A Tripod for Ian
My colleague and friend, Ian Hawes, can spend half an hour diving in 0.5°C water patiently moving a mini-electrode into a microbial mat with a micromanipulator. (He prefers 3°C water or maybe even 20°C water.) However, there are times when his patience is pushed beyond the limits, for example when the geometry of the micromanipulator (attached to a stake pounded into the lake floor) doesn't let him position the electrode properly. After the last field season in Antarctica, when he couldn't measure O2 gradients in the vertically-oriented microbial webs we found, we talked about designing a tripod that would give him more flexibility with the micromanipulator geometry. I've been working on that tripod.
The idea is to have a light-weight tripod that can be placed over an area of interest and to be able to move the (attached) manipulator to measure the O2 concentrations and pH in mat oriented from horizontal to vertical. I'm part way there with my design, particularly after a very useful discussion with Art at the Martinez Home Depot. I used two sizes of C-channel aluminum, a larger one for the tripod and a smaller one attached to the micromanipulator. I filed a flat spot on the outside of a C-clamp, drilled a hole in it, and bolted it to a short section of the smaller C-channel aluminum.
With the attached C-clamp, I can put the smaller C-channel aluminum piece inside the larger one and hold it securely. It can't come off unless you open the C-clamp very wide. Ian will be able to loosen the clamp slightly and then use the clamp as a handle to move the manipulator up and down along a tripod leg.
The weight of the manipulator pulls in such a way that it will not slide down the leg on its own even if the clamp is loosened. However, it is fairly easy to intentionally move up or down by holding the C-clamp (at least on my deck - I hope the same is true under water!). This gives an overall system that is stable and can be manipulated quite a bit.
However, I have two concerns with the current design. First, when it is folded up, the two legs without the manipulator are still quite far apart.
I'm not entire sure that it will be easy for a diver to get the tripod through a hole in the ice. But I like this design because the tripod is very easy to set up since you only have to rotate the leg with the manipulator away from the other two legs to get a very stable geometry. I've sent Ian an e-mail asking if the folded geometry will be reasonable, but he's in the Solomon Islands and has limited e-mail access now. If it isn't acceptable, I'll have to come up with a different way to attache the C-channel pieces at the apex of the tripod.
The second issue is that the manipulator can't actually access the full angular range we want. We can get horizontal to about 30° and 70° to vertical (actually overhanging). I tried adding another C-channel piece that lets the manipulator hang from the tripod with its base plate vertically oriented. This provides access to the full range of mat angles we are interested in. However, this solution proved to have a high frequency resonance. Any bump to the tripod leads to the manipulator shaking horizontally for a very long time. I don't know whether or not this motion would be damped out under water. I do know that if it wasn't, it could break an electrode in a mat.
I think the basic problem with the vertically hanging design is that the manipulator weighs a couple of pounds, and it hangs from a C-channel piece that has some rotational flexibility. The energy from motion of the manipulator gets temporarily stored by torquing the C-channel tripod leg, and then it is transferred back to motion of the manipulator in the other direction, etc., etc., which leads to the "deadly" horizontal motion. I'm not sure how to avoid this problem... I'll have to think about it, and suggestions are welcome. In the mean time, we have something that is better than a stake, but not yet what we want.
I also plan to put discs on the bottoms of the legs to help their stability when they are placed on mat under water rather than on my deck.
The idea is to have a light-weight tripod that can be placed over an area of interest and to be able to move the (attached) manipulator to measure the O2 concentrations and pH in mat oriented from horizontal to vertical. I'm part way there with my design, particularly after a very useful discussion with Art at the Martinez Home Depot. I used two sizes of C-channel aluminum, a larger one for the tripod and a smaller one attached to the micromanipulator. I filed a flat spot on the outside of a C-clamp, drilled a hole in it, and bolted it to a short section of the smaller C-channel aluminum.
With the attached C-clamp, I can put the smaller C-channel aluminum piece inside the larger one and hold it securely. It can't come off unless you open the C-clamp very wide. Ian will be able to loosen the clamp slightly and then use the clamp as a handle to move the manipulator up and down along a tripod leg.
The weight of the manipulator pulls in such a way that it will not slide down the leg on its own even if the clamp is loosened. However, it is fairly easy to intentionally move up or down by holding the C-clamp (at least on my deck - I hope the same is true under water!). This gives an overall system that is stable and can be manipulated quite a bit.
However, I have two concerns with the current design. First, when it is folded up, the two legs without the manipulator are still quite far apart.
I'm not entire sure that it will be easy for a diver to get the tripod through a hole in the ice. But I like this design because the tripod is very easy to set up since you only have to rotate the leg with the manipulator away from the other two legs to get a very stable geometry. I've sent Ian an e-mail asking if the folded geometry will be reasonable, but he's in the Solomon Islands and has limited e-mail access now. If it isn't acceptable, I'll have to come up with a different way to attache the C-channel pieces at the apex of the tripod.
The second issue is that the manipulator can't actually access the full angular range we want. We can get horizontal to about 30° and 70° to vertical (actually overhanging). I tried adding another C-channel piece that lets the manipulator hang from the tripod with its base plate vertically oriented. This provides access to the full range of mat angles we are interested in. However, this solution proved to have a high frequency resonance. Any bump to the tripod leads to the manipulator shaking horizontally for a very long time. I don't know whether or not this motion would be damped out under water. I do know that if it wasn't, it could break an electrode in a mat.
I think the basic problem with the vertically hanging design is that the manipulator weighs a couple of pounds, and it hangs from a C-channel piece that has some rotational flexibility. The energy from motion of the manipulator gets temporarily stored by torquing the C-channel tripod leg, and then it is transferred back to motion of the manipulator in the other direction, etc., etc., which leads to the "deadly" horizontal motion. I'm not sure how to avoid this problem... I'll have to think about it, and suggestions are welcome. In the mean time, we have something that is better than a stake, but not yet what we want.
I also plan to put discs on the bottoms of the legs to help their stability when they are placed on mat under water rather than on my deck.
