Thursday, August 21, 2008

Accelerated Aging - Getting Bad Data Even Faster, Part 3

UV light is a very common culprit in degradation of plastics as these photons have enough energy to cause problems if they are absorbed by the polymer. E = h c/l so shorter wavelengths have higher energy. So the shorter the wavelength, the better, right?

Unfortunately this is also not correct for two reasons. First, the shorter wavelengths become increasingly rarer on the surface of the earth thanks to all the ozone up high. So going with lots of really short wavelength is unrealistic. But secondly, different plastics react differently to the spectra. A plot of degradation rate vs. wavelength will show a peak and it is seldom at the shortest wavelengths. This plot is known as the activation spectra (see Andrady's article for a good review) and is key to running good tests. Polypropylene for example, has a peak at about 350 nm, much higher than other plastics.

UPDATE: The formula for energy has been corrected. I was so excited to get the HTML right that I ignored the content.

Wednesday, August 13, 2008

My only political comment and it's not political at all

Flat out, politics will not be a topic of discussion on this blog, but there is one blog that I want to mention for having an unusually rational look at the daily polling results and predicting the Electoral College Results that decide the election, not nationwide polling.

The site is It can be a little confusing at first to look at, but here is the idea in a nutshell. Various groups publish polling data for each state, resulting in an average and a standard deviation, from which the distribution can be worked out. Using these input distributions, 10,000 Monte Carlo simulations are run for all 50 states with the output being another distribution. As of today, 8/13/08, Obama wins 65.4% of the simulations with the most likely outcome being 299.2 to 238.8, and the most likely outcome of the popular vote being 49.7% to 47.7%. The results change daily (or less frequently if there are no new polls for a given day). Watching the site over the last few weeks, the numbers reported above haven't been changing much: Obama is getting about 300 Electoral votes, winning about 60 - 65% of the simulations and getting about a couple of percentage points more than McCain in the popular vote. There are far more details to get into (older polls are weighted for less), but this is it in a nutshell.

While Monte Carlo simulations are just a tool that can be misused just like any other tool (scientific or otherwise - ever seen a bolt on a bike rounded off by a kid using pliers?), this is a far more rational approach that just looking at the "average" reported by a poll. However, this is the first national election for which the technique has been used, so the connection to "reality" has not be firmly established. I am certainly looking forward to seeing how well this model does in the actual election. And I have to expect that the "reality" will be used to refine the model for future elections.

The operator of the site is Nate Silver, a baseball statistician of renown. He is openly an Obama supporter, but believes that this effort is party-neutral.

Friday, August 08, 2008

Accelerated Aging - Getting Bad Data Even Faster - 2nd in a Series

A very common mistake in accelerated aging is to assume that "more is better", the the "more" applying to the stressing variable such as the lamp intensity. Such approaches can have numerous pitfalls. Accelerated UV aging already has advantages over natural aging since the lamp can be run 24/7 while old Sol runs (for most latitudes) at most about 16 hours, with most of this being at less than maximum intensity. But just like the endless pursuit of faster CPU speed (so that people can make stupid mistakes even faster than ever before!), the need for speed is also relentless in accelerated aging. Lamp intensity can be increased ever further (normal maximum intensity for Miami is 0.34 W/m2 and this is commonly increased to 0.55 W/m2 - all these measurements are most commonly done at 340 nm) but excessive increases lead to other problems. Increased lamp output increases the sample temperature which can lead to increases of reactions that are not of interest.

Monday, August 04, 2008

Oh No! Harold and Kumar...

I'm not going to make reviews of movies a regular entry here, but I was captured by one scene near the very end of "Harold and Kumar Escape from Guantanamo Bay". It was an all around bad movie, but this poem, attributed to David Feinberg, that only a geek could love, made it worthwhile: (BTW, the first two words of the original poem were "I'm sure". I changed it to "I fear", as that what was used in the movie and I thought it was much better.)

I fear that I will always be
A lonely number like root three

The three is all that’s good and right,
Why must my three keep out of sight
Beneath the vicious square root sign,
I wish instead I were a nine

For nine could thwart this evil trick,
with just some quick arithmetic

I know I’ll never see the sun, as 1.7321
Such is my reality, a sad irrationality

When hark! What is this I see,
Another square root of a three

As quietly co-waltzing by,
Together now we multiply
To form a number we prefer,
Rejoicing as an integer

We break free from our mortal bonds
With the wave of magic wands

Our square root signs become unglued
Your love for me has been renewed