Author Topic: How can I seal a small dc motor for water?  (Read 10344 times)

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Offline GrunthanerTopic starter

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How can I seal a small dc motor for water?
« on: February 06, 2011, 09:24:41 PM »
Hi I am a technology teacher and having kids compete with small dc motors (not servo) to make small robots go under water in a small fish tank. I saw methods of sealing servo motors and putting them into small balloons to seal. Besides that are there other techniques? Can we just put the small motors in the water and use them for the small competition knowing they will fail over time if not sealed?


Offline AtomHex

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Re: How can I seal a small dc motor for water?
« Reply #1 on: February 07, 2011, 10:55:09 AM »
 I built a small ROV for under water and used the canisters that film came in, the little plastic ones with snap on lids make a small hole in the lid for motor shaft and one in the other end for wires seal them with Vaseline. Open motors in the water are actually shorting out , IF they work there running at very very low efficiency.


Offline Soeren

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Re: How can I seal a small dc motor for water?
« Reply #2 on: February 07, 2011, 02:05:42 PM »
Hi,

Open motors in the water are actually shorting out , IF they work there running at very very low efficiency.
No they're not!
Water has got a much higher resistance than the motor - some people break in their motors under water.
That said, it would shorten their life some of course.

Please don't present a (wrong) guess as if it was knowledge.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline dunk

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Re: How can I seal a small dc motor for water?
« Reply #3 on: February 07, 2011, 02:20:10 PM »
so distilled water has no resistance so electronics can operate without fear of short circuit under distilled water.

regular tap water typically has enough resistance to mess with digital electronics but i would speculate something like a small DC motor would still operate with reasonable efficiency under tap water.

obviously salt water is at the other end of the spectrum. much more power would be lost due to the conductivity of the liquid.
some sort of mechanical seal would be probably be needed to keep the motor dry if you were using it under salt water.


but electrical short circuit is not the only concern.
if the motor body were full of water the armature would experience far more resistance, meaning the motor would be under more load, draw more current, and generally be less efficient.

on the other hand, water is more thermally conductive than air so a heavily loaded motor would cool down quicker meaning you could allow it to draw more current without damage.

i'm not sure where these 2 opposing factors would balance out... you would need to experiment.

but i see no reason why a DC motor could not operate un-sealed in a fresh water fish tank.
the only question is if it would be more efficient (both in construction time and operating performance) for you to seal it.


dunk.

Offline Soeren

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Re: How can I seal a small dc motor for water?
« Reply #4 on: February 07, 2011, 08:04:20 PM »
Hi,

so distilled water has no resistance so electronics can operate without fear of short circuit under distilled water.

Distilled water is ionized the second you immerse anything in it. That's why the case modders use certain kinds of oil for immersed PC's


regular tap water typically has enough resistance to mess with digital electronics but i would speculate something like a small DC motor would still operate with reasonable efficiency under tap water.

Please see this post for some measurements.
As I mentioned in my previous post, some people break in their motors running them in tap water (apparently to flush out carbon from the brushes as they're ground into shape).


obviously salt water is at the other end of the spectrum. much more power would be lost due to the conductivity of the liquid.
some sort of mechanical seal would be probably be needed to keep the motor dry if you were using it under salt water.

As far as my measurements go and taking a random small motor of say 10 Ohm as an example, I see no indication that it would not run, as less than 1% of the power should be lost to the water.

I should have tried it before I flushed my batch of "sea" water to be sure, but that'll have to be another time.


but electrical short circuit is not the only concern.
if the motor body were full of water the armature would experience far more resistance, meaning the motor would be under more load, draw more current, and generally be less efficient.

Yes, I'll have to measure speed in/out of the water as well.
I don't think the resistance will be that large compared to whatever load it's intended to drive though.


on the other hand, water is more thermally conductive than air so a heavily loaded motor would cool down quicker meaning you could allow it to draw more current without damage.

Yes, and this will allow a much higher power consumed, so maybe it's possible to actually get more power and speed out.


i'm not sure where these 2 opposing factors would balance out... you would need to experiment.

