Hypothetical air compressor tampering

[Diver-writer]

I need an air compressor expert with a creative and devious mind! I'm writing a mystery novel and am trying to figure out a way that someone could tamper with an on-board scuba air compressor so that the tanks are contaminated with carbon monoxide. This is the scenario:

It is a live-aboard dive boat that is out on a multi-day trip. The air compressor was sitting out on deck at a busy marina before the trip, and the boat was unattended for long stretches of time, so it's conceivable that someone dressed as a mechanic would be able to slip aboard and have time to do some fairly involved modifying of the compressor without being noticed. What I'm looking for is a way that the compressor could be sabotaged so that it would either produce or suck in carbon monoxide into the tanks that would be filled during the trip. But I don't want it to be permanent damage, after the divers have gone down with the contaminated tanks (with nearly disastrous results, naturally) and the sabotage is discovered, I want the on-board mechanic to be able to identify the problem and be able to fix it so they can then fill new tanks with good air and continue their dive the next day (once everyone had recovered from their relatively mild cases of CO poisoning).

I did a little research on air compressors and it looks like something in the range of a Max-Air 90 gas air compressor would be a good fit for their needs, but in all honesty it's a pretty battered, cut-rate outfit, so it would be more feasible that they would have some ratty old air compressor, but I don't know what type that would be. I'm open to suggestions!

If anyone can solve this for me, I'll gladly give you a mention in the Acknowledgements section when I get this published someday. Thanks!!

 

[dannobee]

Either loose (or otherwise leaky) exhaust hoses or exhaust manifolds on the gasoline powered generator or main engine would do the trick. And the inlet for the air compressor would need to be near the exhaust leak(s). The gasoline engine would need to be running at the same time the tanks are being filled.

The exhaust leaks could be readily fixed by the mechanic on the boat.

 

[do it easy]

The saboteur could remove the hopcalite and charcoal from the filter stacks and that *might* let CO build up if the compressor wasn't running efficiently. After the sabotage is discovered, the filter stack could be replaced and everything will be normal.

 

[spectrum]

Letting the hot muffler burn through the intake hose always works in the movies. First you get the vapors of burning plastic hose followed by sucking in the carbon monoxide.

 

[ScubaKris]

Keep it simple, use a simple compressor.

This same plot ws used about 48 years ago in a Sea Hunt Episode where the bad guy wanted to get CO into Mike' s tank, the wrong guy got it and Mike saved the day. The compressors of the day are much like the one I use, homemade.

It is a high pressure comrpessor, with a filter to filter out oil vapor and water. I have a 5' hose put up wind or above the compressor so not to get CO. I have a small filter on it to keep out dirt and bugs.

The compressor in the Sea Hunt was the same. The bad guy just placed the intake hose near the engine exhaust while filling Mikes tank, then put it back wher it belonged.

 

[james croft]

C'mon Divewriter, who are you really trying to whack? We promise not to tell...

 

[DA Aquamaster]

I had a US military surplus Stewart Warner "Flamethrower" 3.5 cu ft compressor on my boat. It was designed as a portable compressor for the military and was intended to be used to fill compressed air flamethrower tanks in the field. The frame that supports the compressor and motor did double duty as a pack frame in military use although that is no longer an option given that I routed the filter stack under the engine and secured it to the upper strap fittings. The flow rate is not great but it is compact and unlike most other portable compressors in that size range, it can be run continuously and has good intercooling so it's a good choice for use on a boat on a budget.

For scuba use, I had to add a filter stack and make an intake manifold for it to allow me to put a remote intake hose on it as otherwise the intake is only a few inches from the exhaust. I used a new 5 hp Honda engine on it rather than the original Continental engine (heavier and wimpyer) and I am not sure if the exhaust was that close on the original Continental.

The short story is that if someone loosened the screw holding the manifold and/or removed the gasket under it, it would suck in carbon monoxide from the exhaust. My compressor also has a carbon monoxide detector on it, but it is a small housing containing a small cardboard disc with a treated strip on it that changes color if CO2 is present. So depending how you write the story it could be replaced by a dummy strip or the hard to see strip could be the clue that their was CO in the air.

It would be quick to tamper with and not easy to detect unless the operator was rigorous in checking it's security before use.

A bit more involved and much harder to detect tamper would be to remove the RTV silicone that I filled the screw holes in the head of the first stage that used to hold the old intake on. When I first modified it, I did a leak check and noticed a serious leak but had fun finding it as it was not at all obvious the four holes went through the head and out between the first and second cooling fins. That would be really hard to detect unless someone did a vaccum check to test for leaks.

 

[Diver-writer]

It's going to be a very annoying petty bureaucrat who makes divers' lives miserable...and yes, he IS fictional!

 

[Diver-writer]

Thanks DA Aquamaster - that's awesome! If it's ok, I'd like to e-mail you with some more questions.

 

[Isaac-1]

If you are looking for something more exotic than the classic exhaust fumes leaking into the air intake, you could in theory have someone switch out the compressor oil with some other oil that would combust and produce CO, this would probably not be noticed without a CO monitor which many/most compressors don't have. Compressor oil is forumlated to avoid just this problem, and that is part of why it is so expensive. How clean the air would be after just changing oil is something I will leave for the experts, as the interior surfaces of the compressor would likely be contaminated with carbon buildup, etc. Regardless of the source of the CO you choose it would have to generate enough CO to either overwhelm the hopcalite in the filter stack or to saturate it over time. If you choose a slow source to saturate the hopcalite then you could have clean air in the first few tanks filled. As to the specific compressor model there is really no such thing as a cheap scuba compressor, the Max-Air model you mentioned is one of the cheaper out there for its size, however there are plenty of old battered ones, some by companies that are out of business with limited or no parts availabilty. Personaly I would be vague about the specific model and consider just calling it an old military surplus compressor, as there are many variants (ranging from off the shelf civilian models, semi customized models, to speical purpose built like the stewart warner flamethrower units that can be converted to scuba use by adding filtration). One thing to keep in mind is the smallest of these portable models take about half an hour to fill a tank, and are not designed for continous use.

Ike