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    Model submariner meeting at Osterwald (near Hannover, Germany)

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    Hermann
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    Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  Hermann on Mon May 14, 2012 9:16 pm

    Hi friends,

    on the weekend from 4th May to 6th May a small model submariner meeting took place in the public bath of Osterwald. About 20 submarine modellers most of them living in northern Germany shared this meeting and had a nice weekend although the weather was cold and somewhat rainy on saturday. I have put some pictures in the annex to give some impressions...

    Kind regards
    Hermann























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    nigele(ADMIN)2
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    Re: Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  nigele(ADMIN)2 on Mon May 14, 2012 9:35 pm

    Very Nice Hermann,its so nice to see what is happening in Germany
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    Hermann
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    Re: Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  Hermann on Mon May 14, 2012 10:33 pm

    Hi Nigele,

    yes, its also nice for me to see what is happening in the model submariners community in the UK. I think the scenes are very similar as we share the same hobby and model submariners "are just made from the same wood" without regard where they come from.
    Also the problems (many old men, only few young newcomers) seem to be quite similar...


    I missed to mention that on that meeting we experienced a sudden explosion of a hydrogen/ oyxgen gas mixture inside a model submarine. That model, a Graupner "Shark", was running submerged when a certain sound of explosion could be heard. The model fell to the bottom immediately and had to be recovered. When back to the shore we could see that the big hatch had been lifted by the explosion and the locking straps of aluminium had been bended by the pressure so the hatch wasn't tight any more.
    The model was equipped with nickel-metalhydrate cells which are normally declared as "gastight". I asked that modeller colleague how he used to charge the batteries and he confirmed to charge them with the hatch open! So hydrogen wasn't produced during the charge (due to overcharge). But that modeller mentioned that the battery had become weak before that explosion so I suppose that already one cell of the stack had been discharged completely and was charged in reverse polarity by the persisting load current. In this case the input of electrical energy cannot be stored by chemistry and results in electrolysis of the water portion of the electrolyte generating free hydrogen and oxygen.
    And a concentration of 4 per cent hydrogen will be sufficient for an explosion if ignition is provided by sparke of motors or switches.
    So we should learn not to discharge our batteries too deep just to avoid reverse polarity charge on cells and generation of gas.

    Kind regards
    Hermann

    photographs of the model after the explosion (the hull remained intact because the pressure was released very quickly by lifting off the large hatch)





    Last edited by Hermann on Mon May 14, 2012 10:38 pm; edited 1 time in total (Reason for editing : correction of a typing mistake)
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    Hermann
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    Re: Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  Hermann on Mon May 14, 2012 10:48 pm

    Hi Nigele,

    I just saw that your location Braintree, Essex, is not so far away from Thaxted where my very dear model railway friend Bertie lives at Guelph's Lane. He is the father of a dear colleague in the company where I work (a native Englishman and a citizen of Bremen for more than 30 years...) and also still an active member of the Thaxted Morris Men.

    Kind regards
    Hermann
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    Giovanni LiCalsi
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    Re: Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  Giovanni LiCalsi on Tue May 15, 2012 6:55 am

    Hi Hermann,
    Is the submarine running on a brushless motor?
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    david f
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    Re: Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  david f on Tue May 15, 2012 4:00 pm

    And Hermann,

    I'm interested to see that even NiMH cells can produce bangs! (Maybe Lipos get a bad press?)

    David
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    Hermann
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    Re: Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  Hermann on Tue May 15, 2012 5:52 pm

    Hi Giovanni, hi David and all the others,

    that model had a conventional brush motor (it looked like a Type 540) so with its sparking commutator there was a splendid source for ignition.

    David, also for me it was a new experience although I was always sure that every type of batterie can develope gas under certain circumstances like bad treatment. But I had a special focus on charging in a closed volume so I was surprised to witness an explosion during discharge (although it has to be noted that it must have been very excessive discharge driving atz least one cell in reverse polarity charge).

    So it must be kept in mind that in fact every battery may develope gas under fault conditions or bad treatment.
    Compared to explosions of Lipo batteries an explosion of a hydrogen/oxygen mixture seems to be more harmless as this reaction developes no fire. But the bang is impressive enough and of course there is a potentional danger that persons can be injured by fragments of your model.

    Kind regards
    Hermann
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    Hermann
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    Re: Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  Hermann on Wed May 16, 2012 8:29 pm

    Hi,

    becoming an old man I just forget more and more, for example another experience I had to learn on the meeting at Osterwald (I just reminded now): permanent damage of Reed contacts by coming close to an extreme strong magnet.
    Who had a similar experience yet?

    For my models I use a selfmade switching unit that consists of a normal (monostable) relay, two resistors and two reed contacts (normal open). The first reed contact is used to energise the relay coil in series with the first resistor. When the relay has switched on, the first reed contact is bypassed by the relay contact (the relay holds itself energised), the relay contacts also serve as the switch for the main supply. To switch off the second reed contact is used to provide a short across the relay coil so the relay releases and interrupts the supply path. This for theory of operation (and it operated very well and reliable for many years...)

    But on that saturday at Osterwald, for the second run of my model, I used a magnet of one modeller colleague (he was so kind to offer it when I searched for my own ferrite magnet). And I was really surprised that my model refused to switch on! I tried it several times without success. I opened the model and found that both reed contacts didn't close any more. At last I bypassed the reed contact for a short moment to switch on, closed the hull and made the run. Afterwards I had to re-open the model to bypass the second reed contact to switch off.

    Conclusion: both reed contacts must have been bended too much by the extreme magnetic field provided by that strong magnet my colleague had got from an old hard disk drive (I suppose it must have had a flux density of about 1 Tesla). My reed contacts are close to the pressure hull wall (about 8 millimetres away from the outer surface) and so they suffered from the stron magnetic field...

    So I will take care for my contacts...

    Kind regards
    Hermann
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    Hermann
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    Re: Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  Hermann on Wed May 16, 2012 8:46 pm

    ...here is the basic circuit diagram of the relay switch...

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    david f
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    Re: Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  david f on Thu May 17, 2012 11:29 am

    That is also a very interesting observation.

    I use a reed switch (to control a MOSFET transistor) for the main supply.

    I had no idea that they could be damaged by very strong magnets!

    I control the reed switch through the thickness of the hull so maybe this has prevented the problem.

    David
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    Hermann
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    Re: Model submariner meeting at Osterwald (near Hannover, Germany)

    Post  Hermann on Thu May 17, 2012 4:39 pm

    david f wrote:

    ..I had no idea that they could be damaged by very strong magnets!...


    Hi David,

    it was also a new experience for me. Distance between magnet and reed contacts will of course preserve the contacts from damage. In my application there was only a distance of approx. 8mm (5mm wall thickness of the aluminium hull tube and inside another 3mm to the switch contact).
    I looked in the datasheet of the manufacturer of the contacts as I expected to find some absolute maximum ratings also for the magnetic field to be applied, but there wasn't any rating concerning the maximum field strength (or flux density). There was only a rating given for the minimum sensitivity...

    Kind regards
    Hermann

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