And that's what we live for ;D

Lifetime of the motor would probably go down a bit, but that's just guessing.
There would be no sparks between the brushes and the armature, so no pitting.
I think the stumbling point will be the bearings. Ball bearings is a no-no, but self lubricating (sintered and oil-filled) bronze bushings should not have problems and while there will be oxygen in the water, I think a motor will last quite a while before rust is a problem. The axles would have the most problems with rust, but they're hardened and polished steel, so is quite resilient. Another problem area might be the springs pressing the brushes down, but with a good quality motor...


but i see no reason why a DC motor could not operate un-sealed in a fresh water fish tank.
the only question is if it would be more efficient (both in construction time and operating performance) for you to seal it.

Yes and even a badly sealed motor could be dried out between uses.

However, any kind of sealing will mean a higher load (and now without the water cooling it), so it's probably not safe to judge anything too rigid without some experiments to back it up.


While I was a kid, I had some motors to mount on any flat bottomed toy ship (with a suction cup). They had a rudder to set manually and was encapsulated in plastic with a single AA cell to run them (with a twist housing to start and an O-ring seal). I remember them as fairly water tight.
I imagine they can still be found in some toy stores, as they existed in the sixties, but was still for sale in the late eighties and early nineties (when my sons got some) and in a couple of years I hope to buy some for my (now 2 year old) grandson.
They could be wired to a motor controller (instead of the AA) and have the power wire sealed with RTV silicone, so would be a good choice if a small motor would do.

Now that they pop up in my memory, I seem to recall that I may have one or two of them somewhere, just don't recall if they're in the cellar or where I have stuffed them - I'll keep an eye open for them .
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline GrunthanerTopic starter

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Re: How can I seal a small dc motor for water?
« Reply #5 on: February 08, 2011, 05:23:25 AM »
Wow interesting info guys thanks, I spoke with our physics teacher who is actually a retired engineer from ATT he said use distilled water too, although Soeren that was an interesting point you made regarding the water being ionized once something is immersed in it. Also how some break in motors in the water to round out the brushes but doesn't that build up oxidation inside?

Regarding the film canisters Atomhex good idea if I can still find them these days although I can post a message on the school bbs maybe get a bunch of empties or go to a photo store for donations.

Dunk thanks for your vote on submersing in water too. Is there a way to make some distilled water as my budget is low and the tank is large.

Soeren, I have searched for waterproof small dc motors but haven't found any any ideas to improve my search? I am also the Robotics club advisor and we compete in MATE ROV we use large bilge pump motors for our robot but even if I could get a small bilge pump it would be too expensive for my small summer program.

Thanks again,

Offline dunk

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Re: How can I seal a small dc motor for water?
« Reply #6 on: February 08, 2011, 08:15:57 AM »
Dunk thanks for your vote on submersing in water too. Is there a way to make some distilled water as my budget is low and the tank is large.
hi Grunthaner,
so i think you would be fine with tap water.

distilled water would have virtually no conductivity to start, then increasing slightly over time as Soeren points out.
tap water will be *slightly* conductive but not enough to affect your motor.
salt water is the worst case but Soeren's experiment suggests it would still work.


so, although distilled water would technically be better, it won't make much difference compared to tap water.
i would recommend you tried tap water.


dunk.

Offline Soeren

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Re: How can I seal a small dc motor for water?
« Reply #7 on: February 08, 2011, 01:40:54 PM »
Hi,

[...] break in motors in the water to round out the brushes but doesn't that build up oxidation inside?

I don't think so, as they just run them for a couple of hours max.
To compare, I have had some springs (I took apart a huge 4 color LASER printer last Friday) soaking in water for two days now - not a bit of rust.


Soeren, I have searched for waterproof small dc motors but haven't found any any ideas to improve my search?

I found the kind of motors here and here.
Should be possible to find them much cheaper though.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline AtomHex

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Re: How can I seal a small dc motor for water?
« Reply #8 on: February 08, 2011, 08:49:49 PM »
Hi,

Open motors in the water are actually shorting out , IF they work there running at very very low efficiency.
No they're not!
Water has got a much higher resistance than the motor - some people break in their motors under water.
That said, it would shorten their life some of course.

Please don't present a (wrong) guess as if it was knowledge.


