Guest post: Estimating salinity as a homework assignment

Today I am super excited to share a guest post that my awesome friend Joke Lübbecke wrote for us. Joke is a professor in physical oceanography in Kiel, and we like to chat about teaching occasionally. She has great ideas for exciting tasks for students to do and I bet they learn a lot from her. Here is what she writes (and the photos in this post are the original photos that her students kindly agreed to let us use on this blog. Thanks very much!):

Estimating salinity as a homework assignment

When I gave the second-year oceanography students in my class bottles of salt water and – without any further instructions – asked them to find out what the salinity was, I wasn’t really sure what to expect. Would they just take a sip and guess 35? Would they all use the same approach? So when they handed in their solutions in the following week I was very happy to see how creative they had been and how many different things they had tried to get to an answer. For example, they had

  • Evaporated the water and weighted the dry salt
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Evaporating water from salt water and weighing the remaining salt to measure salinity
  • Used differences in buoyancy between salt and fresh water
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Measuring salinity by comparing buoyancy with known samples
  • Measured the electric resistance of the sample, then tried to mix a solution with the same resistance by adding more and more (defined quantities of) salt to a fresh water sample
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Measuring salinity by measuring the resistance of the sample and reproducing a sample with known salinity and the same resistance

or simply

  • Tasted the sample and compared to water samples with known salinities :-)

The numbers they came up with were as diverse as their approaches so this was also a nice demonstration of the difficulties to accurately measure salinity.

(And of course the salinity of the water sample they got was about 35, but who cares? – the journey is the reward!)

An overturning experiment (part 3)

By popular demand: A step-by-step description of the overturning experiment discussed here and here.

I wrote this description a while ago and can’t be bothered to transfer it into the blog format, so please go and find a .pdf here. This .pdf addresses young children in the first part, and grown-ups in the second part.

Have fun and if you use this in school or with your own kid, please let me know how it went! I love to hear from my readers! :-)

 

An overturning experiment (part 2)

How to adapt the same experiment to different levels of prior knowledge.

In this post, I presented an experiment that I have run in a primary school, with high-school pupils, in a Bachelor-level course and in a Master-level course. The experiment itself was run identically in all cases. However, the introductions, explanations and discussions about it obviously differed.

For example, in the primary school, I introduced this experiment by showing pictures of lions and penguins and other animals that the pupils knew live in warm or cold climates, and we talked about where those animals live. In the end this aimed at how temperatures are a lot colder at the poles than at the equator. This is the differential heating we need for this experiment to work. While this is something that I felt the need to talk about with the primary school kids, this can be assumed as a given with older students (or at least that is the assumption that I made).

With the university-level courses, one of the points that I made sure came up during the discussion are the limitations of this model. For example that we apply both heating and cooling over the full depth of the water column. How realistic is that? Or the fact that we heat at one end and cool at the other, rather than cooling on either end and heating in the middle?

With the university-level courses, we could also discuss other features that we could see during the experiment. Remember this image, for example?

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The thermal conveyor belt experiment.

Let me zoom in on something.

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Curious features in the thermal conveyor experiment. Do you know what this is about?

Do you see these weird red filaments? Do you think they are a realistic part of the thermal circulation if it was scaled up to a global scale?

Of course not. What we see here is salt fingering (oh, and did you guys notice that a diagram of how salt fingering works is displayed at the very top left of my header? I wasn’t exaggerating when I said that it was my favorite process ever!). So basically, this is a process that is caused by the different diffusivities of heat and of the red dye. And while it is pretty large scale in our small tank, you cannot scale it up just like that when talking about the real ocean. And it is also really difficult to get rid of salt fingers for this experiment, in fact I haven’t yet managed. But I am open to suggestions! :-)

Another point that I would talk about with university-level students that I would probably not bring up with primary school kids (- although, why not if I had more time than just those 45 minutes per class?) is that ocean circulation is driven by more than just differential heating. Even when just considering the density-driven circulation, that is additionally influenced by changes in salinity. Put that together with wind-driven circulation and we are starting to talk about a whole new level of complicated…

But anyway. My point is that even primary school kids can benefit from doing this kind of experiments, even if what they take away from the experiments is not exactly the same as what older students would take away.

