 per cartridge and throw it out with the MS and AC. Not that environmentally friendly for sure.

The story that was told here on Scubaboard by Rjack I believe is a good example of why one might want to have a catalyst bed on a Rix compressor in an urban environment. I can't recall if he did have a catalyst bed or not, but he did have a portable CO detector with 95 dB alarm set to go off if the CO concentration on the discharge was > 10 ppm. One day he was filling his tanks in his garage and the alarm went off. When he went out to investigate either his wife or neighbour was running the petrol-powered lawn mower near the intake.

Again it might be just simpler to forgo the catalyst bed and purchase a ToxiRae 3 or Oxycheq CO monitor and plumb that into the discharge air. They will alarm if you are nearby should there be an unexpected lawn mower nearby.


I'm curious if your BS8476 and the Euro breathing air standard EN12021 both require compressed air > 200 bar to have a maximum moisture content of 25 mg/m3 (-52 C atmospheric dew point) then every Rix you sell must be sold with an filter tower packed with molecular sieve in order to meet that standard over there.

What is odd over here in the US and Canada is that for hospitals now the compressed air used on patients at one atmosphere must be produced using an oil-less or oil-free compressor, but in both countries for the hyperbaric chambers in those same hospitals the compressed air used up to 6 ATA can come from an oil-lubed compressor. I think they have it backwards.

In an ideal diving world we would all be using a Rix oil-less compressor.

 

[swamp diver]

Iain,

I am standing at the compressor intake in this photo with the auto assembly plant in the back ground. The prevailing summer wind is from the plant to the compressor intake. Assuming the intake was to a Rix compressor would you recommend an AC bed? This is an actual dive shop fill station but they do have an AC bed on their oil-lubed compressor and test their compressed air quarterly hopefully when the wind is blowing toward the shop.

 

[Wookie]

Wookie, I assume youre question is to me since I started this thread, but Iain really answered it correctly (while I've been out at the stores working up a re-purposing solution for my input filter - I'll post that on the rebuild thread).

But to answer the question, yes I bought a new 16" tower from Jim (used I was afraid of and I don't have access to any buddies in the hydro business...as of yet) AND I bought two LF 65240s to get me going (same as 1801s). The repack i also bought is for later adjustments in the chem pack based on more reading&research, which Iain was kind to provide above before I even asked (thanks Iain!).

So I'm confused why youre confused...which is thus making me double check my math...again!

Yes, I am a tinkering engineer as you know, but I do have my limits....

I didn't realize you had bought a tower and LF filters. Never mind the question. Sorry for the confusion.

 

[iain/hsm]

Iain, Assuming the intake was to a Rix compressor would you recommend an AC bed?

Looking out at that each day? Heck No. I'd recommend he move, and fast to a warm island, next a beach!!!

---------- Post added February 21st, 2014 at 08:55 AM ----------

Iain I hesitated in recommending an AC bed on a Rix however one really needs to look at what is in the proximity of the compressor.

Agree entirely but our guy has bought a 18" tower, when broken down a cartridge 2-1/2" diameter with 8" of useable media length. Even when using the very best of chemical AC and MS
He has nowhere to go in a 8" stack length, to add one AC he removes the other MS and the possibility of adsorbed nasties breaking through.

changed my opinion on not using an AC bed on a Rix for diving applications. As you suggested he only had a 28 inch column of MS 13x The compressor was at his workplace in an industrial strip mall.

Again with a 27" stack length he can from the bottom up go silica gel SG or activated alum AA then MS, AC, MS, he has the length to make this work.

industrial plants that VOC concentrations (i.e. BTEX) can be quite high and while not a problem at 1 atmosphere if diving trimix at 12 ATM they could very well become an issue. VOCs from paints, glues, engine cleaner, etc which can get accidentally sucked into the intake

There is a way to check one's breathing gas prior to jumping in for that 12 ATM dive and that is to use a portable photoionization detector (PID) as seen below. They are finicky to use and not cheap but it can be done if all you have is a oil-lubed compressor but you want to make sure your breathing gas is VOC-free air. I know the military has experimented with using such devices in the field as well.

