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Thursday, October 18, 2007

Leeming SHS: Day 4: October 18


A thermistor is a resistor that changes voltage with temperature. There are kinds where the resistance goes up as temperature increases, and kinds where it goes down. There are kinds that are logarithmic, and kinds that aren't. And there are words like "B coefficient" and "Steinhart-Hart equation".

We need a thermistor to measure the temperature of our motor. Our motor comes with a thermocouple built in, but that's a different kind of beastie, and not the right thing.

James from Tritium stopped by yesterday and told us what kind of thermistor we needed, and we sent Richard out to buy one. (Well, I told him "I need one, so buy two." He came back with four.) This morning, I have to attach it, and calibrate it.

But on the way to the track I realise that I have no idea where I put thermistors Richard bought yesterday, so I phone John and get him to buy more, given that he is in the middle today's Jaycar trip anyway. He buys the last two Jaycar have in stock. (Any other teams who need a 10k thermistor, please see me. Pit 27.)

Attaching it is tricky, because it has to be glued on to the motor, and there's no time for glue to set because we want to take the car on the track. So I settle for running the wires (which is trickier than it sounds, due to my mind having been mushed by the previous several days, and everything I do needing three tries to get right), and then calibrating it.

James has told us how to calibrate it: you need a kettle and some ice. I put the thermistor in ice water, and it reads -66 degrees; I stick it in nearly boiling water and it reads -199.9 degrees (because this is the most negative number the display can display). In other words, we've bought positive temperature coefficient thermistors instead of negative. But that's the only kind that Jaycar sells, so: meh. I create a few mixtures of various water temperatures to get a few more points for a curve of real-world temperature versus imaginary according-to-our-wrong-kind-of-thermistor temperature.

We practice egress from the vehicle: the rules require that each of our drivers be able to escape from the vehicle unassisted within 15 seconds. Someone watches the second-hand on their phone, calls "Three, two, one, go!", and the driver pushes the canopy up two notches on the sawtooth while leaning forward, pulls his feet under him, puts his left hand on the canopy and pushes up while standing up, then steps out of the car backwards. After a couple of practices, they can all do it.

Our scrutineering is this afternoon, so we go through those last-minute things: putting some more stickers on, tidying things up.

We put drivers in the car to do laps. While they are out, I hear from a couple of pits down "Three, two, one, go!" -- some other team are also practicing their emergency egress.

We put the car into the trailer, which involves much grovelling underneath it in the trailer to tie it down, and return to the Alatai for lunch. After lunch, scrutineering.

Scrutineering is, as always, chaotic. I'm sure if you looked at it from the right angle, you'd see fractals. We get weighed: our official weight is 259kg. That turns out to be pretty good: word is that Nuna weigh 199kg, and Michigan weigh 240. The lightest car scrutineered so far is Heliox, at 166kg.

Darren from Willetton appears: they've completed their trek from Perth and are now camped at Hidden Valley caravan park.

We get to the driver egress test, and pass. But there's also a driver ingress test -- the wording on the ingress/egress rule is a little imprecise: "enter and exit the vehicle unassisted in no more than 15 seconds". We were hoping that the exit would be timed, but the entry would not, because it makes sense, right? You have to be able to get out quickly, in case the car is on fire or something, but you don't have to be able to get _in_ quickly. Unless you're planning to use your solar car for ecologically sustainable bank robberies.

But we do indeed need to enter the car unassisted in 15 seconds. We try, and fail. But we get close, so they pass us anyway.

We pass electrical with a requirement to put a plastic cover over our battery safety system and slap a few more "high voltage" stickers around, get our batteries sealed, and we're done.

I simplify: in fact there has been all sorts of to-ing and fro-ing: moving around the official "log" folder with details of our car, mucking around with ballast, discussing the wiring of the car and the number of seats it has, demonstrating our safety gear including our red flag and whistle, and such and so. But we get it done, and return to the Alatai.

After dinner, it's laser tag at Zone 3 Darwin. As I put my shoes on to go to the Zone, I feel something in one of the shoes: I take it off, shake it, and four thermistors fall out.

I have no memory whatsoever of putting them in there. I was wearing the shoes all day.

We play (shoot bad guys, don't get shot), go back to the hotel and the hotel pool, and end the day.

-- Doug, Leeming Hammerhead