Guatemala, Day 18

Well the volcano eruption turned out to be pretty serious, but it seems that the devastation is contained to the villages directly in the line of ash flow. I'm well over 50 miles from the volcano so there's nothing to see here. The locals are all hustling to take relief supplies to the affected areas. People are donating cash and goods, and pickup trucks are being loaded with supplies and driven down. Its a pretty impressive mobilization effort. These are good, good people.

Lab has continued to be a lot of fun. Earlier this week the students finished building their variable power supplies. The supplies convert wall AC into a DC source that can go from 1-29V and source up to 2 amps. We use a full wave rectifier, some big smoothing capacitors, a linear voltage regulator, and a 2A fuse, just to be safe.

The finished product! A variable DC power supply.

The lab took forever, because getting all the connections soldered correctly and safely is a time consuming process. It was getting really late and I was getting really hungry, and so it was a blessing when there was a city-wide power outage and we were forced to pack up in the dark and go home.

Engineering in the dark.

We used our power supplies to recharge some NiCad batteries yesterday. Students had to work out a series of equations in order to satisfy the following requirements:

1. Battery has to charge at a reasonable speed
2. In order to charge the battery at that speed, you need voltage and current. Both those elements have to be in the range that can be delivered by our homemade power supplies
3. There needs to be a current limiting resistor. You can't dissipate more power in the resistor than its max rating, or else it'll burn up
4. You can reduce power in the resistor by reducing the current flow, but then your battery will take longer to charge

Eventually each group settled on a series of parameters and we connected up their batteries and waited for them to charge. No one started any electrical fires in the process, which was certainly appreciated.

We hold our classes in a community work space, where anyone can rent out space. One of our space-mates is a woman who works with a Michigan-based non-profit that builds and places water purifiers around the world. She showed us an example of one of their water systems. It was super interesting! The group is called Clean Water for the World (follow that link to read about what they do and maybe even donate a few bucks). The system is pretty simple (by design) - a thick paper filter followed by a UV light tank that kills all the biologicals in the water. It can do up to five gallons per minute and can be installed in a school or other community center. The specific one you see in the picture below will be installed at the volcano rescue and relief center this weekend. They have 270 of these suckers in use around the world!

The system has a built in counter that lets the locals know when its time to change the UV lightbulb. Units get delivered with enough filters and bulbs to last for two years, after which time the local communities must start buying their own.

Another fun experiment we did was testing to see what percentage of the air we breath is oxygen. This is a useful skill because a common piece of medical equipment in the developing world is the oxygen concentrator. These gizmos pump pressurized atmospheric air into special "zeolite" tubes that sequester nitrogen and allow mostly pure oxygen to pass through. How do you know if your concentrator is working? Well you have to test the percent oxygen that gets produced. We didn't have an oxygen concentrator to work with so we just tested atmospheric air. Here's how the test went:

1. Fill a glass with the gas you intend to test (regular air in our case)
2. Stick a candle to the bottom of a tub, and then fill the tub with some water. Light the candle
3. Flip the glass over the lit candle, and submerge the lip of the glass.
4. As the candle burns the oxygen in the glass, it will suck water up into the upside-down glass.
5. The more water gets sucked up, the more oxygen there was in the glass that had to be displaced as it burned.
6. If you measure the percentage of the glass that fills up, that tells you the percentage of the air that is oxygen

Insanely, this nonsense actually worked, and we found that our glass sucked up 20% of its volume with water, which is roughly the percent of oxygen in the air we breath. We tried doing a follow-up test where we filled the glass with only exhaled air (which is a little lower in oxygen than atmospheric air) but alas our system wasn't sensitive enough to notice the difference.

If you look closely, you can see that there is some water that's been sucked up in to the glass

That's pretty much all the fun news to share. Here are a few other tidbits you might enjoy. Today we were testing whether an IV infusion pump was properly working and we needed something to function as an IV bag for the test. My students came up with a pretty cool solution:

Yes, that's a rubber glove filled with water and tied to the tubing using a hair band.

Next tidbit - I took apart a potentiometer to show the students how the magic inside works. Turns out, its just a strip of resistive material...

This is the guts of a potentiometer. There is a wiper (which I pulled out) that connects a point on the outer ring to the smaller inner ring that's connected to the middle contact.

Tidbit 3: here's the teaching staff. Me, Paul, and Luis. Paul teaches computer science at the University of Kansas and is on his third trip with EWH. Luis is our logistics coordinator, which works out nicely since he's a native Spanish speaker.

Finally, here's a shot of most of our group on our first hospital visit two weeks ago.

We're going to a community health center tomorrow - should be an interesting site visit. Stay tuned for more pictures and stories.