Invention Idea #2

Red increases the pulse and heart rate, and raises your blood pressure. It increases the appetite by increasing your metabolism, which is why red is such a popular color in restaurants.

("color red metabolism" gets hits mainly for People Magazine-level science info, so I don't know how well-established or bunked this is.)

The office across the hall from me just got "natural sunshine" fluorescent bulbs put in. They are much whiter than the regular bulbs, which are reddish-orange. It is causing me to wonder what effect constant immersion in red light is having on office workers. Obesity? Or is the above effect only a differential one? Has this even been studied?

Anyway, my invention: blue-tinted "diet" sunglasses.

Forming Acrylic Mirror

I think all this information exists in previous entries, but it's nice to have all my knowledge, however little that is, dumped into one spot.

Heat forming acrylic (aka plexiglass) is pretty simple. Just get the temperature up around 220°F and it's pliable. I put it into the kitchen oven. Don't rely on the oven temperature, though, use something with a probe to tell if you've got the real temp. For one thing, you can put the probe right down where the mirror is, not just floating around in the air inside the oven. Also, when I was researching this I found some warnings about fumes. I think that's only if you overheat because I never smelled or sensed anything. Maybe I've silently shortened my life by 20 years.

I tried a couple different methods of using a form. The first couple tries were "open-faced," meaning I laid the acrylic on top of something and counted on the weight of the material itself to cause it to sag into the form. It isn't really heavy enough for that, so I moved to a two-part form. It takes longer to heat that way because of the mass of the form itself. Another disadvantage of this method is that you have to be able to force the mirror into shape before you heat it, which means no 2D curves (like a bowl, if you see what I mean). Actually, you probably could do that by putting the acrylic sheet between the halves of the form and then weighing the top part down. Like the open-faced method, but with extra weight. Drill a hole in the form so the temp probe can sit right on the acrylic.

The above should give pretty nicely formed acrylic sheet even in complex shapes. Unfortunately it's totally unworkable for acrylic mirror. I'm not sure if it's the acrylic or the mirror backing, but the heat stresses the material such that you don't get a smooth reflection anymore. For a while I thought it was imperfections in the form, but various experiments ruled that out. (You can get bumps and ridges from a rough form, though, which can be eliminated by loosening the form a little or by lining the form with some kind of bumper material. I used a sliced up silicone baking sheet.)

Also: I never got far enough to have this problem, but eventually you'll run out of space in the oven. Multi-part forms? Some other heating method?

The curve I wanted wasn't 2D, so how about cold forming (aka "bending")? I have not yet determined the point at which the acrylic cracks or deforms. I haven't even determined if there's some point short of complete destruction where the mirror breaks down, a la the heat deformation. It seems to be pretty sturdy and stable, but then I'm not making tiny radius curves. If you imagine bending a yard stick, that's about what it seems like, or maybe a little stiffer. Put another way: I have a square of acrylic mirror that's 2'x2' bent into a curve about the same as the side of a 55 gallon drum and I haven't see any problems with it. (This is the 1/8" thick stuff. There's also a 1/4" thick stuff that I've never tried.)

The trick with cold forming is that you have to hold it in place somehow. Even drilling a hole through the acrylic will deform it, although only in the immediate area surrounding the hole. I started with a sandwich method--cutting two parts and then cramming the mirror between. That works, but you have to have some method of securing the bread of the sandwich that doesn't involve drilling through the mirror. Also, you cover up some portion of the mirror surface.

Of course, you can drill through the mirror, but in that case why even have the top piece of bread? That's what I switched to: Put the mirror on an open-faced form and drill a few well-placed holes to secure it down. Don't overtighten, because the force isn't distributed over a wide area like with the sandwich, so the local area can get quite deformed. Maybe a reason to have the top piece of bread after all. Or maybe just some rubber washers or silicone baking sheet for padding.

You can't get too fancy with this method, but for a single, relatively gentle curve it works great.

Stirling Walking Beam

I made a Stirling engine before, but it wasn't "real" in the sense of "having traditional engine parts, like a flywheel and crankshaft". Also, it would be pretty difficult to scale that one up or extract power from it as is. So this is my first regular ENGINE engine. (This is called a "walking beam" engine--many other configurations are possible.)

One reason I built this one was to prove to myself that I really understood how they worked. For that reason, I designed this all myself. Not that there's SO MUCH there. Also, there are tons of videos on YouTube that are identical to mine.