 My guess is not wrong....electricity can short out in water, water is a conductor, I just didn't express myself in such eloquent terms as you did, and , "  it would shorten their life some of course. " are your words and in my world loss of efficiency includes shorter than life expectancy of a device.


And this part " Please don't present a (wrong) guess as if it was knowledge." even wrong guesses are knowledge.

 And holy crap what a bunch of WIND BAGS including myself....The guy asked

 " How can I seal a small dc motor for water?"

« Last Edit: February 08, 2011, 09:12:03 PM by AtomHex »

Offline Soeren

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Re: How can I seal a small dc motor for water?
« Reply #9 on: February 08, 2011, 11:03:50 PM »
Hi,

My guess is not wrong....electricity can short out in water, water is a conductor,
Yes it is.
To "short out" means having a very low resistance - that is not so, ergo it's wrong.

Water is a conductor, yes, but so is even slightly humid air - somehow I guess you wouldn't claim that a motor couldn't be run in humid air.

A DC motor has got a very low resistance, 100 times (or more) lower than the water, so the potential dividing would be like putting eg. a 10 Ohm resistor in series with a 1500 Ohm resistor - which of them do you think (or guess) will run the show?


I just didn't express myself in such eloquent terms as you did,
This is engineering, not politics, so wrong is wrong, however eloquent or not it's put.


and , "  it would shorten their life some of course. " are your words and in my world loss of efficiency includes shorter than life expectancy of a device.
Efficiency is the ratio of power output/power input.

Whether you decide to put other terms in the same hat doesn't really matter to what's what, it just tells me that you don't understand the subject very well (solution: study study study).

MTBF and MTTF (which is the term for the life expectancy of a component that cannot be repaired) has nothing to do with efficiency, it's only a prognosis for how long the component will last (average).


And this part " Please don't present a (wrong) guess as if it was knowledge." even wrong guesses are knowledge.
Utter nonsense!
If all involved knows that something is wrong, it might be called a proven anti thesis and hence knowledge. If not, it's just plain wrong and has nothing to do with knowledge.


And holy crap what a bunch of WIND BAGS including myself....The guy asked

 " How can I seal a small dc motor for water?"
And your point is?
If people asks questions, it usually means that they haven't got the answer. From that, it only takes a very modest deduction to conclude that the person asking doesn't know all the options available either and hence, it is not always a good idea to just answer a question directly.

If someone asked you "is it possible to build a robot on the kitchen table", the grammatically correct answers possible is just "yes" and "no"... Would you answer that way?


If people are wrong, I don't see a reason to spare those that might be offended. Viral spread of wrong answers is a huge problem - Just look at how many people believe in the "memory" myth - Better P off a few, if it helps the majority.

There's an easy solution if you cannot bear to be called out on wrong answers... Just don't answer something that you don't positively know 100% for sure.
Whenever your fingers itch for a quick answer, ask yourself "am I guessing or knowing".
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline AtomHex

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Re: How can I seal a small dc motor for water?
« Reply #10 on: February 08, 2011, 11:52:28 PM »
I can bear constructive criticism but like I said  HOLY CRAP........O and by the way, how do you cut/past and dissect every little word and sentence and then comment on each statement wrong or right...does anybody else find that as anal as I do or is it just me.

Offline newInRobotics

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Re: How can I seal a small dc motor for water?
« Reply #11 on: February 09, 2011, 12:26:56 AM »
Talking about sealing a DC motor; wouldn't it work to seal it in a similar way a servo is sealed in a tutorial?

What first comes to head is to apply something like silicon on vents and holes of a motor (marked in red); then fill it with oil (as in servo sealing tutorial); finally, use some more silicon to seal the crack which is used to open the motor (marked in green).

This might work or migh not, I'm not a pro  ;D
« Last Edit: February 09, 2011, 06:30:33 AM by newInRobotics »
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Offline rbtying

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Re: How can I seal a small dc motor for water?
« Reply #12 on: February 09, 2011, 01:07:07 AM »
I can bear constructive criticism but like I said  HOLY CRAP........

