Thursday, January 5, 2017

How Do They Work?

By now, you know that these SOCCOM floats open incredible windows into a vitally important part of our climate system, the Southern Ocean, but how exactly do they do that?

Let’s take a look. First off, there are the sensors.

Here’s a photo of the top of one of the floats with the sensors labeled.

To start, take a look at the temperature and salinity sensor. That’s the black tower that has the tall holes in it. Salinity is measured by measuring the water’s conductivity. If the water has higher conductivity, that means there are more ions in the water, which means a higher salinity. If you know the temperature and pressure, you can calculate an exact number for the salinity of the water from this device.

The temperature probe is actually called a “thermistor” not a thermometer. The traditional mechanics of a thermometer use mercury, but a thermistor is actually a resistor (a metal, ceramic or polymer) whose resistance changes very precisely with temperature. Put thermo- and resistor together, and what do you get? Thermistor!

The reason why the pressure sensor is labeled differently is because you can’t actually see it! It’s behind all the other sensors, but it measures the pressure of the water around the float, and from that you can calculate the depth.

Because the float has these three sensors (T, S, and P), scientists will say that this float has a “CTD,” which stands for Conductivity, Temperature and Density, and that’ll get you your bread and butter parameters that you need to know about the water in every profile.

The pH sensor is connected to the temperature and salinity sensor by a tube that pumps water over from the temperature and salinity tower. That means the pH sensor is measuring exactly the same water that just had its temperature and salinity taken. This pH sensor is called an “FET,” which stands for Field-Effect Transistor. You might know that transistors can be used to precisely measure voltage, but what you might not know is that the voltage this sensor measures is directly proportional to the pH of the water! The tricky thing with this sensor is being able to package it in a way that we can get reliable data at different depths, temperatures and salinities. This particular sensor has just the right packaging to keep it going no matter what kind of punch Southern Ocean packs!

Next up is the oxygen sensor, over on the right. It’s an optical sensor, and it works by shining a blue LED into the water. The fluorescence properties of the water are determined by number of O2 molecules around. With no oxygen content, you’ll have maximum fluorescence, but any oxygen molecules in the water will quench a certain amount of the photoluminescence.

The oxygen sensor is kind of a variation on the nitrate sensor, which is short, and looks a little bit like a whistle. Instead of just blue light, the nitrate sensor sends many different frequencies of light into the water and measures the absorption spectrum of the water across the different frequencies.

Last but not least are the backscatter and chlorophyll sensors which we’ll talk about together because they’re wrapped up in one package down at the bottom of the float. (Why are they at the bottom? Because they’re heavy, and we want the float to stay vertical as it rises and sinks!)

Here’s a photo of Steve Riser with Southstar, named by Melvin H. Kreps Middle School. Down by his foot is the chlorophyll and backscatter sensor. The chlorophyll sensor optically measures something called chlorophyll fluorescence, which is slightly different from chlorophyll itself. When we deploy the floats, we calibrate that sensor using samples that we’ve collected from the larger rosette-shaped CTD.

Here’s a photo of that CTD. There are two parts to it—the 24 bottles around the edge, and the sensors at the bottom. This photo was taken right before it went out that door and sunk to a depth of 2000 meters. At different depths, the scientists will close the bottles, one at a time, in order to capture water from different depths, all the while monitoring the sensor readings from the ship.

The cable that holds the CTD from the boat is conductive, so as the rosette sinks, we can watch the data come in from the sensors, meter-by-meter. If there’s something particularly interesting at any depth, the scientists can actively “fire bottles,” or close them, capturing the water at that depth, on its ascent.

The CTD can also be sent all the way to the bottom of the ocean (which sits at around 4000 meters below the surface). Sometimes the scientists like to do “deep casts” like this. We’ve done that twice now, sending along some styrofoam cups. At 4000 meters, they’re under a heck of a lot of pressure. Oceanographers tend to measure pressure in decibars because it just so happens that the numerical value for pressure in decibels is close to the numerical value for depth in meters!

Take at look at this poor old Styrofoam cup!

Next up: we’ll take a look at exactly what these measurements tell us about the Southern Ocean!


1 comment:

  1. This is really neat. I could see PBS doing a tag-a-long to capture the all the different things that you people are doing.

    The big CTD used for calibration looks interesting. It's amazing that it can work at the bottom of the ocean with all that pressure. Have you ever had one of the bottles not close properly?

    At first, when I looked at the bottles on the CTD, I didn't notice that there was a cap on the top AND the bottom. :) I opened the image in a new tab and zoomed in to see the bottom caps. ..With just an opening on the top, I was thinking to myself, "How do they get more water in there when there is already water in there?" Water is compressible, but not very much. Right?

    When the bottles come back to the surface, there must be a lot of pressure pushing out. No? Those are some pretty strong bottles. I know the 2 liter bottles of soda are at around 3 to 4 atm. Right? ... I googled it real quick. "The pressure increases about one atmosphere for every 10 meters of water depth." .. So, 4000 meters / 10 is 400 atm. .. Really? 400 times the pressure than at the surface? Did I calculate that wrong? :\

    And, it looks kinda scary in that deployment room, so close to the water with the door open. She looks pretty secure, but my heart rate would definitely be elevated. :)