It's be really awesome to scale this up. Like with an oil drum for the displacer cylinder. I'd start that right away, except not only do I not have an oil drum, I'd need access to machine tools to make the power cylinder.

Tips

• With the power piston shaft and displacer piston shaft mounted on the same point, getting distances right is a little tricky. It's a big parallelogram this way. Make them separate next time. Maybe even on a rotating collar so the phase angle between can be modified.
• If the shaft tiepoints have a lot of play in them, the engine works jerkily if at all (because all the motion is taken up in using the play).
• I think I overdid it on the height and underdid it on the width. Could have used a little angle-reducing distance on the piston shaft. Alternatively, shorten the stroke.
• The flywheel is a little heavy. The momentum should carry it through the compression stroke, it shouldn't have to be barely sucked in.

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.

Syringe Stirling

I just realized I've never done a post on Stirling engines. How can that be?? Stirling engines are so, so awesome.

Stirlings are a member of the class called "hot air engines". As a group, hot air engines work by exploiting the expansion and compression of a gas when it is alternately heated and cooled. There's a good explanation/animation here.

(That particular engine is a "true" Stirling because of those green cross-hatches. That's a "regenerator" that vastly improves efficiency. However, most people call all generic hot air engines, regenerator or not, "Stirlings".)

Stirling engines are awesome for a lot of reasons, but one of the best reasons is that you can use any source of heat. This isn't an internal combustion engine that can use only one type of fuel that's later impossible to wean away from. This is an external combustion engine. You could use natural gas or wood or ethanol or solar energy or geothermal energy. In fact, some satellites/probes use Stirlings to convert nuclear energy to electricity. That's right, Stirling engines in spaaaaace.

Anyway, back to Earth and me. Because Stirlings are so simple and efficient, you can make even a really crappy one run fairly well. They are very common first projects in machine tool classes, for instance, whereas building an IC engine from scratch would be...challenging for a beginner. However, I'm not even at the level of machine tool student, so even that option isn't open to me.

Fortunately, lots of people have been coming up with plans for engines that don't require machining. A few years ago, I tried one that used water-based pistons but I couldn't get it to run. I really have no idea what the problem was, probably multiple things. More recently, I found some "plans" for a test tube Stirling. It took me a while to actually get it going, because it turns out that glass syringes are incredibly finely made and also very rare nowadays. I actually had to buy one on ebay as an antique!

I think the functioning is clear enough to explain itself. (Note: This is technically not a Stirling since I have no regenerator, although I could stuff a bit of steel wool in there to fix that.)

After viewing the video again, actually that might not be so clear. Here's what is happening:

1. Candle heats air inside left end of test tube.
2. Air expands, pushing "piston" up.
3. Marbles roll to the left, displacing the air to the right.
4. Air cools down, contracting.
5. Pulling piston down and causing marbles to roll right.
6. Air displaces leftward.
7. GOTO 1

The Parable of the Parallel Parabola

OK, so....man, I wish I'd been detailing every little step so I wouldn't have to regurgitate it all up in a huge mass. I'll try to make this short.

First of all, I used my calculation to make a simple parabolic reflector. I just plotted it out on graph paper and then set a few nails as guides to hold the mirror in place. This actually worked really well. (Even more surprising in light of how poorly the (first!) oven-formed one came out. More about which below.)

The one on the right has a black dot where I pre-calculated the focus to be, the one on the left is just a different focal length.

Now then. Having a single strip mounted with nails isn't that useful to me, so I want to mass produce these. Can't use the nail thing as a form since it'll just bend unevenly. I spent quite a few days trying to figure out how to make a jig that would cut a perfect parabola, but it was too hard (I still have some ideas on that, though, but that's another 2 or 3 posts). (And before you tell me, I know all about the T-square and string method of drawing one.) I eventually decided to just freehand follow a line.

So I had my shop assistant cut a parabola for me and I sandwiched the mirror in there. (My shop assistant is my father-in-law down the street who actually owns a bandsaw.)

(Other item of note: I originally wanted to have the mirror soften and sink down into shape, but that creates alignment problems. Instead I clamped the bendy strip cold. But that means it's hard to tell when I've reached temperature. So I put a probe down into the coldest part of the thing. The tip of the temperature probe is resting right on the mirror, so when that gets up to ~210°F, I can stick a fork in it. This takes like 2 hours--wood is a really great insulator, unfortunately.)

