Sunday, October 6, 2013

Plastic Permeability Experiment the iScience Way

When I moved into a house only 12'x18' small I had to go paperless. Instead of notebooks on a shelf I put everything I might want to refer to later on my blog. Easier to search and retrieve.

Last week I wrote a blog about Dr. Means and his attachment to paper in his pursuit of science. And I also addressed questions from his video on National Geographic showing him holding a Ziplock sandwich bag with three small toads inside. It made me curious about these bags. Why don't the frogs suffocate? So I did some experiments. Friday night I tested if the zipper parts leak. I used smoke as a test because I can see it. (Link to Friday test with video) The zipper didn't leak. So that means oxygen must be going in and carbon dioxide must be going out, right through the bag. I don't have any way to measure respiration and permeability, but I thought I could come up with something that would at least produce some gas. I thought about hydrogen peroxide and a potato. That makes oxygen. But I didn't have a potato.  And it might make the gas way too fast. I wanted something pretty slow, like a frog breathing. I do have yeast and sugar. That makes carbon dioxide, and if I use a small amount and don't get it too hot it will be fairly gradual.

Back in the olden days my science teacher was very strict about lab notebooks. They had to be kept in pen, not recopying them for neatness, all original. That's probably still the preferred method because you can't fake it. But I'm not that rigorous and I do everything with my iPhone. I have a complete record to refer to later if I want to. But mostly I do it because it's easy. What's the point of doing science for fun if you make it too much work? I'm going to go through my experiment with you the iPhone way. Realize I'm leaving out a lot of photos for the sake of it's boring.

125 ml (1/4 tsp) instant yeast in 37ÂșC (100°F) tap water
(I would use it hotter for bread but I didn't want
it to go too fast.)
Equal amount white sugar

Stir it up

Put the dish of yeasty water and an equivalent dish of plain water
in baggies and zip them shut. The control is because the air inside
will expand just from heat so I need to isolate that effect from the
carbon dioxide coming from the yeast.

Start the timer. This screen shot shows the start time, 11:36 am
Now I just let the stopwatch run and take a screenshot
every time I take a measurement. I just went about my business
and checked it whenever I felt like it. Science is not the
boss of me. I'm not trying to calculate a rate
 or anything, I just generally want to know what's going
to happen.

First observation was after about 20 minutes. Screen shot the stopwatch.
It has the actual time as well as the elapsed time in the shot.
How convenient!
The baggie with the yeast in it was noticeably puffier than the
control bag. I realized I needed a way to measure this change.
I made up this haphazard way of pinching the top of the bag and
holding a ruler to the zipper line. I should have done it at 0:00:00
but I didn't think of it. Should have equalized them to be identical.
This is why real experiments get done over again.
Control bag with just water lets me pinch 1/2" more than the
yeast bag. This bag is not filling up with carbon dioxide,
the other one actually is.
I was sitting on the front steps making notes (on my iPhone) and
realized I could smell the yeast. Pretty sure that means carbon
dioxide is permeating the plastic. Smells like a beer. Other bag
smells like a bag.

I checked several times during a rather hot afternoon.
I could plot my little measurements if I wanted to. Maybe I will.
Skipping ahead to the end here.
To compare both bags. Condensation in each. Yeasty one still puffier.

It was a bit cooler by now. Control pinches down to
1 3/4"

Experimental one is only pinchable to 3/4"
Minimum at the hottest part of the day and peak
yeast output, about 1/2"
I brought the experiment inside for the night and
checked it again this morning. 
Yeast isn't doing anything now. Bag is pinchable to 1 1/2"
Temperature in the house about 25°C 
Control pinchable to almost 2"
I keep my notes for the experiment right on my iPhone
(It goes longer than this, have to scroll.)
Notes are immediately synced to my computer
and iPad so I can work on notes wherever I want.
Note typos where it says "on" when I meant "one" and
"puffer" when I meant "puffier." This is real,
Mrs. Burns!

So that's how I document science experiments with my iPhone! It's way easier than a paper log book. If nothing interesting happens I don't have to put it on my blog. I develop recipes the same way. Take pictures of the scale and the measuring cups as I'm working, then I have that to refer to when I'm writing it up as a recipe with notes saying it's too sweet or too sour so I can use different proportions next time.

As for the permeability of zipper bags to CO2, I definitely think they are permeable. Overnight the control didn't contract where the one pressurized with carbon dioxide contracted enough to allow another 1/2" of pinching (+50% the previous amount of pinching, 1" to 1 1/2"). I think carbon dioxide definitely got out of the bag.

This doesn't say anything about oxygen getting in the bag, but carbon dioxide getting out would be pretty important to anything breathing inside there. I'm no biologist, but I read WWII adventure books about sailors in diesel submarines so that's the basis for that totally non-expert statement.

I'm glad I did the little experiment because I really wasn't sure what would happen. If I'd had more identical little dishes I would have done more bags with increasing amounts of yeast until I got one to blow itself open. I probably could have mixed the yeast right in the bottom of the bag, but somehow that seemed messy. I liked that my little sushi-dipping dishes held the bag upright.

Just for the sake of seeing how fast I could do it, I made a chart. I used Google Drive. Just like Notes on iCloud it's automatically available now on my iPad or to any of you online if I give you the link. I did it really fast. Spreadsheets are my bitch. Here's a screenshot.

I'm not really sure what my 8th grade science teacher would think about all these ways to create data that literally anybody can alter. I mean, I just clicked a button that says "smooth" and got these lines to look like this. I don't know what it's doing. Since all of this is arbitrary and made up it doesn't matter. But for a real experiment that others will want to replicate or confirm I wouldn't feel comfortable with a tool on the computer like "smooth." I would need to be able to explain the algorithm it was using for that smoothing. 

I think the lesson is that all you see anymore is charts and most of them are not meant to be taken as full scale or even based on an accepted measurement. Inches of Pinch is not a legitimate measurement of pressure. I made that up. Realize these things are only relevant as they relate to changes between the control and the experiment, no relation to the outside world. If I want it to indicate pressure I should probably invert the whole thing to indicate an increase in pressure with a decrease in pinchability.

But my back hurts and I want a snack. I think it's fine if random hacks like myself can do some Saturday Science and whip up a half-assed chart all with software that's free and post it on a blog that's free and give it to you to enjoy, for free. Hey, if somebody reads it, does that make it peer reviewed?!


  1. Sailors? I think they highly prefer the appellation "submariners".

  2. I feel like I would remember if my grandfather referred to the men in submarines as submariners instead of sailors. He mostly called them by their rank. Plebes, midshipmen, or specifically Chief or Captain, but I understood the term for anybody in the Navy was "sailor." All of the books in question are in storage though so I can't check just now.

  3. Love the experiment. Just the act of doing it is not only a great learning/teaching moment, but it's fun. As for the sailors, my step-father, Navy Captain H.C. Hunley (yes, THAT Hunley, speaking of submarines) called those below-the-surface swabbies 'knuckleheads'. But that may be the unofficial term for them!