Creating a continuous stratification in a tank, using the double bucket filling method

Because I am getting sick of stratifications not working out the way I planned them.

Creating stratifications, especially continuous stratifications, is a pain. Since I wanted a nice stratification for an experiment recently, I finally decided to do a literature search on how the professionals create their stratifications. And the one method that was mentioned over and over again was the double bucket method, which I will present to you today.

Two reservoirs are placed at a higher level than the tank to be filled, and connected with a U-tube which is initially closed with a clamp. Both reservoirs are filled with fresh water. To one of the buckets, salt is added to achieve the highest desired salinity in the stratification we are aiming for. From this bucket, a pump pumps water down into the tank to be filled (or, for the low-tech version: use air pressure and a bubble-free hose to drive water down into the tank as shown in the figure above!); the lower end of the hose rests on a sponge that will float on the water in the tank. When the pump is switched on (or alternatively, the bubble-free hose from the reservoir to the tank opened), the clamp is removed from the U-tube. So for every unit of salt water leaving the salty reservoir through the hose, half a unit of fresh water flows in to keep the water levels in both reservoirs the same height. Thus the salt water is, little by little, mixed with fresh water, so the water flowing out into the tank gets gradually fresher. If all goes well, this results in a continuous salinity stratification.

Things that might go wrong include, but are not limited to,

  • freshwater not mixing well in the saline reservoir, hence the salinity in that reservoir not changing continuously. To avoid that, stir.
  • bubbles in the U-tube (especially if the U-tube is run over the top edges of the reservoirs which is a lot more feasible than drilling holes into the reservoirs) messing up the flow. It is important to make sure there is no air in the tube connecting the two reservoirs!
  • water shooting out of the hose and off the floating sponge to mess up the stratification in the tank. Avoid this by lowering the flow rate if you can adjust your pump, or by floating a larger sponge.

P.S.: For more practical tips for tank experiments, check out the post “water seeks its level” in which I describe how to keep the water level in a tank constant despite having an inflow to the tank.

First day of class – student introductions.

How do you get students to get to know each other quickly while getting to know them yourself at the same time?

The new school year is almost upon us and we are facing new students soon. For many kinds of classes, there is a huge benefit from students knowing each other well, and from the teacher knowing the students. But how do you achieve that, especially in a large class, without having to spend enormous amounts of class time on it?

There are of course tons of different methods. But one thing that has worked really well for me is to ask a question like “where are you from?” and have people position themselves on an imaginary map (you show which direction is north, but they have to talk to each other to figure out where they have to position themselves relative to the others). For the first question they are usually a bit hesitant, but if you ask three or four, it works really well. For other questions you could ask which of the class topics they are especially interested in, on which topic they have the most knowledge already, or the least, or where they want to go professionally, or what their favorite holiday destination is – all kinds of stuff. Depending on the level of the class, you can ask questions more on the topic of the class or more on a personal level.

This is highly interactive because you always have to talk to people to find your own position, and it is very interesting to see how the most complex configurations of students form, representing maps to scale even though some people might live in the same city whereas other people are from a different continent, for example.

The best thing is that it is a lot easier to remember stuff like “oh, those two used to live really close to where I am from, we were all clustered together for that question”, “those two are interested in exactly the same stuff as I am because they were right next to me when the question was x”, … than to recall that information from when everybody had to introduce themselves one after the other.

I really like this method, give it a try! And don’t be discouraged if students are hesitant at first, they will get into it at the second or third question. And getting them up and moving does wonders for the atmosphere in the room and makes it a lot more comfortable for you, too, to stand in front of a new class.

How to ask multiple-choice questions when specifically wanting to test knowledge, comprehension or application

Multiple choice questions at different levels of Bloom’s taxonomy.

Let’s assume you are convinced that using ABCD-cards or clickers in your teaching is a good idea. But now you want to tailor your questions such as to specifically test for example knowledge, comprehension, application, analysis, synthesis or evaluation; the six educational goals described in Bloom’s taxonomy. How do you do that?

I was recently reading a paper on “the memorial consequences of multiple-choice testing” by Marsh et al. (2007), and while the focus of that paper is clearly elsewhere, they give a very nice example of one question tailored once to test knowledge (Bloom level 1) and once to test application (Bloom level 3).