One of the main messages the primary school kids got seems to have been that you need to take curd cheese beakers for your warming element (look here for some reports (in german)). Not exactly my main message, but at least they were very observant of how the experimental setup was designed ;-)

An overturning experiment

A simple experiment that shows how the large-scale thermally-driven ocean circulation works.

Someone recently asked me whether I had ideas for experiments for her course in ocean sciences for non-majors. Since most of the experiments I’ve been showing on this blog were run in the context of Bachelor or Master oceanography-major courses, she didn’t think that the experiments were as easily transferable to other settings as I had claimed.

So here is proof: You can do pretty complex experiments with non-university level students. To prove my point, let’s go to a primary school.

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Experimenting with a primary school class.

The experiment we are running here is the global (thermal) conveyor belt. In a long and narrow tank filled with water, a heating and a cooling element are inserted at either end. Dye is added onto the elements to visualize the flow of water.

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Adding dye to visualize the thermally driven flow in the tank.

In the image above you see that there is something blue near the bottom of the tank, and I am adding red dye to the other side. Blue is used to track the cold water and red to track the warm water (intuitive color-coding goes a long way, no matter how old your students are!)

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The experiment as seen by the teacher.

What you see here is the cold blue water sinking to the bottom of the tank and spreading, and the warm red water rising to the water’s surface and spreading there. As the warm water reaches the cooling pads, it gets cooled, becomes denser and sinks. Similarly, the cold water reaching the warming pads becomes less dense and rises, closing the loop.

Modeling the Denmark Strait Overflow

Ha, this is a bad pun. We are modeling the Denmark Strait Overflow – but in a non-numerical, small-scale-and-playdough kind of way.

More than a year ago, Kjetil and I ran that experiment with a group of high-school students and when writing a post about a much more sophisticated version of this experiment I realized I never documented this one in the first place. So here we go!

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The set-up: Tupper ware with a modeling clay ridge (“let’s call it Greenland-Scotland-Ridge”) across, filled with water to a level above the ridge, cooled with a sport’s-injury cooling pack in “the North”.

Dye is added to the “northern end” of the tank (i.e. the end where the water is being cooled by a sport’s injury cooling pack). As the water cools, it becomes denser and fills up the reservoir on the northern end until it spills over the clay ridge.

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The overflow. See the blue, dense reservoir on the left and the dense water spilling over the ridge.

This is a very simple demonstration of how overflows actually work.

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Kjetil, his Master student Eli and some of the high-school students. Can you see the sketch of the Denmark Strait Overflow on the slide in the background? (Plus, for everybody who is interested: This is the food coloring I have been using right there in the front right!)

Traveling circus

Stuff that I  brought to Isafjördur to teach the intro to oceanography.

I’ve been a fan of minimalistic travel for a while. And apparently I was ready for a new challenge: Minimalistic travel but with the full equipment for experiments in oceanography! Sadly I didn’t manage to carry on even though I tried…

Stuff that I’m bringing with me to teach “intro to oceanography”.

It might not look like too much, but you’ll be pleased to know that with this equipment, I can show every experiment I’ve shared on this blog so far (with the exception of the ones in the long internal lee wave tank) plus at least a dozen or so that are still in the pipeline to be published on here (I have lost track of what I have shared and I’m too lazy to look it up now, sorry).

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Granted, I did send a list of stuff that I’ll need to Isafjördur, too, and asked them to organize those things for me. But on that list there are only things like paper towels, empty 1.5l bottles or matches – hence things that are very easy to obtain anywhere, but a pain to travel with. I’m bringing all the fancy stuff like high-intensity non-toxic dyes, modeling clay, clear straws (surprisingly difficult to find!), split pins, wooden tongs, heating&cooling pads, an inflatable globe and many more.

So who wants to invite me to come teach at their place? I also “train the trainers” if you want to learn how to do all of this awesome stuff and then teach your students yourself!