For 12 ATA dive on air enjoy the ride, I doubt VOC's would be a problem more voices in your head at that ppN2 The ppO2 is pretty near the mark also 7ATA 200FSW or 60 MSW being about the max

Now for the controversial part. Im against all electronic detectors, Also No to CO detectors, No nothing.

My reasoning is quite simple really one point………customer base.

Now at work I have a whole pile of electronic junk, both battery
and mains, some cost probably about that of a good car.
Paramagnetic 02, hyperbaric gas detection, the works

However the customer base we have on the second hand ex military surplus SA-6 pumps will not, cannot afford most even a simple oxygen sensor, so what you do?

Practical advise and guidance using the tools to hand and the kit they have already, a compressor that even the USN seal teams doesn't use filters with? Its a hard sell to advocate a twin filter BPR and interstage gauges when this will cost more than the pump did.

Yes ideal world that "Ion Science Pro check" is a cool piece of kit but it costs more than the SA-6 did, so were back to the tail wagging the dog and similar to these air test labs were in 10 years of testing
You will have paid for your SA-6

Added to which when compared with a conventional oil lubricated compressor with expensive oil change every 100 hours, lower operating filter life and testing you spend more money just on the throw away stuff than a brand new SA-6 would cost over ten years.

Conversely all these test labs are doing is putting in place a placebo to dive shops to continue in running inadequate poor quality equipment for the price of a piece of paper on the wall.

And to the dive shop $130 shoot even $200 to have paper on the wall is a cheap price to pay even every six months than to buy a competent compressor.

I do think your idea of having a separate CO tower and bed is a good one but clearly adds to the upfront expense. For most it is just easier to pay the extra $5 per cartridge and throw it out with the MS and AC. Not that environmentally friendly for sure.

Crunch the costs of a lab air analysis, at a lab of your choosing say $130 per test and every six months over two years is $520

Now to buy outright that small CO filter bare tower $288 ex works UK. So a tad over two tests and your paid for. Your call but my customer base all self fill, don't have money for fancy electronics but can see the advantage in sound engineering at an affordable price. For a piece of kit to do a job each day every fill, each fill. Than a piece of paper on the wall of some test done six months ago.

Further all are repacking and would not pay the extra for hopcalite in the throw away cartridge. I was told by one that throw away cartridges is akin to computer printer cartridges market, poorer quality and a higher price.

Interesting point the 33 inch filter on a Rix SA-6 will run over 250 hours using this EU non bound non carcinogen MS cost around $24 to refill. If using cheaper China imported MS about half the cost and half the duration. Even that causes a split in customer support

I'm curious if your BS8476 and the Euro breathing air standard EN12021 both require compressed air > 200 bar to have a maximum moisture content of 25 mg/m3 (-52 C atmospheric dew point) then every Rix you sell must be sold with an filter tower packed with molecular sieve in order to meet that standard over there.

Yes quite true, a filter is added for breathing air sales for BS 12021 and BSEN 8476 applications.

The Rix passes the old BS4001 standard for breathing air without any filtration.

In an ideal diving world we would all be using a Rix oil-less compressor.

In my ideal world we would not use compressed air at all, but mix it with a mix maker using pure oxygen and nitrogen, 60/40 or 40/60 and 32.5% depending on depth. No need for filters or filter chemicals but then we would need soda lime…..then another wants it with an O2 analyser Grrrr Iain Middlebrook.

 

[KD8NPB]

8. As for the filter I agree never buy a used tower of some old scrapper hydraulic unit, hydro is a of little use with the unknown pressure cycling of the shell.

Funny story about that, the shop has actually gone through about 4 surplus Bendix 737 hydraulic accumulators. None of them experienced a casing failure.

HOWEVER.

The cast metal ends that the threads are cut into? We've split 3 of them, and retired the 4th one before it had a chance to happen.

Luckily, the compressor is in a shipping container in the back lot, so it's safely away from the shop in case anything decides to go boom.