For testing knowledge, they describe asking “What biological term describes an organism’s slow adjustment to new conditions?”. They give four possible answers: acclimation, gravitation, maturation, and migration. Then for testing application, they would ask “What biological term describes fish slowly adjusting to water temperature in a new tank?” and the possible answers for this question are the same as for the first question.

Even if you are not as struck by the beauty of this example as I was, you surely appreciate that this sent me on a literature search of examples how Bloom’s taxonomy can help design multiple choice questions. And indeed I found a great resource. I haven’t been able to track down the whole paper unfortunately, but the “Appendix C: MCQs and Bloom’s Taxonomy” of “Designing and Managing MCQs” by Carneson, Delpierre and Masters contains a wealth of examples. Rather than just repeating their examples, I am giving you my own examples inspired by theirs*. But theirs are certainly worth reading, too!

Bloom level 1: Knowledge

At this level, all that is asked is that students recall knowledge.

Example 1.1

Which of the following persons first explained the phenomenon of “westward intensification”?

  1. Sverdrup
  2. Munk
  3. Nansen
  4. Stommel
  5. Coriolis

Example 1.2

In oceanography, which one of the following definitions describes the term “thermocline”?

  1. An oceanographic region where a strong temperature change occurs
  2. The depth range were temperature changes rapidly
  3. The depth range where density changes rapidly
  4. A strong temperature gradient
  5. An isoline of constant temperature

Example 1.3

Molecular diffusivities depend on the property or substance being diffused. From low to high molecular diffusivities, which of the sequences below is correct?

  1. Temperature > salt > sugar
  2. Sugar > salt > temperature
  3. temperature > salt == sugar
  4. temperature > sugar > salt

Bloom level 2. Comprehension

At this level, understanding of knowledge is tested.

Example 2.1

Which of the following describes what an ADCP measures?

  1. How quickly a sound signal is reflected by plankton in sea water
  2. How the frequency of a reflected sound signal changes
  3. How fast water is moving relative to the instrument
  4. How the sound speed changes with depth in sea water

Bloom level 3: Application

Knowledge and comprehension of the knowledge are assumed, now it is about testing whether it can also be applied.

Example 3.1

What velocity will a shallow water wave have in 2.5 m deep water?

  1. 1 m/s
  2. 2 m/s
  3. 5 m/s
  4. 10 m/s

Example 3.2

Which instrument would you use to make measurements with if you wanted to calculate the volume transport of a current across a ridge?

  1. CTD
  2. ADCP
  3. ARGO float
  4. Winkler titrator

This were only the first three Bloom-levels, but this post is long enough already, so I’ll stop here for now and get back to you with the others later.

Can you see using the Bloom taxonomy as a tool you would use when preparing multiple-choice questions?

If you are reading this post and think that it is helpful for your own teaching, I’d appreciate if you dropped me a quick line; this post specifically was actually more work than play to write. But if you find it helpful I’d be more than happy to continue with this kind of content. Just lemme know! :-)

* If these questions were used in class rather than as a way of testing, they should additionally contain the option “I don’t know”. Giving that option avoids wild guessing and gives you a clearer feedback on whether or not students know (or think they know) the answer. Makes the data a whole lot easier to interpret for you!

Rainbows and refraction II

Taking the same graphics as in this post, but presenting them differently.

In the previous post, I presented a screen cast explaining, in a very text-booky way, how rainbows form. Today, I am using the same graphics, but I have broken the movie into six individual snippets.

I’m starting out from the schematic that concluded last post’s movie and ask  five questions that you could ask yourself to check whether you understand the schematic:

Ideally I want to link the other five of the movies into the one above, but I haven’t figured out how to do that yet, so here you go for the answers:

What do you think of this way of presenting the material? Do you like it better than the textbook-y movie? I’m curious to hear your opinions!

For both this and the other way of displaying the material, I am toying with the idea of adding quizzes throughout the movies, in a programmed learning kind of way. But considering all the pros and cons, I haven’t made a final decision on it yet. What do you think?

Rainbows and refraction

Why is a rainbow always red on top and blue at the bottom?

We always talk about prisms and refraction and stuff, but be honest – would you be able to explain the order of colors in a rainbow without pausing and thinking first?

