K14 Elevated CO2

[Russjstewart]

This is a bit tricky to answer without knowing all the parameters, but here are a few things that might help.

• Ambient carbon dioxide (CO₂) levels currently hover around 430 ppm. In large industrial cities, however, concentrations can easily climb to 1,000 ppm or higher. That’s already double the limit for compressed breathing air, which is capped at 500 ppm under standards like EN12021:2014.
• A compressor can also introduce carbon monoxide (CO) into the system, especially oil-lubricated models like the K14. This typically happens when some of the lubricating oil begins to "burn" due to excessive friction, often caused by worn or damaged parts. Depending on your filter cartridge, it may contain Hopcalite, a catalyst that converts the toxic CO into CO₂. While this reduces CO, it can further elevate CO₂ levels in the breathing air.

Even with everything working properly, your location alone might be contributing to high CO₂ levels. I suspect that in the future, dive shops everywhere will need equipment like the Bauer Aeroguard to stay within spec. Alternatively, the standards may be revised. But either way, it's a growing issue that needs addressing.

The fact that you failed the moisture test is also not nothing. The current moisture limit is around 35mg/m³ for pressures above 200 bar, which is usually quite achievable. Your filter has only 10 hours on it, but keep in mind that most of the water is removed before it even reaches the filter. It's possible there’s a minor issue with one of the cyclone separators, or perhaps the PMV isn’t functioning as it should. I’d recommend removing the valve and taking a peak inside your cylinders.

A dumb question here but is it feasible to bypass the filters to capture an air sample to be tested for CO before the air goes through the hopcalite filters, I assume the pressure would be an issue here. Alternatively can you get a HP CO monitor to be inline before the filters?

 

[nadwidny]

Hey, I have a Bauer K14, ex dive shop compressor built in 1995 has a little under 5k hrs. I have had it 13 years, and do about 15-20hrs a year, not much use, mostly filling cylinders to run a thirsty booster, topping off bailouts and dil for my rebreather, a few air fills now that my daughter is diving.

With my daughter now diving thought I would get the air tested for peace of mind. First test came back failed due to high moisture, unexpected, they suggested I submit another sample, emptying the tank slowly and filling opening the condensate dumps more often. Second test failed, moisture was fine this time failed CO2, suggested I submit another sample this time run the compressor for 10min before filling tank for sample. Third test passed though moisture and carbon dioxide content were close to the maximum acceptable limits.

Any suggestions as to what might be going on here? The filter cartridge has around 10hrs on it.

How do compressors concentrate CO2? curious to know what is going on here. The compressor is kept in open garage with air intake outside, so not concentrating CO2 from elevated level in the environment. The Bauer service teck at my LCD suggested that compressors can periodically dump CO2, suggesting running the compressor for 10min before filling and fill into a bank so any CO2 dump is diluted.

What were the values on your samples? That would help to determine how bad your numbers were. I also run a K14 of similar age and annual hours as you.

When I worked at a lab that did air testing, high CO2 (above 500ppm) was always due to having the intake inside a building. If your intake is outside check and make sure you don't have any leaks in the intake line. Our method for sample collecting also mandated a 10-15 minute run before collecting the sample, especially on a new filter. I suppose it depends on the filter but our customers (mostly fire halls running a similar size compressor to a K14) would typically get CO2 and water values of 5ppm or less. Apparently new filters also absorb CO2 for an hour or two.

What is more important is what your CO reading was.

Moisture problems (ugh, so difficult) could be either an old filter or the cylinder with the sample wasn't purged properly and still had residual moisture inside. Apparently the molecular structure of the cylinders can trap moisture which is why we told our customers to make sure they weren't drawing any air from their cascades. Or it could be a leak somewhere in the system. Or, or , or.....Moisture was always a bear to track down the cause but we could usually get it solved after a retest or two. Being in Tassie I wouldn't worry about moisture much, depending on how bad it was. Moisture is a problem when freezing temperatures are encountered.


Just got home from diving a week with Eaglehawk. You guys have some nice diving down there. I wish I had brought my rebreather because it got more spectacular below 25m.

 

[Tanks A Lot]

[...]Apparently new filters also absorb CO2 for an hour or two.[...]

That's a great point, I completely forgot to address that in my earlier response!

Certain molecular sieves are indeed capable of absorbing CO₂, but there’s a catch: they only do so when they’re nearly perfectly dry. During the first few hours of a filter’s operation, it absorbs trace amounts of water and oil that slip past the cyclone separators. The rest of the sieve, bored, so to speak, with nothing better to do, happily captures CO₂.

However, as the filter continues to run, it accumulates more and more water and oil. This mixture gradually displaces the CO₂ that was previously adsorbed by the "bored" sieve.

As a result, at the beginning of a filter’s life, CO₂ levels in the output gas will be well below ambient. But toward the end, those levels will inevitably rise above ambient, as the filter starts to release some of the CO₂ it had earlier retained.

 

[Tassi Devil Diver]

Thanks everyone for your contribution some terrific and very helpful info.

Re compressor location, cool temperate climate, in a large spacious shed with air intake outside. The shed is in a low density rural residential setting, so I don't think environmental CO2 is the issue.

Re moisture, I semi regularly knock the valves off the air tanks I have and look inside, never seen any moisture issues. Today I took the valve off the tank I have dedicated to driving my booster, the tank that gets filled most often. I have never checked this tank, looking inside I saw no evidence of moisture issues, no rust etc. I usually drain the condensate once during a tank fill at the halfway mark and at the end of the fill, around every 5 or 10 minutes, I note a sticker on the compressor indicates to drain every 15-30minutes.