Considering that Soeren is one of the most experienced EE's on this forum (and one of the most helpful on the internet), I'd say that he knows what he is talking about. 

Someone who resorts to a large-font to swear, uses more than three '.'s in an ellipsis, spells "paste" as "past" should probably look to correct their own faults first before pointing out others' - it's something everyone has to go through.  We're not too picky about grammar in SoR, but we're quite picky about facts (I've accidentally mixed up facts and guessing before, and learned from my mistakes). 

A rule of thumb:
Code: [Select]
  #define [i]term[/i] (A_TECHNICAL_TERM)
If you don't know the technical (usually mathematical) meaning of term, please a) avoid using term and/or look up the definition of term.  If a Google search of your terminology gives the OP relevant results, you have succeeded.  Otherwise, it will only suffice to confuse the OP (and yourself) and drive the thread around in circles.

O and by the way, how do you cut/past and dissect every little word and sentence and then comment on each statement wrong or right...does anybody else find that as anal as I do or is it just me.

The "cut/paste and dissect [everything]" method is simply the most efficient method to address all the points presented in a post directly.  It's actually quite common for fora.  It's really what the "quote" system is for, as few people really want to have the entire post quoted (unless it is short).

In my opinion, if you have decided to critique something about another person, do it in such a way that you at the very least sound like the person's intellectual equal and/or better in text - it lends credence to your argument. 

Just my $0.05.  I do not claim to represent the opinion of SoR or any other members.  I apologise in advance if it sounds excessively harsh - I've had a frustrating day.

Offline AtomHex

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Re: How can I seal a small dc motor for water?
« Reply #13 on: February 09, 2011, 10:07:11 AM »
Hey, you guys are right...I give up

 I can't communicate on anyone's level...
 I'm very obviously stupid about everything...
 I don't know how to properly express myself...
 I can't spell....
 I used the word crap for a colorful mete-fore so now I'm swearing...
 I enlarged a font, that seems to be worth being criticized for also...
 This post is probably not laid out in the correct manner...
 And I'm sure there are many other things that someone will point out to me that aren't right..

 So,  I apologized for being inadequate for this forum and not up to everyones standards of perfection and I will try harder in the future to make sure that my grammar and technical remarks/statements/guesses are to the absolute best of my knowledge.

 

  
 
« Last Edit: February 09, 2011, 02:21:49 PM by AtomHex »

Offline garrettg84

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Re: How can I seal a small dc motor for water?
« Reply #14 on: February 09, 2011, 11:25:21 AM »
I'm kind of curious if anyone has experience with exposed DC circuits in pool water (highly chlorinated) and how much that affects conductivity. I don't have a pool handy to test in, but some of the kids in my robotics club will be doing a submersable soon and the main competition will be in a pool.
-garrett

Offline madsci1016

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Re: How can I seal a small dc motor for water?
« Reply #15 on: February 09, 2011, 01:53:35 PM »
Wow, this has been an interesting read.

Anyway, I've been sealing small DC motors for several months now with a program called SeaPerch.



Which is an federal funded outreach progrm for kids where they learn and build a ROV by the end of the camp.

Instructions for build can be found @ http://www.seaperch.org/teacher_tools

Though the jist of the motor part is to wrap the motor in electrical tape, install it into a film canister, and back fill with hot (toliet bowl or similiar) wax.

obviously salt water is at the other end of the spectrum. much more power would be lost due to the conductivity of the liquid.
some sort of mechanical seal would be probably be needed to keep the motor dry if you were using it under salt water.

As far as my measurements go and taking a random small motor of say 10 Ohm as an example, I see no indication that it would not run, as less than 1% of the power should be lost to the water.

A wha??? Soeren, this is dead wrong. 

Average sea water static resistivity is about .2 Ohm * meters. So for the distance between motor contacts of about let's say .05m (5 cm) the resistance is 0.01 Ohms. Versus your 10 Ohm motor, who is going to win?

Salt water is VERY conductive, I've seen massive robots taken down by a few drops of it in a leaky connector. In fact, most if not all sea bearing robots have special circuitry/sensors to detect "GFI" of eletrical systems to the sea water as an early warning indicator before things start shorting out.