Screen shot 2014-07-13 at 3.34.19 PM
Rainbow. Picture taken in Laufenselden in 1996

As I said the other day, I am currently experimenting with screen casts. This is my very first attempt – I didn’t write a script so it is pretty chaotic, I have a cold (which you can hear from my voice), my handwriting sucks, the movie is, at 4 minutes, about twice as long as I wanted it to be – it is not perfect and I will certainly modify it before using it in teaching. But I would be very interested in your feedback so I can improve it!

As you noticed, this is a very textbook-y screen cast. I’ll present an alternative model for the same topic in my next post.

P.S.: As you might have noticed from the watermarks in the video, I have continued experimenting with screen cast programs and am currently using Doceri. And I am very happy with it!

Examinations via Skype.

My experience with an examination via Skype.

In 2012, I taught two lectures via Skype at the University Centre of the Westfjords, while actually physically sitting in Norway. That experience is described in this post. When writing that post, I remembered that I also have experience in doing examinations via Skype. Except that experience was as a student, not as a teacher. In 2011, I defended a Master’s thesis at the University of Hamburg while, again, being physically located in Norway. How did that work out?

Defending a thesis via Skype is not that uncommon these days and actually a very easy, cheap and environmentally friendly way of defending when you no longer live in the place where you studied (or when you cannot travel there for other reasons). The way it worked in my case was that I had two opponents on the call, and since we were all to cheap for the upgrade, we could only hear each other and did not have a video connection. Which made it less stressful for me – when I am video-skyping, I tend to focus on my own video way too much, and thinking about how weird my hair looks or how I should sit in a specific position to block something behind me that would otherwise be visible. This tends to take away brain power from the topic I should be focussing on. Since I knew both their voices, there was also not an issue with knowing who was speaking at any given time (if you are ever on a call/skype with a group of people and there is even one person who doesn’t know everybody else really well: Please make sure to always announce who you are when you start speaking!).

I had to give a presentation, which I did by sending them the slides in advance and asking them to look at specific slides while I was talking about them. Thanks to my friend Nadine who let me borrow her apartment, I had a fast internet connection and privacy. What more do you need?

The only stressful time was waiting for them to call back after the exam when they were discussing my grade, but I guess that is a really stressful time no matter the setting.

So yes – examinations via Skype are actually a good option! No bad experiences here.

On drawing on the board by hand in real time

Drawing by hand on the board in real time rather than projecting a finished schematic?

It is funny. During my undergrad, LCD projectors were just starting to arrive at the university. Many of the classes I attended during my first years used overhead projectors and hand-written slides, or sometimes printed slides if someone wanted to show really fancy things like figures from a paper. Occasionally people would draw or write on the slides during class, and every room that I have ever been taught in during that time did have several blackboards that were used quite frequently.

These days, however, things are differently. At my mom’s school, many classrooms don’t even have blackboards (or whiteboards) any more, but instead they have a fancy screen that they can show things on and draw on (with a limited number of colors, I think 3?). Many rooms at universities are similarly not equipped with boards any more, and most lectures that I have either seen or heard people talk about over the last couple of years exclusively use LCD projectors that people hook up to their personal laptops.

On the one hand, that is a great development – it is so much easier to show all kinds of different graphics and also to find and display information on the internet in real time. On the other hand, though, it has become much more difficult to talk students through graphics slowly enough that they can draw with you as you are talking and at the same time understand what they are drawing.

Sketch of the mechanisms causing westward intensification of subtropical gyres – here the “before” stage where the symmetrical gyre would spin up since the wind is inputting more vorticity that is being taken out by other mechanisms.

The other day, I was teaching about westward intensification in subtropical gyres. For that, I wanted to use the schematics above and below, showing how vorticity input from the wind is balanced by change in  vorticity through change in latitude as well as through friction with the boundary. I had that schematic in my powerpoint presentation, even broken down into small pieces that would be added sequentially, but at last minute decided to draw it on the whiteboard instead.

Sketch of the mechanisms causing westward intensification of subtropical gyres – here the “after” stage – the vorticity input by wind is balanced by energy lost through friction with the western boundary in an asymmetrical gyre. Voila -your western boundary current!