 

[swamp diver]

Looking out at that each day? Heck No. I'd recommend he move, and fast to a warm island, next a beach!!!

I showed you the better side of that fill station's neighbours! To the north about 250 metres away is an 8 lane highway. You can be sure the triplex cartridges are working overtime.

---------- Post added February 21st, 2014 at 08:55 AM ----------

Agree entirely but our guy has bought a 18" tower, when broken down a cartridge 2-1/2" diameter with 8" of useable media length. Even when using the very best of chemical AC and MS

He has nowhere to go in a 8" stack length, to add one AC he removes the other MS and the possibility of adsorbed nasties breaking through.
Again with a 27" stack length he can from the bottom up go silica gel SG or activated alum AA then MS, AC, MS, he has the length to make this work.

While I don't have a cartridge in front of me I believe you have underestimated the length of the cartridge in an 18 inch tower. The cartridge should have about 10" of length for chemical media and 2" of pads. I'd forgo the catalyst and still put in 8" of MS and 2" of high quality coconut-shell derived AC. As long as one has a humidity disc with a 10/20/30 RH indicator there will not be any roll-up of water displacing the VOCs. If you look at the studies on VOC roll-up due to rising RH in an AC bed most of the well adsorbed compounds don't get displaced until the RH in the bed hits 30% to 40%. It is actually heat due to poorly installed compressors that is the real problem for desorbing the stored VOCs. Keep the compressor well ventilated and have a visual humidity indicator and there will not be a problem. I think some of your RIX marketing bias has crept into this discussion!

For 12 ATA dive on air enjoy the ride, I doubt VOC's would be a problem more voices in your head at that ppN2 The ppO2 is pretty near the mark also 7ATA 200FSW or 60 MSW being about the max

Sorry that was my bad and initially referred to breathing gas (trimix) but when we were doing some pilot studies with the Ionscience Phocheck and Rae Systems PID we were using air tanks with grab samples from various local fill stations. Believe you me you want to have an AC bed in your compressor during the hot summer months especially on the oil-lubed pumps. If you can't afford a Phochek you should probably bring along your teenaged child to sniff for VOCs as their detection unit, cranial nerve #1, is far more sensitive than the olfactory system in us old farts.

Now for the controversial part. Im against all electronic detectors, Also No to CO detectors, No nothing.

My reasoning is quite simple really one point………customer base.

Now at work I have a whole pile of electronic junk, both battery
and mains, some cost probably about that of a good car.
Paramagnetic 02, hyperbaric gas detection, the works

However the customer base we have on the second hand ex military surplus SA-6 pumps will not, cannot afford most even a simple oxygen sensor, so what you do?

Practical advise and guidance using the tools to hand and the kit they have already, a compressor that even the USN seal teams doesn't use filters with? Its a hard sell to advocate a twin filter BPR and interstage gauges when this will cost more than the pump did.

You overestimate the cost of the available technology on this side of the pond. One can purchase a very fast Rae System ToxiRae 3 to monitor CO contamination with a minimum detection limit of 1 ppm and resolution of 1 ppm for only 124 British pounds which will monitor the compressor output in realtime for at least 3 years. The battery is user replaceable and one just throws the unit out when the sensor is gone, although I am told in SE Asia a sensor replacement is available. One can set the alarms at whatever the local standard requires and at 95 dB the alarm can be heard over the compressor. There are many many 'cheap' but safety orientated divers out there using this technology which is probably a better system for avoiding CO contamination than just having a catalyst bed in the purifier which often does not work as intended if the bed is cold or wet.
ToxiRAE 3 - Single Toxic Gas Monitor for Carbon Monoxide and Hydrogen Sulfide - RAE Systems

For those that cannot afford the ToxiRae 3 there is a cheap US made version now on the market that we have tested and its performance is right up there with the high end units plus the battery and sensor are user replaceable. The only real negative with this unit is the alarm is barely audible so it is better for checking tanks after they have been filled by the noisy compressor. Cost is only 81 British Pounds.
Carbon Monoxide Inspector - Portable CO Meter & Tester, Made in USA

We've tested the BW Tech, Analox, Sensorcon, and Rae units for CO detection in diving and all work well but the Sensorcon unit is a game changer for the sport diving market given the price point.

