New Results

Latest solar trough configuration:

A few details:

1. Those are just simple, band-sawn parabolic arches. The mirror is attached with velcro. Works awesome. Ly.
2. I suspended the mirror from the pipe thinking that would keep the focal length constant while allowing simples changes of elevation. Why do I always go straight for the complicatedest solution in the universe?

And data from same:

The highest temp on there is almost 140°C. That's over 280°F. According to my rough calculations, the four steepest upward slopes indicate powers in the range of 20-25 watts. (Some of those dropouts are me messing around with the setup, some are cloud cover. Also, I later discovered that true solar south is like 15 or 20° east of where I've been pointing.)

Two big changes from the last run, other than the already-mentioned one of shortening the excess pipe.

1. Painted the pipe flat black.
2. Used a laser pointer to adjust the focus. I mounted a frosted glass square (I happen to have a bunch I bought for just such a use as this) at what I thought would be the focal length. Then I stood back and aimed the laser pointer in a roughly perpendicular way and looked where the point fell. This really needs a System to keep it perpendicular, but anyway I was able to determine that my focus was off by over an inch.

I'm thinking the Mark III will be the last iteration. Pointing in the right direction, and simplifying the pointing a little, may let me add another 10-20 degrees to the peak but that will be about as far as I can go with these simple materials and using an open-air design.

Belated Mirror Squeezer Results

I did one "live" run of data collection from the parabolic mirror squeezer, but after I did it I realized there was a major problem. The pipe was much longer than the mirror width, so it overhung. Too late, I realized that this meant there was unheated oil in there, making the temperature measurement invalid. And I don't even know what direction the error was worse in, since the overhang could also act as cooling fins. Anyway, here's the graph of the results:

It looks very similar to the original run in a hotbox:

One major difference: Check the x-axis. The parabola run is at least 4x steeper.

Anyway, if I get some sun this weekend I should be able to re-run with more accurate results.

Things That Don't Work vs Things That Do

I've never been completely happy with my temperature logger. It's a bit fragile in the sense that if anything at all goes wrong, and there's no way to tell that at the time, I lose the entire run. I lost a run Memorial Day weekend.

Instead of storing the measurements on the Arduino, I'd like to instantly beam them onto my computer far inside the safety of the house. That way I can track things realtime as well as be assured that I have them. Coincidentally, for my birthday, I got both another Arduino¹ and Making Things Talk.

The book describes a great number of schemes to make microcontrollers talk to each other and to computers. You can use wireless networking, bluetooth, XBee, etc (I have only the vaguest notion what some of these are). Naturally the easiest protocols require the most expensive hardware. I only need one way, slow communication, so I got a simple RF module.

Claim:

It works just like a serial port! Just connect the transmitter to the TX pin and the receiver to the RX pin! It Just Works(tm)!

Reality:

No.

Perhaps my unit was faulty. I found many tutorials and guides across the internets and while results varies, I can't ever really say it worked. I did see data appear for a short time, but mainly what I saw was noise. Or nothing at all, which is even less explicable.

Last night I had a brainwave. Or brainstorm. Something happened to my brain and it resulted in an idea. Why not use a wireless laptop as the go between? The kids have these OLPC dealies. The laptop has a USB port and does WiFi. I have a WiFi router (specifically purchased, used, to work with these laptops). About 30 minutes and 10 lines of Python later, I was reading values from /dev/ttyUSB0 and sending them out over a socket to my desktop to another 15 minutes and 20 lines of Python.

The guts of the entire scheme are already there. But with so much success so fast, I'd like to add features. For instance, instant graphing of values on both ends. A protocol so that the laptop knows if there's been an error and can tell me, out in the field. When I have more than one sensor, I'll need a way to indicate which sensor had what value. It'll be like a complete Science Sensing Station!

¹If you are at all interested in robots, sensors, controlling stuff with computers, electronics or just plain messing around, I highly recommend the Arduino. That SparkFun item is all you need, assuming you have a USB port (and possibly a cable). Well....you may also need some external electronics, depending on what you want to do. LEDs, resistors, motors, etc.

What Price A Stamp?

So stamp prices are going up again. And by "going" I mean "went, like 5 months ago". The wheels of Project Potpourri grind slowly, but they grind exceedingly fine.

Now they have a Forever Stamp. You buy it today and it will be good until the end of time regardless of future price increases. Sounds like a great way to save literally hundreds of cents, right? My wife (or maybe it was me--these human details are so much dross) posed this question: If I buy a 41 cent Forever Stamp in 2007, am I really saving money vs a 53 cent stamp in 2020 or does it come out in the inflationary wash?

IIRC, inflation is something like 3% per year, but of course we don't have the information on what future stamp prices are going to be. What about history? How closely have stamp prices tracked inflation in the past? Using a history of stamp prices and relative dollar values since 1913 I was able to answer this question. (Notes: All this is based on the first ounce only. I didn't try to go back farther than 1913. For each year, I only use the average dollar value for the year, I didn't break it down by month. For years with more than one price increase, I only report the last one. Since 2007 isn't over yet, I used the last dollar value available, which is September.)

Year	Apparent price that year	Stamp price in 2007 dollars	% diff relative to today
1917	0.03	0.49	16
1919	0.02	0.24	-70
1932	0.03	0.46	10
1958	0.04	0.29	-42
1963	0.05	0.34	-20
1968	0.06	0.36	-14
1971	0.08	0.41	0
1974	0.10	0.42	3
1975	0.13	0.50	18
1978	0.15	0.48	14
1981	0.20	0.46	10
1985	0.22	0.43	3
1988	0.25	0.44	6
1991	0.29	0.44	7
1995	0.32	0.44	6
1999	0.33	0.41	0
2001	0.34	0.40	-2
2002	0.37	0.43	4
2006	0.39	0.40	-1
2007	0.41	0.41	0

It used to fluctuate quite a bit, but for almost the last 30 years, they've been within a few percents of the same price. And given that even a 10% difference is only $.04, I'd call it fairly constant over the last 100 years (except for a few years).