By Sam Gartzman, University of Rhode Island
“Fire bottles 23 and 24” is the phrase to know when the CTD is coming on board. The CTD (Conductivity, Temperature and Depth) is a water profiling and sampling package that we regularly send down to close to the bottom of the ocean. There are 24 different bottles to collect samples on it and once the last bottles are fired we know it will be on deck soon!
Many of the science groups are interested in the water that we get from the CTD. Everyone rushes to the Baltic Room (the big room where the CTD comes in) with their bottles and caps to get the water before it warms up (many of the properties and gasses are temperature sensitive). However, I am not carting around a box of bottles. I march in with a pile of copper tubes. Now that is odd, what would you use copper tubes for? We are studying the noble gas (Helium-3, Helium-4, Neon, Argon, Krypton and Xenon) concentrations of the water. Since noble gases are largely unreactive we use them to help understand mixing and air to sea exchange. Since some noble gasses can diffuse through glass or plastic we use copper to be able to seal them inside for analysis back in the US.
Recipe for a copper tube sample:
1. Roll out and cut the copper – The copper comes in large rolls. Soon before each cast we will cut a large amount to 24 inches (Two 12 inch samples) and cap the ends to keep the salty air out of the inside of the tube.
2. Flatten a small piece of the copper— As the water warms up in the tube it wants to expand. This way we are shrinking the volume of the tube slightly while the water is cold. After sealing we will re-round the tube to allow for expansion of the water inside. This makes sure the seals stay in tact. When I say flatten we aren’t making it completely flat, but just not as round as it is from the roll.
3. Label your tubes— How are you supposed to know what water went in each tube?!
4. Put rubber tubing on each end of the copper to transfer the water from the CTD bottles (known as niskin bottles) to the copper.
5. TURN UP THE TUNES— To ensure proper copper tube sampling the samplers should be cool, calm and relaxed. Tunes aid in this process.
6. Check the niskin Bottle— Since noble gases are the most sensitive to changes in temperature and atmospheric exposure, we are the first to sample from each bottle. Before taking any water we open the bottle to see if there was a good seal (indicated by no leaking water). If there isn’t a good seal it is unclear if the water in the bottle is actually from that depth and we tend to not use those bottles.
7. Flow water into the copper— The copper tubes are parched and want some good saltwater to cool them off.
8. Get rid of any bubbles— This is the most time consuming part. We take a rubber mallet and knock every side of the tube to push any bubbles out. Although blowing bubbles were one of my favorite childhood activities, bubbles would cause error to the concentrations.
9. Seal the tubes— This is by far the coolest part of the process. We close the ends of the rubber tubing to keep the water inside the copper. We walk over to the cold welding system to seal the tubes. This system is essentially a hydraulic press that has a set of teeth that crimp the ends of the copper shut. The hydraulic pump can apply up to 10,000 psi of pressure so be sure to keep your fingers out!!! We get two samples from each niskin bottle. It makes a pretty cool noise when it crimps too, just in case you were wondering.
10. Repeat steps 6-10 for each bottle being sampled (Typically 8-12 bottles on each CTD cast).
11. Re-round the tubes— As explained in step 2 this will give the sample some room to expand. Right after re-rounding we shake each tube. You know you have a good seal on the tube if it makes a nice popping sound, which are the dissolved gasses in the water filling the extra space in the tube. We make note of the troublesome ones that don’t make this popping noise since they may not have gotten a good seal.
12. Rinse the tubes— Each tube gets a nice warm bath in some deionized water (ew, gross ions) to remove any excess salt water that could cause corrosion.
13. Pack the samples up— Once dry the tubes are packed up in bubble wrap. The corners are quite delicate and sharp so we take extra care to protect them.
PHEW! That was a lot of steps. While it does seem like a lot, it generally goes pretty quickly. Once back in the US these samples will be analyzed on a mass spectrometer, which is able to give high precision measurements of these noble gases of interest.
You can find this recipe and more on channel 73 on Nathaniel B Palmer closed circuit television.