RE: [PSUBS-MAILIST] Snoopy life support test

Hi everyone,

OK, let's try to answer some of all these questions. First, my daughter is twelve but she's extremely tall for a twelve year old, being already the same height as her mother. She would still be on the light side for an adult, but she's definitely more than half an adult. If I were diving with a crewmember of about my size, my guess (and that is all it is) initial O2 setting would be 1.25 lpm instead of the 1 lpm used in the test with my daughter.

I agree the initial concentrations of CO2 are high, and I think they are due to the time taken up by getting into position and dogging down the hatch. I was working in the sub up to about ten minutes before the first test, so there was probably some lingering CO2 from my earlier presence too. The getting-ready process was probably about 4 minutes, the last minute or so being with the hatch closed. Just to make sure the instrument is not reading high, I tested it outside and it reads 0.02% CO2 and 20.2% O2.

In terms of the permissible concentrations of CO2, the air at the end of the unassisted dive did not yet feel at all stale. We were at 1.6% CO2. But according to Phil's life support white paper, the Navy's standard for manned submersibles is to keep CO2 under 2.5%, while NASA's standard is 1% for indefinite exposure and 3% for up to one hour. I could not tell any difference in air quality between the three dives, other than by looking at the instruments.

Yes, my O2 meter did track the Sub Aspida readings, which is reassuring for when I ship the latter back to Jon! Problem is, mine has one percent discrimination while the Sub Aspida goes to a hundredth of a percent. On the unassisted dive mine went down to 19% and then 18%, at which point its lower alarm went off, but on the two life support dives mine didn't have the precision to show anything meaningful at all, and was at 20% the whole time. Its functional enough to tell you if you're in trouble, with a backlit display and low-high alarms, but useless for a test of any accuracy. By the way, with something as accurate as the Sub Aspida you have to be a bit careful in the measurements. If you aim a breath in its direction, the reading shoots way up. If you raise it into the dome, it shoots way up. Just walking from outside into my home, the CO2 increases by 300%.

Regarding the radial fan current versus battery capacity, I've a new bit of information. Yesterday I noticed that the fan's spec sheet lists the current draw as 0.22 amps, while the fan has 0.36 amps printed on it. Given the conflicting figures, I put down the higher one. But Cliff's email prompted me to go out and measure the current draw to settle the matter. It turns out the fan pulls only 0.2 amps with the scrubber full, so I guess the spec was right and the motor sticker wrong. Another detail here is that Snoopy, being a little K250, is a 12 volt boat with a single battery bank. The original K250 design has three batteries wired in parallel. In the recent redesign I was able to expand that with a fourth battery for a total of 316 amp hours, but I still have the original single-bank configuration. Of course, without a separate hotel battery, my reserve for the fan will depend on the battery charge condition.

If anyone wants the specifics, the radial fan is a Delta Electronics model BFB0712H. Here are some photos attached of scrubber and fan.

Final point... I was not surprised by the fan results. What really did surprise me was the stability of the cabin pressure, in particular in view of the precision to which the Sub Aspida was measuring it (1 millibar), and how easy it was to use a system with no feedback loop regulating the flow. I wonder if that is typical or lucky?

Thanks,

Alec

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From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jon Wallace
Sent: Sunday, August 07, 2011 4:25 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Snoopy life support test

The CO2 readings are definitely high, especially in the test without life support. I know that when I used the monitor out in the open the CO2 percentage was between .01 and .03 so I don't expect a calibration error is the cause of the high readings unless something happened to the device during shipment. That should be easy enough for Alec to test however by simply going to an open area and checking the CO2 reading.

Because the oxygen levels are also low it sounds feasible that the initial environment was the result of respiration while getting both people settled in the cabin and starting the test, although it doesn't really explain the drop in CO2 from 1.6 percent at the end of the no-life-support test to the beginning of the radial-fan test.

It looks to me like even the radial fan had a problem keeping up with the CO2 levels. It rises from .2 to .3 percent in the first four minutes and then goes back down to .2 percent at 15 minutes and then starts rising again. Given the CO2 data from both fan tests it might appear that the scrubber is not efficient enough to handle two people in the cabin. Perhaps another scrubber is necessary or a larger one, or there is something wrong with the airflow over the sofnolime (packed too hard, obstruction, or fan is not powerful enough). I would have expected the scrubber to handle not only the respiration of the occupants but also reduce the CO2 concentration of the existing atmosphere from the time the test started.

I find the no-life-support CO2 levels very interesting given how quickly the levels accumulate. At about 43 minutes the CO2 level would have been at 3% or the maximum short term exposure allowed by OSHA. Simultaneously, the O2 level would have hovered around 16%, borderline survivable and certainly requiring immediate surfacing. What it really illustrates is that getting into a situation where you cannot surface at the end of your planned dive, is really going to ruin your day. Without life support, there's no room for error, accident, or circumstance.

Jon

On 8/7/2011 2:37 AM, Cliff Redus wrote:

Alec, thanks for posting the life support test for Snoopy. I quite enjoyed seeing actual test data. Your results of axial vs. radial fans mirrored my results and my conclusions were the same; i.e., radial fans are the way to go. Have you done any test or calcs to see how long a 0.36 amp current load will take to burn your backup batteries? My guess is that at this current, you would not make 72 hours. IF they don you may have to find a radial fan that draws less current.

I note from your test data that your CO2 readings look high. Atmosphric air has about 300-400 ppm of CO2. As there are 10,000 ppm per 1%, this translates to 0.03% to 0.04% . At the point you initiated each test, you were seeing 0.385, 0.25 and 0.27% for the base case, radial fan and axial fans, respectively or in terms of ppm, 3800, 2000 and 2700. This could be becuase your CO2 sensor is out of calibration or there was a lot of exhalation of CO2 in the boat prior to the start of each test. When I did my in the garage life support test, I found that it was necessary to use my air compressor with a nozzle to purge the cabin to get the CO2 level back to normal air for the start of each test. If the CO2 sensor readings are correct, then with the axial fan, you reached the ABS maximum allowed CO2 level of 5000 ppm (0.5%) reading at 4 minutes. If the CO2 sensor had a 1600-1800 ppm bias error, then the axial fan would have also kept the cabin below the 5000 ppm for the duration of the test.

Thanks again for posting.

Cheers Cliff

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