To the OP: start the compressor and keep looking at the pressure gauge after the last filter.
If it gradually increases pressure, it's not good.
It should stay near 0 and then suddenly climb to ~130bar once the compressor has been running for a few minutes.

Took note of this today and stays near 0 then suddenly climbs as described after running for a few minutes.

Could someone explain how the PMV should operate. Should the compressor remain pressurised between use? I do note that pressure in the compressor drops over time and have noticed occasional air escaping from the third stage inlet or outlet fitting, seems intermittent.

Can you determine the condition of a filter by change in weight? I do track this, in 12 months after 10hrs of use the filter gained 75g in wt.

What were the values on your samples?

I don't have the values of the original samples that failed, just notified that the test failed and asked to provide another sample, in the final report Moisture: 48.9 , CO2: 595 , CO: 0.6, oil: 0.05.
Great to read you have spent some time diving Eaglehawk Neck, it is a great place to dive with the high cliffs plunging into deep water.

 

[Wookie]

Could someone explain how the PMV should operate. Should the compressor remain pressurised between use? I do note that pressure in the compressor drops over time and have noticed occasional air escaping

The PMV (pressure maintaining Valve) maintains the filter tower at 120ish Bar when the compressor is not running. On a K-14 your final discharge pressure (not 4th stage) should never drop below 100 bar ever. As I posted, nothing destroys a purification filter faster.

Took note of this today and stays near 0 then suddenly climbs as described after running for a few minutes.

If your final pressure is at 0, your PMV is leaking down.

 

[Miyaru]

Could someone explain how the PMV should operate. Should the compressor remain pressurised between use? I do note that pressure in the compressor drops over time and have noticed occasional air escaping from the third stage inlet or outlet fitting, seems intermittent.

The Pressure Maintaining Valve is basically a strong spring that pushes against a seat, closing airflow.
Once the pressure on the other side of that seat reaches ~130bar, the force is strong enough the push the spring back, and the air starts flowing through.

The reason:
Air at ambient pressure can hold a lot of moisture. Increasing the pressure causes the moisture-holding-capacity to drop. By keeping the pressure high, 99% of all the moisture in the (compressed) air will condensate on the internal filter walls and drips down.

That last 1% of humidity is finally removed by the molecular sieve material inside the filter (the white grains). To make your filter last a long time, the pressure inside the filter needs to be maintained.
That is achieved by a one-way-valve (a.k.a. non-return-valve) after the last stage and before the final separator-and-filter,
and by the PMV that is places after the filter and before the fill-whip or bank connection.

The separators between the compressor stages are depressurised/opened when the compressor stops, and that's when all the water drips out. The final filter always stays pressurised, and is only manually depressurised when changing filters.

If the filter (or molecular sieve) is in contact with air at ambient pressure, it will continuously collect moisture from the air and reach saturation within days. From that moment on, the filter is useless. Depressurising the filter housing has the same effect.

There is another component in the filter housing: active carbon. It's job is to take out anything toxic (bad smells). But it can only do the job if it stays dry. Any humidity will render the carbon inactive.

What the active carbon does not do: take out carbon-monoxide. If you're running the compressor and your neighbors are having a bbq within 100m, the CO level in the compressed air already rises above 1ppm. To take out the CO, another filter housing with hopcalite can be used. Hopcalite is rendered inactive by the slightest humidity, so if you have a hopcalite filter, keep that one continuously pressurised as well. Hopcalite is a catalyst (meaning that the hopcalite itself doesn't chemically change) that turns CO into CO2 and when it does, the reaction process produces heat. A rise in filterhousing temperature is an indication that the compressor is taking in elevated levels of CO.

 

[CG43]

That last 1% of humidity is finally removed by the molecular sieve material inside the filter (the white grains). To make your filter last a long time, the pressure inside the filter needs to be maintained.
That is achieved by a one-way-valve (a.k.a. non-return-valve) after the last stage and before the final separator-and-filter,
and by the PMV that is places after the filter and before the fill-whip or bank connection.

The separators between the compressor stages are depressurised/opened when the compressor stops, and that's when all the water drips out. The final filter always stays pressurised, and is only manually depressurised when changing filters.

If the filter (or molecular sieve) is in contact with air at ambient pressure, it will continuously collect moisture from the air and reach saturation within days. From that moment on, the filter is useless. Depressurising the filter housing has the same effect.

"To make your filter last a long time, the pressure inside the filter needs to be maintained."

Ok, as long as the compressor is running tank-next tank... that is correct.
But does this also apply at rest for a few days ?
I have doubts about that !

When the compressor stops, the air temperature is approximately 50° C, the pressure 220 bar, the relative humidity after the final seperator is 100%.
The water vapor pressure at saturation is exclusively a function of the temperature here 50° C .
When the temperature drops due to rest , water condenses out and the relative humidity remains at 100%. This is not very good for storage .

If, on the other hand, if we release the pressure from the filter, what happens ?
The watervapor pressure decreases with the filter pressure ( it is a partial pressure of the total pressure and the concentration does not change due to the release ) .
However, the saturation water vapor pressure depends exclusively on the air temperature.
So the relative humidity decreases with the decreasing partial watervapor pressure.
And that is exactly what we want, the filters full of very dry air.
Of course we then close the valves so that no new air from ambient gets into the filters.
That's the same you do when you want to store tanks.
Fill and then drain down to residual pressure.