And I am convinced that that was a good decision. Firstly, drawing helped me mention every detail of the schematic, since I was talking about what I was drawing while drawing it. When just clicking through slides it happens much more easily that things get forgotten or skipped. Secondly, since I had to draw and talk at the same time, the figure only appeared slowly enough on the board that the students could follow every step and copy the drawing at the same time. And lastly, the students saw that it is actually possible to draw the whole schematic from memory, and not just by having learned it by heart, but by telling the story and drawing what I was talking about.

Does that mean that I will draw every schematic I use in class? Certainly not. But what it does mean is that I found it helpful to remember how useful it is to draw occasionally, especially to demonstrate how I want students to be able to talk about content: By constructing a picture from scratch, slowly building and adding on to it, until the whole theory is completed.

Q&A pairs

Have students group in pairs, develop and answer questions.

It is really hard to come up with exam questions (or even just practice questions) that have the right level of difficulty so that students feel challenged, but confident that they will be able to solve the questions.

One way to develop those questions is to not actually develop them yourself, but have students develop them. So what I did in CMM31 was to ask students to group in pairs of two and develop questions that they thought would be fair exam questions. So they should be difficult enough that students have to think and employ a lot of what they learned during the course, but they should not be so difficult that they are impossible to answer.

You would think now that students would come up with really easy questions in order to trick you into giving an easy exam, wouldn’t you? There is a way to avoid this: After students have developed the questions in pairs (and made sure they know the correct answer), you can go around the room and have everybody share their question with the rest of the group (see? now having a difficult question makes you look smart!). The rest of the group answers the question, the person who asked the question has to say whether they are happy with the answers, or add to the answers if they feel like important aspects were not mentioned. Plus since there is an instructor in the room, he or she can always comment on the answers.

I usually say I give the students 10 minutes to come up with the questions (so 5 minutes each) and it then ends up being something like 6 or 7 minutes each. Since I’m sitting in the same room and listening in on the conversation, I can adapt the timing so it works best. Then it usually takes about 3 minutes to answer each of the questions so that everybody, including the instructor, is happy. So depending on the size of your group you might want to split the group into smaller groups so that exercise doesn’t take up too much time.

I find that using this Q&A pair method gives me a pretty good insight into what concepts students perceive as difficult, and how well the group as a whole can answer the questions. Since it is not the instructor asking the question, it seems to be much easier for students to throw in ideas (and I make sure that as the instructor I am not standing in front of the class, and when students start talking to me rather than the group, I point out who asked the question and that they should be talking to that person).

It does take up a lot of class time, but it is using class time for concepts that students feel are important and worth talking about.

Long-distance teaching.

My experiences with giving a lecture via Skype.

As I mentioned in yesterday’s post, I taught two lectures at the University Centre of the Westfjords, Iceland, in 2012 while physically being in Norway. How did that work out?

Teaching via Skype is a great option for when travel is not in the cards, be it for environmental, economic or other reasons. But I can tell you – it is a lot more stressful than teaching in person because you miss out on all of the non-verbal clues that tell you whether or not students are following. But I would do it again any time!

Why did it work out well? I think there were several important factors. In no particular order:

1) I over-prepared. I tend to be over-prepared, but in this case I put a lot of time into preparations, and I even talked through both lectures with a friend to make sure they were structured in a way that was easy to understand.

2) I had all the important key words on the slides. I always try to make sure to have key words on my slides so students can write down any weird technical terms that I might use and forget to explain, but in this case I defined everything on the slides.

3) I had an ally physically present in the class room. I think this was probably the most important reason for why things worked out really well and why my stress levels didn’t go through the roof when we realized that the internet connection was too weak for a two-way video. When departing for a research cruise from Reykjavik and visiting someone at their marine research institute, I happened to walk into the lab of the person who was responsible for the course, Hrönn. Hrönn and I clicked immediately and so while I was on Skype talking to the class, I knew I could rely on her to make sure things went well on the other end and to give me all the crucial information that would otherwise not have been communicated – if students got bored, if students looked like they did not understand, if everybody had left the room and left me sitting there, talking, if the connection was so bad people couldn’t understand me, etc.. Even though in the end she did not have to do anything, it helped enormously to know that she was there and would let me know if things went wrong.

4) I introduced myself to the students. I put up a picture of myself, talked about my background, where I was living, why I was interested in oceanography, why I was skyping in to give the lecture. During the lecture, I mentioned examples of how the topic was relevant to my personal life and told stories of my own experiences. Teaching via Skype adds a lot of distance – I tried to still be visible as a person and connecting on a personal level as much as possible.