Offline Soeren

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Re: How can I seal a small dc motor for water?
« Reply #16 on: February 09, 2011, 05:48:59 PM »
Hi,

[...] and back fill with hot (toliet bowl or similiar) wax.
I'm probably gonna regret asking for a clarification here... So I won't  ;)


As far as my measurements go and taking a random small motor of say 10 Ohm as an example, I see no indication that it would not run, as less than 1% of the power should be lost to the water.
A wha??? Soeren, this is dead wrong. 

Average sea water static resistivity is about .2 Ohm * meters. So for the distance between motor contacts of about let's say .05m (5 cm) the resistance is 0.01 Ohms. Versus your 10 Ohm motor, who is going to win?
No it's living truth.

When did you dip a set of probes?
Or perhaps more relevant... What was the are of the probes? ;)
Or... when are you gonna realize what resistivity is and why it isn't to be equated directly into resistance?  ;)  ;)

If I were to present it like you, I could say...
Copper has got a resistivity of ~17 nOhm-m (that's nano-Ohm per meter), so a wire wound 1,470.6 times (give or take a couple of meters) around equator will be 1 Ohm.
I think you see now, where the error crept in and who of us are wrong?  8)
(Yeah, I know... I'm a bastich  ;D).


Salt water is VERY conductive,
No, it's not. It's a puny 5S/m.


I've seen massive robots taken down by a few drops of it in a leaky connector.

Hah, so much for "build in the US of A"   :P  :P  :P   ;D
(Oh sorry, I forgot, you don't like me yanking your patriotic chain ;D)

Seriously, one thing is sea water in low power high impedance circuits (bad mixture), but sea water against a low impedance like a motor... No problem!

Did you read my post containing data on "sea" water (35g/l) and the area of the probes, which was larger, and the inter-distance shorter than what you'd see in a motor?

Now go dip some probes in salted peanuts and ponder the numbers over a smooth Scotch (or whatever your favorite peanut wash down may be) - Except for the peanuts, the rest might be good for you  :)
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline dunk

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Re: How can I seal a small dc motor for water?
« Reply #17 on: February 09, 2011, 07:03:10 PM »
all this ranting got me thinking...
so i always thought sea water was more conductive than Soeren's experiment implies...
it is possible that the ions in sea water are much more conductive than those in his table salt.

but i have no data. it's just a hunch based on how unreliable electrical systems tend to be on boats.

so i've just been doing a little searching online and it's tough to find an answer.
2 things that are clear:
1. the conductivity varies with temperature.
2. the salinity varies with pressure (and therefore depth).

Wikepedia suggests a figure of 4.8 Siemens/meter ~ 0.2ohms/meter.
http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity
but i don't trust Wikepedia. it's mostly made up on the spot to support arguments.

these guys are selling "standard" sea water:
http://www.oceanscientific.com/Products/SeawaterStandards/tabid/113/agentType/View/PropertyID/53/Default.aspx
but they aren't doing a great job of explaining the product.
they quote "S = 35" and then later say "The label value is quoted in salinity and in specific conductance (mS/cm) with a confidence of ± 0.2%."
so if they mean 35mS/cm ~ 3.5S/m that is in the same range as Wikepedia's 4.8 Siemens/meter.

what i don't understand about these figures is that they don't take into account the surface area of the immersed probe.
common sense would imply that more probe area would lead to lower resistance (or higher conductivity if you want to use the Siemens scale).


hmm,
maybe i'll pop down to the ocean tomorrow if i can skip work.

on the other hand,
Soeren is usually correct if you catch him sober so i might just cave.


dunk.



[edit]
i just found this:
http://www.seawatersolutions.com/osil-seawater-conductivity-standards-28-p.asp
if you click on the image of the bottle you can just about see "25C 53.07mS/cm"
which is 5.307S/m = 1.9 ohms/m at 25 Centigrade.

it still doesn't answer the query about the probe surface area though.

who would have thought there was a market in selling sea water right?
« Last Edit: February 09, 2011, 07:09:17 PM by dunk »

Offline madsci1016

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Re: How can I seal a small dc motor for water?
« Reply #18 on: February 09, 2011, 07:20:28 PM »
I'll do you one better. Cup of water from my saltwater fish tank. Probe surface area roughly what you would find exposed inside a small brushed motor. 12V across 2" conducts about 1 Amp of electricity.

