Vacuuming chambers

[MNDeepROV]

Hey all, this is my first post ever on this fantastic forum....a plethora of information! Thanks to all the tinker-masters who partake!!

I am working my way through the forum...however, one specific question has been nagging at me...

First, to set the mood:
I am designing an ROV to go to depths of 300-400'. Deep for a Minnesota native I know! However, we have some pretty deep mine pits, etc., that beg to be explored...not to mention Lake Superior! Having that 300' depth goal in mind, my main question relates to buoyancy...of course.

Without further ado...
Does air in chambers affect buoyancy at depths due to changes in the water temp that will drop the internal air pressure?

I'm referring to potential air in the camera or electronics chambers, etc. For shallow depths I imagine this is not too much of a factor as the temp may not vary as much. However, when water temps are at or getting near 33 degrees the internal pressure drops regardless of the pressure the container can take, no? I'm a diver and notice this with my tanks...maybe it's not too much of a factor in an container with uncompressed air (less volume to change), or perhaps just doesn't affect buoyancy as the water displacement isn't changed? Just need to get my head wrapped around this...

I talked to someone at Seabotix a while back and although I wasn't 100% sure I was talking to a tech savvy person they did tell me that they vacuum all the air out of the unit.

Does anyone vacuum any of their electronics or camera compartments? If so, do you have any good tips as to the techniques used and how to ensure the air is out without creating too much of a negative pressure?

Additional questions would be around the electronics and the performance in a vacuum chamber...sensors, relays, etc.

Appreciate thoughts anyone may have on the subject! I might get to sleep easier again soon. My mind is just wheeling on design options and can't wait to get down to my first project. Many questions to answer and research to perform....that's part of the fun of it though.

Thanks!
~newbie and tinkerer extraordinaire
long post I know...but it's an an intro one...

[rossrov]

Welcome. Yes, because the displacement doesn't change, the buoyancy won't be affected. The manufacturer could vacuum the air out to "pre-load" the seals and so provide a low-pressure/shallow depth leak test. I'm not a diver, but have seen small pumps for dive camera enclosures intended to do this and have done similar on my own project. With the insides under vacuum, or partial vacuum, you can hold the enclosure in a bucket of water, look in through the camera window and see any water being pushed in. Also, but not sure about this, less air in the enclosure may reduce the likelihood of condensation. I was once told Lowrance or Eagle Fish-Finders had the air replaced with nitrogen for this reason. Not real sure about electronic components in a vacuum - things with gas or air in them likely to be trouble. Hope that helps

[MNDeepROV]

Thanks rossrov,

I think I just needed someone to confirm that for me. After typing out the post I kind of had an ah-ha moment, but you're reply helps a ton...gets my mind off that little hurdle. On to the next. I'll look into the vacuum pumps. I think removing the air for condensation issues could be helpful, if its a problem. Haven't seen much else out there for posts related to condensation so maybe it's not so much of an issue. I'll find out!

[bigbadbob]

I work with big AUV's and we vaccum test and then back fill with dry nitrogen to about 2psi.
This is especially important in humid climates like the Bahamas. but is worth doing in any climate.
I'll be doing this with mine as i have access to the gear to do it, would be OTT for homebuilt stuff unless you have the gear.
Don't leave it under vaccum as any heat build up around speed controllers won't be conductred away without air or inert gas in there.
Hope that helps.