Author Topic: 2021 International Year of Caves and Karst: BCRA online seminars  (Read 2792 times)

Offline pwhole

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Re: 2021 International Year of Caves and Karst: BCRA online seminars
« Reply #50 on: March 11, 2021, 02:45:27 pm »
It says they're 'rubber'? I'm guessing that means some sort of synthi-rubber, at that price.

Offline ChrisJC

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Re: 2021 International Year of Caves and Karst: BCRA online seminars
« Reply #51 on: March 11, 2021, 03:02:09 pm »

Does one need permission to pour bright plastic balls into a watercourse?

not entirely sure it's environmentally the best thing to be doing, with already far too many micro-plastics in the rivers/seas.

It does concern me a bit. With electronics, you will always have some dodgy substances in there, although in small quantities. It might turn out that the biggest obstacle is not technical, but bureaucratic! You would have to find a way to get a 100% retrieval rate, otherwise it's not allowed...

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Online JoshW

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Re: 2021 International Year of Caves and Karst: BCRA online seminars
« Reply #52 on: March 11, 2021, 03:14:20 pm »
I've also just clocked one of the key sensors you wanted in there was pressure (as a digital depth gauge presumably), yet to maintain a constant buoyancy at any depth you'd need to Captain Nemo to be rigid - which would then negate any changes in pressure inside the ball.
All views are my own and not that of the BCA or any clubs for which I'm a member of.

Offline ChrisJC

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Re: 2021 International Year of Caves and Karst: BCRA online seminars
« Reply #53 on: March 11, 2021, 03:18:34 pm »
The reference from Andy Farrant had a solution to that - there was a hole in the casing to permit the sensor to access the outside. Of course it would need sealing very well!

And yes, the pressure sensor was to measure sump depth, and differentiate between vadose and phreatic conduit.

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Offline mikem

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Re: 2021 International Year of Caves and Karst: BCRA online seminars
« Reply #54 on: March 11, 2021, 03:28:33 pm »
You would only want a constant buoyancy in a vadose system, for phreatic you would want it to be stay in the flow rather than rise to the roof.

It says they're 'rubber'? I'm guessing that means some sort of synthi-rubber, at that price.
At one point it does say they are synthetic (which still covers a multitude of possible sins).

Online JoshW

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Re: 2021 International Year of Caves and Karst: BCRA online seminars
« Reply #55 on: March 11, 2021, 03:32:51 pm »
You would only want a constant buoyancy in a vadose system, for phreatic you would want it to be stay in the flow rather than rise to the roof.


by constant buoyancy I mean constant neutral buoyancy i.e. neither positively or negatively buoyant, but just hovers in place in still water, but follows the flows/currents
All views are my own and not that of the BCA or any clubs for which I'm a member of.

Offline ChrisJC

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Re: 2021 International Year of Caves and Karst: BCRA online seminars
« Reply #56 on: March 11, 2021, 03:48:28 pm »

by constant buoyancy I mean constant neutral buoyancy i.e. neither positively or negatively buoyant, but just hovers in place in still water, but follows the flows/currents

Agreed. A smidge of positive buoyancy would be required I think to prevent it getting stuck at the bottom.

I think that because water is not compressible, then the density of water must be constant irrespective of depth, therefore a rigid object would have constant buoyancy regardless of depth. This means we could determine the buoyancy in a bucket of water and be confident that it would remain like that throughout the system.

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Offline mikem

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Re: 2021 International Year of Caves and Karst: BCRA online seminars
« Reply #57 on: March 11, 2021, 04:04:33 pm »
You have approx 1 atmosphere more in pressure for every 10m deeper you go, so there is a slight increase, plus any flow creates turbulence, which will be particularly effective if it's dragging air down from the surface (that will reduce buoyancy). However, temperature has a much greater effect on the density than pressure.
« Last Edit: March 11, 2021, 04:27:47 pm by mikem »

Offline JoW

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Re: 2021 International Year of Caves and Karst: BCRA online seminars
« Reply #58 on: April 09, 2021, 03:48:54 pm »
An interesting thread of conversation! For those that missed John's talk there will be another showing of it at some future date, but this hasn't been scheduled yet - watch this space along with our Facebook page and website for more details.

And a reminder that our next talk in the series is on Monday (12th April) at 7pm. This time we will be hearing Andy Farrant talk about chalk caves :)


 

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