Author Topic: Will a capacitor change value under higher air presure?  (Read 3416 times)

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Offline ErikTheNorwegian

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Will a capacitor change value under higher air presure?
« on: April 21, 2014, 09:49:48 am »
I'm sending Down 4  fluke 8846a down, deep into the sea. They are inside a closed camber and the inside pressure  goes from 1 bar to 6 bars during the desent.
Anyone knows what that does to the acuracy of the meters? Firs of my concerns is do the capasitors change value undér presure? They use capasitors as presure sensors. So how do Flukes capasitors behave under pressure?
« Last Edit: April 21, 2014, 09:53:03 am by ErikTheNorwegian »
 

Offline Rerouter

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Re: Will a capacitor change value under higher air presure?
« Reply #1 on: April 21, 2014, 10:24:37 am »
If the seals fail on any of the electrolytic caps (which is highly likely for 6 bar) it will likely work for decent but may rupture on the return (while rising back to the surface any air that got in will likely seal the hole)

Most online sources mention avoiding the issue, but dont actually say how

here is a source that should answer your thoughts on the values changing at least.
http://books.google.com.au/books?id=2JE3EtDdZmMC&pg=PA73&lpg=PA76&ots=z9jrE9weg0&focus=viewport&dq=electrolytic+capacitors+underwater
 

Offline ErikTheNorwegian

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Re: Will a capacitor change value under higher air presure?
« Reply #2 on: April 21, 2014, 10:56:58 am »
Yepp, confirms my tests so far. But how to avoid it would be my NeXT quest. I have filled after som mods all the Electronics in a "glue". But i think i make myself a test Chamber and make som measurments that i can use as a correction factor for the real life measurments.
« Last Edit: April 21, 2014, 12:18:36 pm by ErikTheNorwegian »
 

Offline G7PSK

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Re: Will a capacitor change value under higher air presure?
« Reply #3 on: April 21, 2014, 02:28:15 pm »
What do they do on submersibles,pot tall the electronics I would think. As the capacitors are vented I would have thought that the pressure inside would equalise to that outside but I would expect that the seals would be compromised on the way up, perhaps the thing to do is drill a very small hole in any electrolytic capacitors just before they go down it would take quite a long time for them to dry up.
Just looked it up seem's that deep sea electronics are in a tank of silicon oil.

http://deepseachallenge.com/the-sub/systems-technology/
« Last Edit: April 21, 2014, 02:37:36 pm by G7PSK »
 

Offline theatrus

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Re: Will a capacitor change value under higher air presure?
« Reply #4 on: April 21, 2014, 09:51:17 pm »
Immersing them in a liquid bath (non-conductive) sounds like a good idea, if you have a reasonable guarantee that there is no air or other gases trapped in any components. I wouldn't think this is true for eletrolytic caps.
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Offline MatCat

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Re: Will a capacitor change value under higher air presure?
« Reply #5 on: April 21, 2014, 10:15:04 pm »
I am really curious, the pressure from going down below sea level is because of the weight of the water, if you are in a sealed container how can pressure build inside?  Secondly to this how does someone like James Cameron manage to go down and mitigate pressure issues if this somehow happens? (The only way I can see it is if the outside vessel literally shrinks enough to cause compression inside?)
 

Offline theatrus

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Re: Will a capacitor change value under higher air presure?
« Reply #6 on: April 22, 2014, 12:00:33 am »
You can do it one of two ways:

- Big heavy steel pressure vessel with a normal 1atm inside.
- Lightweight vessel with either equalized pressure inside (so 6atm in this case), or a fluid (not nearly as compressible)

Scuba is effectively number 2 (as you need to maintain positive air pressure vs water to breathe), while the bathysphere is the first option.
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Offline ErikTheNorwegian

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Re: Will a capacitor change value under higher air presure?
« Reply #7 on: April 22, 2014, 09:53:21 am »
The Equipment is a "stowaway" passenger in a free Space in a watertight container. The working environment inside this Space is 6 bar, due to the pressure lines in and out of it. It’s a safety measure due to some reason. We have been granted use of that free Space to my customers test gear that I have to control is working correct by using the four 4486a. It’s a study of temperature, vibration, gas, etc. in technical Equipment. What’s happening is that small currents, builds up, of uncertain reasons and make the readings of the sensors uncertain. The Fluke 8846 measure different values (Current, voltage and capacitance) different places in the POD, umbilical power, string power, battery backup, but also the different Earth/ground differences that comes and goes. Platform runs their own generators, fase problems occur, and makes problems. All this adds up to electrical fen omens that we need to uncover.  Problem is, that we need accuracy of the measurements and have no umbilical that we can use for Remote control or reporting up. The equipment goes Down, stay there for days and months.    I do this for a student in underwater drilling Technology. We have come to very good conclusions so far, so on track, but accuracy could be better. Later on I’m promised that i can show Pictures and video of some of the work as soon as his doctorate is done and presented.

Sorry about my bad spelling, but spellchecker that was here before is gone.
« Last Edit: April 22, 2014, 11:19:55 am by ErikTheNorwegian »
 

Offline poorchava

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Re: Will a capacitor change value under higher air presure?
« Reply #8 on: April 22, 2014, 11:45:49 am »
I would be more worried about the display. Those are VFD, and I think too high external pressure could cause them to crack. I'm doing some underwater stuff from time to time, and the rules include: "no electrolytics, tantalums, LCD, VFD, ceramic caps >1206, mechanical relays, unmolded reed relays". Although that's for 300m+ depths. (>30bar)
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Offline Rerouter

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Re: Will a capacitor change value under higher air presure?
« Reply #9 on: April 22, 2014, 12:30:59 pm »
While unlikely to get a detailed reply, you could also ask fluke via email, as for submariner technicians i would assume at least 2-3 of there models have been characterized for a few atmospheres diving depth,

Having looked over the images in this thread https://www.eevblog.com/forum/reviews/fluke-8846a-teardown-9409/ there are 8 large electrolytics all in the power supply segment, there may possibly be some SMD tantalum's, but i am not seeing any electros or dipped tantalum's anywhere near the measurement side, meaning the most vulnerable components are only to do with supply input filtering, though accuracy could still shift according to that earlier documentation with different types,

The other thing is whether or not those 2 lovely fuses are sealed or can breath, the higher pressure will increase there rating as a negative but will quench the arc if it pops much faster, equally your input impedance that was contributed by the air will make the impedance higher than on land, while a small effect if your measuring uV it could matter,

 

Offline ErikTheNorwegian

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Re: Will a capacitor change value under higher air presure?
« Reply #10 on: April 22, 2014, 01:09:24 pm »
I would be more worried about the display. Those are VFD, and I think too high external pressure could cause them to crack. I'm doing some underwater stuff from time to time, and the rules include: "no electrolytics, tantalums, LCD, VFD, ceramic caps >1206, mechanical relays, unmolded reed relays". Although that's for 300m+ depths. (>30bar)
VFD `s are out and on my shelf as spares :-)

Its the electrolytic capacitors that I’m worried about.. Can’t use fluids like silicone, because of the risk og what we divers call "reverse block" what goes into the caps can stay there.. The front/rear switch is latced over and removed.
« Last Edit: April 22, 2014, 01:13:23 pm by ErikTheNorwegian »
 


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