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Rough Science 5 Zanzibar: Ellen McCallie's diary: Beneath the waves

Updated Wednesday, 23rd March 2005

The team have to devise an underwater breathing apparatus for presenter Kate Humble.

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Day 1

Mike’s and my challenge is to determine how much weight Kate needs to wear in order to move around at will in the ocean. If she gets in just as she is or, even worse, with a dive suit, she will float to the surface of the water and will have a tough time staying down for a dive. If we put too much weight on her, she’ll sink to the bottom, which isn’t so good for looking around. We need to determine the amount of weight to give her such that she can move around comfortably without floating to the top or sinking to the bottom.

When Mike and I were first given this challenge, we talked about the procedure. We knew what to do, based on readings and classes we’ve had in the past. But why does this work? It sounds a bit like science lab, doesn’t it—you know what to do but not why. What do you get when you submerge someone in water and collect the water that runs out? Mass? Weight? Volume? A heck of a lot of water? In these situations, my strategy is to place the challenge in my subconscious, roll it over and over, and wait for something to pop out. In the meantime, I have a cup of tea and eat chocolate biscuits.

Now, it seems obvious. Stick with me and see what you think. When we dunked Kate, good sport that she is, in a bathtub full to the brim, the water had to “make room” for her. In this case, the water “got out of the way” by spilling out of the tub through a hole, where we collected it. This is the amount of water that “got out of the way” for Kate. Or, said differently, the amount of space Kate took up, which is her volume.

Now, if Kate were made of sea water, we wouldn’t have to do this at all. There would be no point in the challenge. Sea water can move around in sea water. But Kate, and especially Kate in her dive suit, is less dense than sea water. Said another way, if we put Kate in her dive suit on a balance and put sea water on the other end of the balance—equivalent to the amount of space Kate’s body and dive suit take up, the sea water will weigh more. It has more mass. By putting just enough lead weights on Kate to make the balance balance, Kate will be the same density of water, be neutrally buoyant, and be able to swim around without issue.

This one is conceptually so hard. Perhaps one just needs to try it! Fill a bucket to the top with water. Put the bucket in a larger pan. Push a toy into the water and submerge it. Collect the water that runs over. Build a balance and see if the amount of water that has run over is heavier or lighter than the toy you put in the water. Unless it is a metal truck or something, the amount of water will probably be heavier, because water is pretty dense. To make the toy not sink or float, you’d need to add enough additional weight to the toy to make the balance balance.

Thus, Mike and my antics with a lot of wood, rope, buckets, and Kate.

Day 2

We didn’t have a scale, so we can’t talk about pounds or kilograms. Instead we just added enough of the lead weights that were in the trunk to make the balance balance.

This was the really fun part—melt down the lead and reforming it, which is called casting, right?

Melting lead is incredibly beautiful. It’s shimmery, silvery in the light. It’s also extremely dense, which makes a panful extremely heavy. Plus, lead melts at a pretty low temperature for a metal. We couldn’t have used iron, that’s for sure, as we wouldn’t have been able to get the temperature high enough just by burning wood.

Making the casts in sand was almost as good as the pleasure of pouring in the lead, waiting a couple of minutes for the lead to cool, and popping the lead out. The first time I tried to pick up the pan to pour the lead, I couldn’t lift it. It wasn’t that it was too heavy. I just didn’t expect a pan to weigh quite that much, so I hadn’t tried hard enough. I got into practice of giving it all I had each time the pan needed lifting. Will I ache tomorrow?

Another thing to think about was safety. The fumes are toxic, hence Mike’s clever funnel devise to capture and move the fumes away. Second, this is being filmed, so there are other people around, particularly Keith, the cameraman, who wants to be as close as is safely possible. Hot lead will burn people or ruin a camera. So we had to be careful not to get the lead too hot or to get water anywhere near it—it causes the lead to splatter as well. An art as well as a science.

I could have made designs in sand and cast them all day it was so much fun. Kathy’s idea to make lead “feet” was hilarious!

Day 3

Jonathan and Kathy’s build was absolutely amazing. Here Kate was breathing underwater—and moving around comfortably as she was neutrally buoyant. Hoorah!

How does it feel at the end of a challenge? We’re all exhausted—physically, mentally, and emotionally. We put everything we’ve got into each build. It can be really tough. Like Day 1 of this challenge. I had such a hard time getting (and keeping) my head around what we were doing and why. Plus, lifting all that water—the balance took us hours and hours to get to work well. Emotionally, you really have to monitor yourself, especially if we’re tired mentally and physically. Keeping an even keel is crucial for the momentum of the challenge. We’ve got to believe that there are solutions and that we will find at least one of them, even when the balance, for example, is falling down around us. Take breaks. Look at the sea. Laugh. Is it obvious that we love it? We do.





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