5) I sent the slides before the call. This might seem obvious, but it really helped to know that they had the slides in Isafjördur already and that in the worst case if the internet were to break down, I could just deliver my lecture via speakerphone.

6) The slides were numbered with clearly visible numbers in one corner. Again, it might seem obvious, but it was really helpful to be able to say “go to slide 16” rather than having to go through “go three slides back, see the diagram? No? Then try going back one more. Still no diagram? I’m talking about the slide with ….”.

7) I made sure I could see the students. Since the internet connection was very slow, we could unfortunately not have a two-way video call for the whole duration of the lecture. But what we did was this: They showed my slides via a projector (thankfully they were numbered!), my video stream was initially, until the connection became too slow, shown on a laptop that was moved to face the class, and I could see the class via that laptop’s webcam. I could only see shapes and not distinguish facial expressions, but when I asked them to nod or shake their head in response to a question, I could see them respond. Next time, I would maybe even try using the ABCD card method or some other way to get more direct feedback in a Skype lecture.

8) We had tested the technology before. We knew what part of the classroom was visible via the webcam so we could ask the students to sit there, we had tested connecting via Skype, we had the telephone numbers on hand as a backup and we “met up” in Skype a couple of minutes before the lecture was supposed to start. But maybe this should go under the “over-prepared” heading.

All in all – I can’t stress the importance of preparation enough, and if you are to teach via Skype: Make sure you have someone in that class that you know and trust to be your ear on the ground to let you know if things don’t go the way they are supposed to.

And have fun! In the evaluation of that course, people explicitly mentioned my lectures as a highlight of the course, and I got really positive feedback. So teaching via Skype might be a bit of a hassle, but it is definitely possible to teach well via Skype.

Melting ice cubes – what contexts to use this experiment in (post 4/4)

What contexts can the “ice cubes melting in fresh water and in salt water” experiment be used in?

As you might have noticed, I really like the “ice cubes melting in fresh water and in salt water” experiment. Initially, I had only three posts planned on the topic (post 1 and 2 showing different variation of the experiment and post 3 discussing different didactical approaches to the experiment), but here we are again. Since I like this experiment so much – here are suggested contexts in which to use the experiment.

1) The scientific method.

No matter what introductory class you teach, at some point you will talk about the scientific method. And what is better than talking about the scientific method? Correct, having students experience the scientific method! This experiment is really well suited for that, because you can be fairly sure that most students will come up with a hypothesis that their experiment will not support.

2) Laboratory protocols.

For courses that include a laboratory component (like mine does), at some point you will have to talk about how to document your experiments. Again, since the hypothesis will typically not be supported by the results of the experiment, this is a great example on how important it is to write down the hypothesis and how you are planning on testing it, and then noting all the observations, not only the one that are along the lines of what you suspected. Also recording the little errors that occur along the way (“someone swapped the cups with the ice cubes, so we are not sure any more which one is which”) is very important, and if you have a class doing this experiment, you can be sure that at some point someone will make a mistake, not write it down and then be very confused afterwards. Great teaching and learning opportunity!

3) Different teaching methods.

If you are teaching about didactical models, this experiment is very well suited for this, too (see my post 3 on the topic and the Lawrence Hall of Science resource). Just have different people work on the experiment using the different methods and then discuss what and especially how people learned using those methods. The Lawrence Hall of Science resource mentions a fourth method (and I didn’t want to give the impression that I am recommending it, therefore I omitted it in my post 3) – the “read and answer” method, where students read about density, stratification and density-driven circulation and then answer questions like “what is density?” or “what is thermohaline circulation”. Again, not recommended for your oceanography class, but adding this option might be very relevant if you are teaching students or educators how to (not) teach.

4) Oceanography and climate

Yes, this is probably the main reason why you are doing this experiment in class. Now you can talk about salt in the ocean. About density-driven currents (and are there other things that drive currents apart from density differences?). About the importance of ocean currents, heat transport, the global overturning circulation, fresh water and many more.

Can you think of more contexts for this great experiment? Let me know! (Depending on your browser, you can comment on this post in the “leave a reply” box below or, if you don’t see that box, by clicking the speech bubble next to the title of the post.)