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Old 10-31-2012, 02:25 PM   #11
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Correlation is important in the experiment you are trying to do. If all three thermal couples read the same value in the same environment then you should be good even if you are off a couple of degrees from the absolute temperature.

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Old 10-31-2012, 03:33 PM   #12
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If all he wants to see is correlation then they don't even have to read the same in the same environment as biases are removed in computation of the correlation coefficient:

E{(T1 - E{T1})*(T2 - E{T2}}/normalization

but I don't think that's what he's after. I believe he wants to know the temperature of his fermenting beer.

In either case if you go into a brewery you won't find thermocouples on the fermentors - you'll find RTD's. As far as implementation most controllers allow for RTD, themocouple or thermistor input so in terms of the configuration of a system it doesn't much matter which is used and you might as well go for accuracy.

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Old 10-31-2012, 05:30 PM   #13
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This stuff is normally used where RTD level accuracy isn't necessary. That said, I typically order the supplies in the special limits grade which specs to (in round numbers) +/-1C in the temperature range this would be. We have a calibrated thermocouple generator so I can validate pretty high accuracy in the instrumentation. What I don't have is a calibrated hot cell and cold cell, but usually I'll use an ice bath and boiling water as simple red-face checks for the complete system.

Bear in mind I'm not an Engineer, but my general function is to investigate and provide information so that we can decide if Engineering or other involvement is necessary.

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Old 10-31-2012, 06:06 PM   #14
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Whoaru99, For "not an engineer" you really know your stuff.

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Old 11-01-2012, 03:58 PM   #15
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Temperature monitoring is very interesting to me and also part of my day-job for testing refinery and power generation equipment performance. Most test thermocouples or RTD's are special tolerance with calibration records at a select temperatures. Also, most test instruments are bought for a single project and never touched again.

I keep a 2-channel data logger on my fermentation chamber (2-relay controller) with type-K thermocouples - example graph below. I usually log data every 3 minutes.

graph.jpg

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Old 11-01-2012, 05:11 PM   #16
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Wow DSmith,

That's a very interesting plot. It looks like you can control the fermentation temperature to one or two degrees of stability. That's very nice. By "two relay system" do you mean that you have heating and cooling elements involved in the control? I would love to look at the raw data that made the plot. I can PM you my email address if you could send the file.

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Old 11-01-2012, 06:07 PM   #17
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Yeah, that's basically what I'd be doing sans control from the wort temp/feedback. My control would be non-feedback/indirect through whatever is going on with the house's HVAC system.

Initially I was thinking sample at something between 1 and 5 minute intervals but in hindsight the slowest sample rate for my thermocouple module is 1Hz.

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