Radiation vs Conduction

Driving home this evening with the top down, I noticed an interesting phenomenon.  It was hot, really hot today, although by the time I was driving home after sunset, the air had started to cool (that wonderful dry, 90ish balmy weather).  But every time I drove under a concrete overcrossing, the radiant heat from above was noticeable. That concrete had soaked up the heat during the day, and it was happily radiating in all directions.

So when I was on the open road, my head radiating through dry air to the clear sky, it was probably seeing a sink at a few tens of K, but when zooming under the overpass, looking at a radiator at 320K plus..

That T^4 factor is huge.

Adventures in Sous Vide

I've been intrigued by the idea of sous-vide for a while.  I was thinking about getting one of the ready made boxes that do this (e.g. SousvideSupreme, a package deal was advertised at Costco) but we don't have a lot of counter space. So why not the classic PID controller + rice cooker/crock pot approach?

There's a few ready-made PID controllers out there (Auberins, SousVideMagic), but I figured, hey, I probably have the control module out in the garage, I know where my solid state relays are, this should be hacked together instantly.  Except I couldn't find the controller in the garage (prompting a bit of garage detritus removal). Back to the ready made controllers.  But I'm not ready to spend $150+ on it. Plan B, buy a $30 PID controller and appropriate temperature sensor, wire up the SSR, get a suitable heating pot, and I'll be done.  A few minutes work after browsing eGullet.com and some googling, and I ordered the PID controller and sensor: I got the platinum RTD, rather than using a K or T or E thermocouple (bunches of which I actually have) because of the whole cold junction compensation thing. (does the PID controller know what temperature it's at? Or does it just assume 25C or something?)

I'm now the proud owner of a JLD618 controller from lightobject.com, along with a companion RTD (Platinum 100 ohm) for a sensor, and the cheapest crock pot they had at Target (a real "CrockPot brand" 7 quart unit for $21) when I stopped by after work.  I wasn't wild about using a crock pot; rice cookers have lower thermal mass between heater and sensor, so transient response should be better.  However, the local Target didn't have any decent sized rice cookers... I guess we're not in a rice eating part of the state.

Wired it all up, and it sort of works.  Lots of overshoot (as in more than 5 degrees C), so I think the controller needs tuning (that thermal mass of the crock pot isn't helping things).  You can see (maybe) that the controller is doing it's autotune thing.

Here's some practical observations:
The lightobject.com website/forum is a good resource, but you have to go hunting.  Hook the RTD red to terminal 8, blue to 9, 10.

Set the sensor type to P10.0 instead of P100 to get tenth of a degree displays and setpoints.