I repeated the experiment in saltwater and moved the probes closer together. At about 1/4" distance in-between (getting to real distances between brushes in a motor), the reaction got violent and my 3A power supply went into CC mode and started dropping voltage.

I admit I mis-understood resistivity, but I'm not wrong about DC motor in saltwater = big trouble. Not just "1% loss".
« Last Edit: February 09, 2011, 09:25:24 PM by madsci1016 »

Offline madsci1016

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Re: How can I seal a small dc motor for water?
« Reply #19 on: February 09, 2011, 07:25:50 PM »
who would have thought there was a market in selling sea water right?

There's a huge market for Salt Water for fish tanks.

I have a 40 gallon, that I just used in my above experiment. 

Offline Soeren

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Re: How can I seal a small dc motor for water?
« Reply #20 on: February 09, 2011, 07:49:44 PM »
Hi,

I'm kind of curious if anyone has experience with exposed DC circuits in pool water (highly chlorinated) and how much that affects conductivity.
Well, thanks for reminding me... The "sea" water I made from tap water should have been made from distilled water (which would have resulted in a higher resistance), as the tap water (here) contains chlorine and thus have a larger number of ions than sea water when 35g salt is added per liter.

The conductivity will depend on who maintains it, or rather how much they chlorinate it (and the temperature not to forget).
I read somewhere, that to keep a pool clean and bacteria free, you need 1.5 to 3.0 ppm, but I'm too rusty in chemistry to make anything out of it and too lazy to read up on it :-[ right now, so I cannot make an equivalent to measure.

The 'meters they use to quick check a pool is actually measuring conductivity for determining chlorine (or other ions) in the water, so perhaps a pool keeper can tell you (if those instruments are calibrated in a usable way),


I don't have a pool handy to test in, but some of the kids in my robotics club will be doing a submersable soon and the main competition will be in a pool.
Neither have I. It's below freezing here, so only indoor pools will be open for business (unless you bring an axe) and the keepers of the indoor public pools in DK would kick you out, if they spotted you geeking around with instruments (or motors) in the water - Heck the security is stricter than an airport, you need to strip and wash down to get into the water - not even bubblegum could be smuggled in, so you're left to swim or kick a$$  ;D


Oh well... I just got too curious and dipped the same probes I used for the other tests, but only ~3mm apart, about 60 to 70 mm submersed in a bottle of concentrated house hold chlorine. I don't know exactly how much chlorine it contains, but you'd probably die from swimming in it (makes your respiratory system go haywire and returns your dinner for recycling in much lower concentrations), so it's not rocket science to see that a swimming pool must have much less ions per unit mass.

It exhibited the same curious short (split second) low-ish (like a few hundred Ohm) like the other tests (got no explanation for that), but the resistance rose quickly and in about a minute or so, it was a bit over 6 kOhm and still rising with a fair speed.
I didn't pursue it further, but from that (remember, this was highly concentrated), I'd think that even a heavily chlorinated swimming pool would be closer to tap water than to sea water.

If it's something that's gonna happen on a recurring basis, I'd be more worried with another chlorine issue... it's highly corrosive and a number of roofs over indoor pools have crashed down when the chlorine have weakened the stainless steel pillars (or what itøs called) that held the roof - some plastic types is weakened by chlorine as well, so if you have the possibility, it might be worth it to gather a range of materials  in a small net and get permission to have it in the pool for a week or so, just to be sure.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Soeren

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Re: How can I seal a small dc motor for water?
« Reply #21 on: February 09, 2011, 08:38:20 PM »
Hi,

what i don't understand about these figures is that they don't take into account the surface area of the immersed probe.
common sense would imply that more probe area would lead to lower resistance (or higher conductivity if you want to use the Siemens scale).
That's because resistivity is a "cubic measure" (not sure if this is the correct term in English), so the "probe area" is inbuilt - 1 square unit each and 1 unit apart. Unit can be meter, centimeter, mm, inch or whatever.

