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Wednesday, April 6, 2011

2nd Day - Low Temperature Oven Test

Yesterday, I worked on the software until I got it complete enough to use. It's not 'production ready' yet, but close enough for use as a test.

I spent several hours running tests without any meat in the oven. The first test had a 100 watt bulb placed into the oven, but it was unable to get the oven temperature up past 175 dF or so. I needed a new, higher wattage bulb. Since we are on a vacation island (Edisto Island), things are a little difficult to come by, but I was in luck - the Ace Hardware store, located about a mile off-island, had 250 watt heating bulbs (the guy said they were for keeping biddies (young chickens) warm). I bought one, along with a 200 watt bulb (total was $11.50!) and came back and plugged it in. With the 250 watt bulb, I was able to take the oven up to about 230 dF, so since my starting setpoint was going to be 200 dF, I was in business!

So, at 10 PM, I got the beef covered in a rub, then popped it into the oven.

The starting temperature is 200 dF. I intended, after about 3 hours, to lower the setpoint to 140 dF (when I get home where I have more electronic components, I intend to automate this step too). The oven, which had been at 200 dF from an earlier no-meat test, took 1.5 hours to come back up to 200 dF. This effect was caused by: 1- having to leave the door open a long time while I worked with getting the meat in the door (the bulb takes up the bottom half of the oven - it's a very small condo studio oven), and 2- the meat, at 6.1 pounds, is a big heat soak. So anyway, the oven was back at 200 dF at about 11:30 PM.

I let the meat cook at this temperature until 2:00 AM, at which point I lowered the oven temperature setpoint to 140 dF.

Observations on the first test:
1- Ovens aren't all that efficient. They could use more insulation.
2- the temperature controller I had designed, coded and built, worked perfectly.
3 - The 'dawn-to-dusk' (DTD) controller I was using as an optical switch (my controller outputs light from an LED taped to the DTD photocell) to turn on/off the oven heat source (the 250 watt bulb), has a long delay built into it. For instance, if I turn my LED off, which will result in the DTD turning on it's switch to supply power to the oven bulb, there's about a 1 minute delay before this happens. It's the same in the other direction - when my controller turns its LED on, the DTD takes about 1 minute before it turns the oven lamp heater bulb back off. I think this is probably designed on purpose into the DTD, as it makes the DTD less sensitive to transient light in it's designed use: e.g., a passing car's lights wouldn't cause the DTD to cycle off-on as it passes. When I get home, I'll pop this device open and have a look at the electronics - I'm expecting a simple capacitor-resistor timing circuit to be in place to slow down the response time.

This built-in lag time on the DTD is resulting in larger-than-desired deadband, or temperature swings between high and low. The deadband is about 10 dF. With a setpoint of 140 dF, the lowest reading is about 134 dF, while the highest reading is about 145 dF. This is too gross a control, so I'll improve it before I'm finished with this project. I want to have a deadband of +/- 1 dF.

I'm planning on letting the test run for a total of 48 - 72 hours. I would like to go the whole 72 hours but we've got some friends coming in to visit, and the 72 hours may not be a good fit, so will have to wait and see.

In the meantime, the test is going, and smelling(!) great!

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