Testing stuff!!!!!!!!!

Whew! 2 solid days of testing. Monday was spent testing different places we could place the CO2 sensor.
First, I tested with the CO2 sensor on the bottom of the enclosure.

I had expected the CO2 to settle on the bottom and give a much larger reading than the true reading. I knew how much CO2 I was putting in because I was measuring it with a large syringe. I would put the mouth of the syringe in the bottom of a bucket that had dry ice in it for quite awhile. I would slowly draw the gaseous CO2 into the  syringe, and then set the syringe in a vertical orientation so that when it came to room temp, it would push the excess CO2 out of the top end of the syringe. I made some calculations to arrive at 5mL of room temp CO2 was adding 80.3ppm by mass (79.7 by volume) to the container we were using. Anyway, back to my hypothesis. I figured that if I added CO2 to the enclosure (to bring it up to 1000ppm), it would pool in the bottom where the sensor was and give a reading far above 1000ppm. I was half right. If I can put a copy of the graph in this I will. The reading from the grey wolf spiked high every time I put CO2 in (50mL per injection), but after about 6 minutes, it would achieve a steady state.

The steady state value almost agreed with the ppm I thought I was putting in (700ppm compared to the 800ppm I believed i was putting in), but I had not anticipated this. My interpretation of the spike goes along with my initial hypothesis. The CO2 is just really heavy compared to air. However, I thought it would stay at that high level. I was really surprised that the CO2 diffused throughout the enclosure.

Next came the middle test where the sensor was located vertically in the middle of he chamber.

I kept the 50mL addition the same every time, but the initial spike from the low test was smaller, and the time to reach equilibrium was much longer, about 15 minutes.

Even though I was adding 800ppm every time (from calculations based on the syringe), the steady state averaged out to about a 500ppm increase each time....hmmm.

High test

The grey wolf was put at the top of the container as well as our sensor, to do some initial voltage measurements.

I was concerned about this last graph, because I thought something had gone wrong. I couldn't find anything wrong with the set-up, so it must be accurate. As Julius Sumner Miller stated "Experiments do not fail, you must provide Nature the necessary provisions, or she won't do what you want her to do."

We added 160ppm at 3:51 and got a reading of about a 21ppm increase 3 minutes later.

We added 320ppm at 3:54 and got a reading of about a 400ppm increase 8 minutes later.

We added 320ppm at 4:03 and got a reading of about a 260ppm increase 7 minutes later.

We added 480ppm at 4:10 and got a reading of about a 410ppm increase 6 minutes later.

We added 800ppm at 4:16 and got a reading of about a 630ppm increase 11 minutes later.

The ideas of what was going on was starting to coalesce in my head. I had imagined the CO2 like it was water, sinking to the bottom of the container and staying there. That was the model in my head of the system. To paraphrase Kenn Amdahl, when pretending the universe is like a watermelon, with the seeds like stars, don't forget that the universe isn't actually a watermelon. What I had failed to account for was that CO2 is a compressible fluid and water is not. This new-found realization crystallized and it made sense of the data. What actually was occurring when CO2 was added, was an initial plunge as the CO2 dropped to the bottom of the container. As the gas diffused, it also set up a density gradient, with the highest density at the bottom, and decreasing as you go vertically upward. That at least partially explains the discrepancy between the ppm injected and the ppm measured. I need to do a little research a little into this phenomenon. The fluid mechanics class I took was a long time ago, and I'm not sure if we covered compressible fluids.

Dave out