Yes, sea water have got other ions than sodium-chloride, but an ion is an ion in this respect, so I'd imagine that it shouldn't matter how you get to the salinity (whether it's made up from various salts or a single one).


on the other hand,
Soeren is usually correct if you catch him sober so i might just cave.
Ha, I'm best with a burner  :P
(Oops, ran out of highland brain oil a couple of days ago, so might be wrong today ;D)


[...] which is 5.307S/m = 1.9 ohms/m at 25 Centigrade.
Decimals... 5.307S = ~0.19 Ohm


it still doesn't answer the query about the probe surface area though.
Hope I did then.


who would have thought there was a market in selling sea water right?
Oh, every 6th. week I get 300ml of it directly into a vein (with some horrendously expensive - about GB£16k for a 1 year supply - "powder" dissolved in it... (to help the other meds I get lower my immune defense, together keeping my arthritis to manageable levels).

"sea water" a.k.a. isotonic salt water, is used extensively in health care, as it's the "same" as the water which makes up around 40% IIRC of a human.
Good for flushing "dry" eyes and nostrils as well.
I've even heard of people drinking it for around 24 hours until they actually hmmm "outputs" nothing but salt water - sort of a cleansing of the body... I use brain oil for that of course... Gives a nice warm fuzzy feeling while it cleans.

Reminds me... I believe I have a 300ml sealed "bag" of isotonic somewhere - when I get around to making a more controlled measuring system, with stainless steel plates in a rigid submerged arrangement, I'll have to try that too.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline madsci1016

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Re: How can I seal a small dc motor for water?
« Reply #22 on: February 09, 2011, 09:27:58 PM »
I'll do you one better. Cup of water from my saltwater fish tank. Probe surface area roughly what you would find exposed inside a small brushed motor. 12V across 2" conducts about 1 Amp of electricity.

I repeated the experiment in saltwater and moved the probes closer together. At about 1/4" distance in-between (getting to real distances between brushes in a motor), the reaction got violent and my 3A power supply went into CC mode and started dropping voltage.

I admit I mis-understood resistivity, but I'm not wrong about DC motor in saltwater = big trouble. Not just "1% loss".

CRAP!!! I hit modify instead of quote.

Offline Soeren

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Re: How can I seal a small dc motor for water?
« Reply #23 on: February 09, 2011, 09:35:04 PM »
Hi,

I'll do you one better. Cup of water from my saltwater fish tank. Probe surface area roughly what you would find exposed inside a small brushed motor. 12V across 2" conducts about 1 Amp of electricity. So using your favorite equation, V = IR, about 12 ohms of resistance.
Great, now it's starting to look like engineering.
Except for the "Probe surface area roughly what you would find exposed inside a small brushed motor" Experimenting is good, undisclosing important engineering parameters are not.

Your result differs quite a lot from what I found, so please reveal the numbers and we can all have a go at verifying it or not.


So how exactly does this equate to only about 1% power from a 10 ohm motor? Have you actually ever dunked a motor in saltwater?
As you have no doubt read, it equate to the numbers I got, with a precisely described area and distance, so I gather it's a rethorical question.
No, I have not yet run a motor in water of any salinity, which should be clear to anyone reading my posts - I still have to find a suitable tank, make totally sealed sensors for temperature, RPM and an electronically variable load and a data logging system for it - pondering putting it on a web server. Still in the sketching state, as there's a bit of more important stuff that takes priority, but I'll get around to it.


No, but since you always like to bring it up, it's pretty clear you have some sort of "non-America inferiority/jealously" complex. Kinda like a kid who's jealous he can't have something so instead he takes every chance to make fun of it as a way to cope.

Also, I didn't say they were American made. I've personally seen UUVs from other countries fail as well. Or would you like a list of the countries so you can think up irrelevant jokes for them as well?
Oh, you really need to lighten up...
Apart from the extreme imperialism, which DK was guilty of back in time as well, I really have no beef with the US and judging from a lot of US citizens I have heard through time, I sure cannot recognize feeling inferior in any way... How large a percentage is illiterate in the US these days by the way? ;)

Jealousy... That should be towards your low prices on most stuff ;D as DK is an extremely expensive country - a Cafe Latte is close to $10 at several of the café's in Copenhagen (not that I ever touch that poison).

