Re: [PSUBS-MAILIST] Humidity control

Jim

As an example, attached is a plot of one one of my life support test in my shop. The data was logged manually over a 8 hour test. For this test, I was evaluating my manual O2 makeup backup system and not the automatic system. On the left y axis, I plot the ppm of CO2 and the cabin pressure in mbar. On the right y axis, I plot the RH, cabin temperature, O2 concentration and the O2 flow rate (0.5 lpm when on). For this test , the RH at the time the hatch was closed was 60%. After about an hour and a half, it stabilized at 90% and stayed at this level for the balance of the 8 hour test. At about 7.5 hours into the test, the solid state cabin pressure sensor failed. I have sense replace this with a conventional pressure transmitter. If you look along the bottom of the plot,you can see when I manually turned on the O2 makeup valve. I would turn the O2 makeup valve on when the O2 concentration dropped to 20% and turn it off when it reached 22%. The cabin temperature was fairly constant at about 79 F during the test. My CO2 concentration varied from about 3700 ppm to 4300 ppm (0.37% to 0.43%) over the 8 hour test. My conclusions from the test were: 1) manual O2 make-up system worked , 2) solid state barometric sensor was not robust enough for this duty 3) I needed to increase the flow rate past the CO2 sensor to help dampen response. 4) the centrifugal radial fan was working fine and 5) the current being drawn by the fan was low enough enable the life support system to run 72 hours the emergency battery bank.

I did not run any test in which I started at 80 RH. I am not sure if it would climb to 100% RH or stabilize at some value between 80 and 100%. On your dive at Ft. Pierce, I don't think you were down long enough

to get much in site into how the RH would trend in your boat.

Cliff

but by perseverance.
Samuel Johnson

Cliff Redus
Redus Engineering
USA Office: 830-663-6445
USA mobile: 830-931-1280
cliffordredus@sbcglobal.com

From: "JimToddPsub@aol.com" <JimToddPsub@aol.com>
To: personal_submersibles@psubs.org
Sent: Wed, March 16, 2011 9:13:09 PM
Subject: Re: [PSUBS-MAILIST] Humidity control

Cliff,

When you have time, could you share more regarding that experiment? Perhaps when the starting point for RH is 50%, injecting dry air wouldn't be worth the effort. What are your thoughts regarding that practice when you're starting out with 80%+ RH? I would think that RH would be rather common is southern US coastal waters.

My experience is rather limited, but I noticed on the coast at Ft. Pierce that it was very humid and hot on the surface. When submerged below 20 feet for a while I expect the temp in the sub might drop, but even at a steady amount of water vapor in the air, that would have resulted in a rise in RH even with the dew point remaining the same. Of course the water vapor would not remain steady because three bodies (in that case) were breathing and perspiring profusely. We weren't down long enough or deep enough to achieve any significant cooling from the ambient water temp.

Thanks,

Jim

In a message dated 3/16/2011 2:39:14 P.M. Central Daylight Time, cliffordredus@sbcglobal.net writes:

Sean, I also think this is true also. I have a relative humidly sensor in my boat. When I did an 8 hour test in my boat in which I do not have any RH control, at the time the hatch was closed, the RH was 50%. After about one hour, the RH climbed to around 80%. Over the next 3 hours, the RH climbed to 85% and stayed at this level for the duration of the test. This leads me to believe that the primary source of condensed water vapor in the boat is due to exhalation of 100% RH air. I think that purging the pressure hull with dry air prior to hatch closing would bring down the initial RH to near zero but quickly for a small psubs like mine, it would climb rapidly. As the chemical reaction in the scrubber needs water vapor to work, this concept would seem to me to be counter effective to scrubber operations.

In the spirit of KISS, I am not sure it makes sense to try and condition the air to regulate RH anyway.

Cliff

Cliff Redus
Redus Engineering
USA Office: 830-663-6445
USA mobile: 830-931-1280
cliffordredus@sbcglobal.com

From: Sean T. Stevenson <cast55@telus.net>
To: personal_submersibles@psubs.org
Sent: Wed, March 16, 2011 8:08:26 PM
Subject: Re: [PSUBS-MAILIST] Humidity control

I would think that, with the possible exception of very short dives, in a
small space (PSub size) the moisture present in the surface air that gets
enclosed in the vessel is less significant than the moisture introduced by the
occupants breath over the course of a dive?

-Sean

On Tuesday 15 March 2011 23:00:13 you wrote:
> We should probably write a white paper on this sometime since it seems
> to come up once or twice every year. I always find condensation easier
> to understand in terms of dew point. Simply put, and in all cases, if
> the temperature falls at or below the dew point, you're going to have
> condensation. You can also get a sense of the relative humidity by
> comparing the current temperature against the dew point. If the air
> temp is 80F and the dew point is 70F then you've got high humidity and a
> sticky humid day on your hands. If the air temp is 80F and the dew
> point is 45F then you have low humidity and a nice dry day on your hands.
>
> The problem starts at the surface when the air in your submarine is
> filled with the atmospheric conditions of whatever the current day is
> like. Now consider that when a sub is in water the cabin air is
> eventually going to reach equilibrium with the water temperature. If
> the water temperature is at or below the dew point of the atmospheric
> air, you can expect to have condensation issues as the air temperature
> in the sub begins to drop due to water conducting heat from the cabin
> through the hull.
>
> In a high humidity situation (warm humid day, cool water temps), I think
> there's a simple and cheap solution that might go a long way to
> controlling condensation during the dive. By emptying the air from an
> 80 cubic foot scuba tank into the submarine cabin just prior to closing
> the hatch, the dry air from the tank should displace enough of the humid
> air in the cabin to considerably lower the dew point inside the cabin
> and reduce the chance of condensation. This should be easy enough to
> test without the sub even in the water by simply waiting for a humid day
> and monitoring the relative humidity in the sub before and after the air
> from the scuba tank fills the submarine.
>
> For warm climates such as Florida, if you have room for the small
> "cooler" air conditioner that David Bartsch was working on last year, I
> think that could be an ideal solution for not only controlling heat in
> the cabin but also the humidity.
>
> Calcium Chloride is cheap and easily available, and is suppose to act as
> a desiccant although I've never tried it. The southern boys out there
> probably don't know about this stuff, but us northern boys usually have
> plenty of it on hand for melting snow and ice in the winter.
>
> Jon

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