(Oh also: You can't see it, but there's a little alignment peg sticking out of the convex part of the form. There's a corresponding hole in the concave part so it can stick through. There's also a hole in the middle of each mirror. If I put each mirror on the peg, then after I'm done with all of them, I can line them up perfectly. So clev.)

The result: Not so great.

How could that possibly be? How could a few nails hastily thrown together at a few points make a better parabola than a careful, full-contact form?

Then a phrase floated up out of the darkness¹. The curve parallel to a parabola is not another parabola. Just think about that for a minute. If you have a parabola and you want to make a curve parallel to it, you can't just take the same parabola and shift it up. Nor can you use some other parabola. (Read the gories yourself, it's pretty cool. If you like that sort of thing.)

So if you cut a parabolic form and sandwich it around a mirror, FOR EXAMPLE, then you are probably going to get the wrong shape because the two halves want to be parallel (i.e. separated by the thickness of the mirror) but can't. Wellity wellity wellity.

I took the equations in that paper and made a little program² that would generate an SVG file of the shapes I wanted. Now I can take those back to my shop assistant and have him cut it out again.

(Note to anyone who actually reads this far, runs the program, examines the output and starts wondering: The curves aren't really all that different. I think the issue isn't so much that the curve is wrong, but that the poor alignment doesn't provide even pressure across the entire mirror. So it ends up wibbly-wobbly rather than smooth. Then again, the freehand wood parabola isn't all that smooth either, so maybe THAT'S the source of the error. The nail method at least creates a smooth curve, even if it isn't mathematically perfect.)

¹I think it came from Practical Conic Sections, a really rip-roaring tale that I've been reading to the kids at bedtime. But seriously, it's very clear and pretty practical.

²

#!/usr/bin/python

# p1 and p2 are parallel to the parabola, i.e. a constant distance
# away *along the normal to the parabola*.

# For a curve C with generated by the function y = f(x), the parallel
# curve C' is given parametrically by:

#                 y'
# X = x - k -------------
#           sqrt(1+(y')^2)
#
#                 1
# Y = y + k -------------
#           sqrt(1+(y')^2)

# where k is the distance of the parallel from the curve.

# For derivation, see "The Curve Parallel to a Parabola is not a
# Parabola" by F. Max Stein.

import math

print '<?xml version="1.0" standalone="no"?>'
print '<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"'
print '"http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">'
print '<svg xmlns="http://www.w3.org/2000/svg"'
print '    width="8.5in" height="14in">'

focallength = 2.5
a = 1/(4.0 * focallength)
mirrorwidth = .125

vertoffset = 7
horizoffset = 2
phorizoffset = 2
prevx = 0
prevy = 0
pprevx = 0
pprevy = 0
first = True
x = -5.5
while x <= 5.5:
    y = a*x*x
    px = x - (mirrorwidth * 2 * a * x)/(math.sqrt(1 + (2*a*x)**2))
    py = y + (mirrorwidth * 1)/(math.sqrt(1 + (2*a*x)**2))
    if not first:
        print '<line x1="%.2fin" y1="%.2fin" x2="%.2fin" y2="%.2fin" style="stroke:black;stroke-width:2"/>' \
              % (prevy+horizoffset, prevx+vertoffset, y+horizoffset,x+vertoffset)
        print '<line x1="%.2fin" y1="%.2fin" x2="%.2fin" y2="%.2fin" style="stroke:red;stroke-width:2"/>' \
              % (pprevy+phorizoffset, pprevx+vertoffset, py+phorizoffset,px+vertoffset)
    prevx = x
    prevy = y
    pprevx = px
    pprevy = py
    x += .125
    first = False

print '</svg>'

Miscellaneous

1. I had a long, long thing here about how following my plan was so easy, but it kept reading like Tighter Buns in 30 Days While Eating Pizza, so let's leave it at this: Down by almost 27 pounds.
2. Way back when, I did some solar experiments. I said I'd come back to that. I'm still working on that. The problem is, the design I came up with is kind of crappy.

   What I want is a parabolic trough mirror focused on a central pipe. I've tried using mylar sheets on various surfaces before, but it didn't come out too well. This time I tried strips of mirror laid in a wooden parabolic form. I haven't tested it yet, but it doesn't look too convincing on the workbench. Lots of gaps, not much total area, not well focused, etc. (No picture, because seriously.)

   While I was wondering what to do about all this, I came across this video. The guy comes across as a little infomercially, but his ideas look pretty good. In particular, I didn't know you could "drape form" plexiglass (aka "acrylic") mirror. That changes everything! Almost zero work and much higher efficiency.