No, I would've thought that you knew me better by now, but I just like to yank your chain every now and then and this is the easy way I've learned... Don't get so mad, it builds character (and hopefully some thicker skin)  :P

But perhaps you'd rather that all non US citizens were a bit disgruntled being left out of the promised land?
Believe me, as a Free Thinker, I have no time or want for patriotism, which, in my optics, is nothing more than what a government want's to "brain wash" into it's citizens, to make them happy, or at least willing, to pay and die for a piece of cloth - it's a coincidence where you're born after all, and I don't care for peoples nationality, political standpoint, beliefs, sexual orientation or haircut... as long as it (or actions born out of that) isn't so on other peoples account... Only how they conduct themselves realy matters to me and I guess the only types that really P's me off is what I call self-inflicted morons.

So, it's not out of patriotism, but I sure wouldn't like to be without the Danish health care system or the much lower crime rates we have.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline Soeren

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Re: How can I seal a small dc motor for water?
« Reply #24 on: February 09, 2011, 09:42:35 PM »
Hi,

CRAP!!! I hit modify instead of quote.
Yeah well crap happens, but luckily you can still modify a modified.
Just don't post if your pulse is too high ;D

Anyway, bedtime a few hours ago and a buddy needs help with a washing machine and a drier in about 6 or 7 hours and he lives at 4th. floor, so I better catch a nap now. Hoping to read some more about your experiments in a day or so.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline madsci1016

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Re: How can I seal a small dc motor for water?
« Reply #25 on: February 09, 2011, 09:56:44 PM »
Except for the "Probe surface area roughly what you would find exposed inside a small brushed motor" Experimenting is good, undisclosing important engineering parameters are not.


It's a pair of small alligator clips dipped in roughly 1/8" to 1/4" tip first. Not the easiest geometric shape to measure surface area, but I doubt you think it's much more then what is in a DC motor. And the 1 Amp was at 2" distance apart.

Measure Ohms in saltwater is different then actually applying power. My hand-held meter reports kOhms measured resistance in the same sample.

I'm a bit put out now, as the 10 seconds of experimenting has now ruined my alligator clip tips from corrosion. Next time it's just bare wires.

Quote
No, I would've thought that you knew me better by now, but I just like to yank your chain every now and then and this is the easy way I've learned... Don't get so mad, it builds character (and hopefully some thicker skin)  Tongue


As much as I respect you Soeren, I hope i'm not the first one to say this, but your 'tone' always seems to come off a bit offensive when you make jokes in this regard. I'll try harder to not read into it in the future, but maybe find something else to pick on me about?  As an American, I hear cracks against us all the time, as you might imagine. It gets old.

Quote
How large a percentage is illiterate in the US these days by the way?


Too large, one of the reasons why I donate time and effort (and now a bit of personal funds since defense funding was cut) into education outreach programs. I frequently visit schools to inspire kids.

Quote
but luckily you can still modify a modified.


No help when you lost the source, and I can't type it out from memory.

Offline dunk

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Re: How can I seal a small dc motor for water?
« Reply #26 on: February 10, 2011, 02:54:36 AM »
As much as I respect you Soeren, I hope i'm not the first one to say this, but your 'tone' always seems to come off a bit offensive when you make jokes in this regard. I'll try harder to not read into it in the future, but maybe find something else to pick on me about?  As an American, I hear cracks against us all the time, as you might imagine. It gets old.
haha.
wow.
you do know Soeren is a clever AI script running on a large snow cooled datacenter near the arctic circle somewhere?
unfortunately the guys writing the program never got round to finishing the user interface as they discovered it was more fun to drink the alcohol meant for fueling the generators.

i wonder if we could submit a patch to the original code making it less full on with new users and delicate Mad Scientists?

as for the American sense of humor, in my experience, the one place it is almost alway missing is about being American.
i've given up on trying to wind them up about it in text as it's hard for them to tell when i'm joking.
in person however all bets are off.


dunk.
(almost always joking about something even if it's not as obvious as this time.)

 


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