Added to which when compared with a conventional oil lubricated compressor with expensive oil change every 100 hours, lower operating filter life and testing you spend more money just on the throw away stuff than a brand new SA-6 would cost over ten years.

Ok now this is some real good Rix marketing bias at this point. The recommended oil change interval on a Bauer compressor running an ester synthetic is 2000 hours. The typical Bauer K14 oil-lubed compressor requires 3 litres of oil which costs about $US45 for the Chemlube 800. Across the pond if you don't purchase your synthetic Shell trimellitate ester oil from Bauer it can be had for about the same price. Most dive shops would be lucky to do a 1000 hours a year so that would be about $50 a year in oil. Now on our nitrox block we change our oil out every 250 hours so about $100 a year in oil. I'm not sure which manufacturer is recommending 100 hour oil changes. We even send our oil off to the lab once a year to ensure it is within spec on change out. Cost is $50 to run ICP metals, total acid number, and viscosity at 40 C and 100 C.

Conversely all these test labs are doing is putting in place a placebo to dive shops to continue in running inadequate poor quality equipment for the price of a piece of paper on the wall.

And to the dive shop $130 shoot even $200 to have paper on the wall is a cheap price to pay even every six months than to buy a competent compressor.

Crunch the costs of a lab air analysis, at a lab of your choosing say $130 per test and every six months over two years is $520

Pretty much every dive shop if they have been in business long enough will have had an air test fail for one reason or another. While not as good as real time monitoring these regular lab-based tests do pick up problems.

I know a large commercial operation that sent their compressor out for service and decided to run a air quality test afterwards. The test revealed 10 ppm of trichloroethylene (TCE) which would have killed the diver at the depths they were working at. The maximum allowable concentration is 1 ppm for air diving up to 6 ATA and at that low concentration the human nose cannot detect the contaminant. The compressor service technician forgot to rinse a few of the stage head valves out after cleaning them in brake cleaner.

We had a moisture failure once due to some new cascade bottles that had just come from the factory and still had atmospheric moisture bound to the inner steel walls. A few flushes with dry air fixed that problem but had it been winter time we might have seen an increase in regulator free flows.

Biannual or quarterly lab testing keeps the compressor operators alert and educated to potential contamination problems.

In my ideal world we would not use compressed air at all, but mix it with a mix maker using pure oxygen and nitrogen, 60/40 or 40/60 and 32.5% depending on depth. No need for filters or filter chemicals but then we would need soda lime…..then another wants it with an O2 analyser Grrrr Iain Middlebrook.

That has been tried over here but there have been too many accidents and close calls. I think it was in Texas where the commercial gas supplier was asked to send over a few K bottles of synthetic air for the hazmat team. Unfortunately the guys did not analyze the air prior to use and a few guys died during a confined space contamination incident due to a hypoxic mix. At least with compressed ambient air you know that the oxygen content is 21% which is why for supplied air respirator use synthetic air has been banned in both Canada and the USA. Only compressed air will do.

I know you are referring to mixed gas diving but the cost of pre-mixing would be too high. Many of the large commercial ops are now using membrane systems out on the boats. One can just dial up the % oxygen depending on depth. Put some sodalime on the hp compressor intake and one can keep the CO2 concentration down to sub 200 ppm levels.
http://www.experts.com/content/articles/Barsky_Nitrox_Article_ADCI-UW-2.pdf

The US Navy still does quarterly testing on all its shipboard and land compressors (about 3000 samples a year) but at some point will move to real-time testing for CO, oxygen, CO2, VOCs, and moisture. Analox and your UK firm Geotech both have units out which can monitor those analytes in real time. The Analox unit looks promising. It is time though for the UK to move from those antiquated Factair tube testers and start to use some of this modern technology now available at a reasonable cost.

Peter