3. Which brings me to the third misc item. WhereTF do you find acrylic mirror at a reasonable price? I've found it as low as $4.50/sqft, but you have to buy at least $50, not to mention shipping. Plenty of ebayers, but the price with shipping never comes out lower than ~$12/sqft and you have to buy several sqft to get that. Lowe's can special order it, but you have to buy 5 48"x96" sheets and it's still $8/sqft. McMaster-Carr, despite their awesome website, doesn't reveal shipping information even if you ask a live human being, which, HELLO.

   This kind of mirror is used in a lot of children's products because it's shatterproof, so I've considered repurposing a baby mirror, but the cost is still pretty high there due to packaging, frames, etc. I've even wandered around Home Depot and Lowe's to see if I could find a bathroom/decorative/whatever acrylic mirror on some other product. The sole success was a really, really crappy medicine cabinet with attached acrylic mirror. The whole unit was $12 and the mirror was 2 sqft.

   I would just go with that, but the fact that it's attached to something else only proves that I should be able to get the mirror alone for cheaper. Also, I hate to buy something specifically so I can throw it away. In the mean time, I ordered a set of these to experiment with. With the shipping, even amortized over several other items in my order, the price per sqft comes out at lalalaicanthearyou.

Tornado in a Box

1: Cut a hole in a box
2: Put your.....no

1: Make a large, square(ish), cardboard tube. Mine is about a meter high and maybe .25 m x .25 m at the base. This is actually two boxes taped together. They aren't even the same size--I just blocked the holes with cardboard and duct tape.

2: On each side, make a slit near the right edge. Or the left edge. But the same for all 4 sides. It doesn't matter which you pick, since you can reverse it by flipping the tube end for end.

The exact width and distance from the edge don't matter too much and you can see I wandered all over the place. Hey, cutting cardboard is kind of hard!

3: Boil some water inside. I went to WalMart for a hotplate but the cheapest one was $20. I tried it on the stove, but that's dangerous and it was hard to see. Then I thought of the bottom of the rice steamer.

Position the tornado box under a light to maximize the reflection from the droplets.

We found that when the steamer was going full....steam, there was too much steam in there swirling around (steam steam steam). So if you turn it on and off every few minutes it might work better. Also, we tried using a steam humidifier but we got nothing at all. I think the steam jet might be coming out too fast and hot. (An ultrasonic humidifier probably has better visibility, but since it isn't hot you'd be missing another vital ingredient.)

The payoff at the end: I asked the Numbers, now that they'd seen a tornado being made, when and where would hurricanes be most likely? In the winter at the North Pole or in the summer at the equator. Ooooooooh, I get it! they said.

The Temperature of What?

So I had all these posts about temperature logging and one post with the actual logged temperatures...but the temperature of what?

A solar hot box!

You probably already know what this is, but just in case you don't: It's basically a tiny greenhouse. Or like a car left in the sun at noon in August. Only it's even hotter, since it is insulated, painted black and pointed right at the sun.

Inside the box I put a jar with 250 ml of cooking oil and poked a hole in the lid for a temperature probe. That's what these temps are.

Why did I choose cooking oil? Because I didn't want evaporation to be a problem. For one thing, it would fog up the inside of the glass. For another, it would cap my max temperature at 100°C (not that that turned out to be a problem in this case). And lastly, it would change the amount of water in the bottle and I needed that to be a constant because I did some calculations with it.

Knowing the amount of oil and the temperature change (plus looking up the specific heat of vegetable oil), I can calculate the rate at which energy is entering the oil. For the above graph, I got 2.5 watts for the steepest part of the curve. However, I see that the site I just linked to has the specific heat of veg oil as 1.67 kJ/kg K and I was using 2.5. So maybe the power is really more like 3.7 watts.

Knowing the area of the collector I can also calculate the amount of power falling into the box. That's about 75 watts. So the end-to-end efficiency was only about 3-5%. Not that great.

Imagine if you put a cup of water on the table and then turn the furnace thermostat up to 90°. How much energy are you going to waste before the water gets hot? This illustrates the 3 main problems:

1. Air passively surrounding a container of liquid isn't going to heat it very fast.
2. There's a lot of volume of air being heated uselessly.
3. During all this time, heat is escaping the cracks, windows, chimney, etc. In the case of the hot box, the glass front gets very hot and is radiating a lot of the energy right back out.

If a hot box is like an oven, the next version will be like a microwave. Don't heat up the air, just beam energy right into the substance.