A little less data tonight

OPC bits

The OPC in pieces in the dry lab, and the tools we are using to revive it.

The Scanfish OPC (Optical Plankton Counter) has been doing some funny things, so the Planktoneer is playing electrical engineer in the middle of the night, with some encouragement and help from the rest of the science crew. We noticed some data drop outs and strange data coming into the OPC computer this evening, and did some troubleshooting.  It seemed for a short time that the problem was in the cable coming into the lab which connected our computer (which saves all the data streaming in) to the Scanfish/OPC. Alas, the problem was underwater, and specifically in the OPC which had some water in side the pressure housing. A bad sign. You can see from the picture we’ve dismantled the whole thing, and while it’s tough to tell from that picture (though the orange toothbrush is a tip-off), we cleaned all the connections and fixed a few corroded wires. The instrument is 15 years old and gets dragged around in saltwater for hours or days at a time. We’ll cut it some slack. I’m cautiously optimistic it’ll live to count plankton another day because seem to have isolated the leak, which was small (things were damp but no water came pouring out when we opened it up) and the water came in through an auxiliary connector that we don’t use because we have so many sensors on the Scanfish.

OPC guts.

OPC guts.

Oceanography gear is complicated. The Scanfish, and most other electronic equipment we use, gets plugged into a conducting cable, which runs through a winch, which has a slip ring, which is plugged into a deck box, which goes into a computer. Whoa. That’s a lot of cables and connections that have to be functioning if we are to successfully collect data. It’s all been worked out over the years though, so things are pretty standard in many cases, and the people who use the gear tend to know a lot about it and can keep it operating. For those interested I’ll do a brief walk through of the Scanfish system and describe how this all comes together to get our data onto the computer.

It starts with the instrument itself. There are some underwater connectors on the control unit, buried inside the bright yellow body, to which we plug underwater cables that fit into the tow cable. These underwater cables have special fittings made of rubber that help keep the contacts dry. Still, we usually use some vacuum grease, o-rings, screwed on covers, and/or electrical tape to make sure the contacts stay dry. Better safe than sorry, so when you hear of a big oceanographic initiative, buy stock in electrical tape companies.

These underwater cables are then plugged into the termination on the end of the tow cable. The termination is a cone shaped piece of metal that is hollow and has a loop or some other piece that allows a hard connection to the instruments. It’s hard to describe, and the terminations vary, but they allow us to attach the instrument for towing and also allow us to feed the conducting wires to the instrument to gather the data in real time. The tow cable itself is usually either about 1/4” or 1/2” in diameter and has 3-7 copper wires inside of it. These wires are shielded with a sheath of steel cable, double wrapped around the conductors. The sheath acts as a ground for a lot of instruments, effectively giving us 4-8 wires to send and receive data from the underwater instruments. I’m sure there are some theoretical limits to the length these conducting tow cables can effectively be, but I’ve seen them as long as 1000 m. But then, I’ve not done work in water deeper than that, and that’s pretty deep anyway.

The tow cable is wound onto an oceanographic winch and fed through the drum into a slip-ring. This is a very important piece of the puzzle because it allows the drum to turn but still maintain an electronic connection to the computers in side. Cables are plugged into the slip ring and run into the lab where they go through a deck box.

Scanfish Control

Scanfish deckboxes on the left, with the laptop controlling the sensors next to that, and the engineering laptop and separate screen on the right.

Different instruments have different deck box configurations, and depending on the instrument they may send power down the conducting cable or just allow communication. The Scanfish deck box sends 120 volts down the wire to power the engineering side of the instrument – the part that moves flaps up and down to keep it swimming properly. In addition there is a deck unit that sends another 45 volts (I think) down the wire to power the sensors. The OPC deck until sends another 74 volts down the line to power that part of the instrument. All of these deck boxes are plugged into another box that sorts out the connections and keeps them straight. Each of the deck boxes – engineering, sensors, and OPC – have computers plugged into them running various software programs to archive the data and talk to instruments. In the case of the OPC this software just listens to the deck box and deciphers the data stream. For the other two parts of the Scanfish system, the computers can actually send commands to do certain things. For instance, we’re about to get to a station where we will “park” the Scanfish so we can do a CTD cast (more on that instrument later). To park it, the operator just clicks on a button on the Windows program, which tells the Scanfish to “Swim” to the surface and wait for another command.  After the cast, we’ll resume Scanfish undulating.

Well, that’s a brief tour of how the Scanfish system is set up. There is a lot that could go wrong, and as you can see, often something does, but almost as often we can fix it.  Stay tuned to see if that’s the case this time.  And enjoy this picture of the sunset.


Sunset, Sept. 4, 2010.


About planktoneer

I'm a zooplankton ecologist who studies how individual behaviors and variability affect populations of copepods in marine and estuarine systems.
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