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Old 05-09-2012, 11:36 PM   #1
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Default DS18B20's or K-Type thermocouples

Which temp sensor is going to give the best performance. The DS's have a much narrower range and output directly in centrigrade. simple + and - 5 VDC and a digital out. The K-Type thermocouples are are 2 wire through the sensor wiring set up with a much larger range. Is the large range going to make reporting accuracy at the lower end of mid level mushy? Or maybe a combination of the two. K-type in the tuns and DS's in or on valves and fittings. That would add a lot of data colection points for way cheap. I got 10 DS's for 9 dollars and 6 K-Type for about 35 dollars. any ideas on placing them in the most effective pattern? Or better yet any place to avoid putting them to prevent damage or leakage. Or even a different temp sensor that works better all together? Comments or opinions?
Thanks
WCB

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Old 05-10-2012, 01:41 AM   #2
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I use the DS18B20s (as does the Brewtroller) and I guess I haven't come across a reason to really look at anything else. I have mine in the stainless probe ends from "Brewers Hardware" along with the compression fittings and connected via some M12 cables.

I think kladue has had some concerns regarding the conversion time with this sensor so hopefully he'll chime in.

In regards to conversion time as it relates to resolution, see Table 1 here: http://www.maxim-ic.com/app-notes/index.mvp/id/4377

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Old 05-10-2012, 01:52 AM   #3
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I have used 100 ohm RTD's and type J thermocouples with the opto22 hardware, but these take additional interface circuits to render useful outputs for anything else. There is a voltage output sensor with 0-5Vdc output that looks interesting http://www.analog.com/en/other-produ...s/product.html. This would play well with a 5V reference analog input board, and 0-5V output pressure sensors for level measurement.

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Old 05-10-2012, 05:06 PM   #4
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digital IC temp sensors and thermocouples are very different, and picking one over the other really depends on what hardware you are reading it with or what you are doing with the data. each of them have their accuracy, range and response time in the spec sheets. its up to you to pick whats best for your application.

if i were using any type of microcontroller or computer (not for just a digital panel meter), i would go with a DS18B20. a digital signal is much less prone to interference from near by electrical loads and variations (heaters, pumps, sensor wiring, legnth of wires). with a resistance or voltage (analog) measurement of a thermocouple, you might have some interference that skews the readings only slightly, or non-linearly, which can be hard to spot or correct. with a digital signal, you either get the correct sequence of 1s and 0s, or you get a corrupted string which immediately tells you something is wrong.

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Old 05-10-2012, 06:12 PM   #5
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The DS sensors are sooo slooow in reading values out that more than a couple and a fast changing process PID loop is in trouble. With the I2C analog system I can read and decode about 16 - 12 bit points by the time one DS responds to a command and returns a 12 bit response.

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Old 05-11-2012, 04:12 AM   #6
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Quote:
Originally Posted by kladue View Post
The DS sensors are sooo slooow in reading values out that more than a couple and a fast changing process PID loop is in trouble. With the I2C analog system I can read and decode about 16 - 12 bit points by the time one DS responds to a command and returns a 12 bit response.
In reading some of the Brewtroller code and documentation, it seems like the 750 ms for the DS18B20 is only in parasitic mode and the time is more like 10 ms when powered; they're also only reading the first two bytes of data to speed up the read.

The newer Brewtrollers are using the DS2482; not sure if this helps the conversion time or is just a bridge to I2C.

Any idea what the read time is on the sensors you're using? I'm kind of curious to check these out and see if it makes sense to switch.
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Old 05-11-2012, 11:53 AM   #7
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The AD22100 sensors are voltage output devices, they are connected to a 8 input 12 bit I2C analog chip. the measure and read commands take 25 Ms each to scan an input channel, 400 ms for all 8. The analog chip is using an external 5V reference so it can handle 0-5v output pressure, flow, temperature devices with industry standard outputs. The hex values returned on the read response are passed to an array for use in another loop for conversion which is running with about 5 Ms intervals for each conversion and transfer to the output array.

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Old 05-11-2012, 03:51 PM   #8
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IMHO, 12 bit temperature resolution is silly for what we're doing. If the sensor is returning results in Celsius, that's 0.06 of a degree.
At least for my system, 10 bit (0.25 of a degree), or 11 bit (0.125 of a degree) are more than accurate enough, and when using DS18B20 in powered mode provide max read times of 180 and 375 ms, respectively.
These numbers are not cumulative unless you're polling each sensor individually, which is foolish. Poll the entire I2C collection, get all the results (at close to the 180 and 375 numbers plus 10 ms or so for transmit time), and apply your logic to the array.

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Old 05-11-2012, 05:36 PM   #9
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Quote:
Originally Posted by kladue View Post
The DS sensors are sooo slooow in reading values out
what is your definition of "slow"... because a few hundred miliseconds isnt slow to me. a thermocouple in a thermowell is going to take entire seconds to fully respond to changes in temperature (not just begin to register the change). either way- i dont think an extra second here or there makes any difference.
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Old 05-11-2012, 07:25 PM   #10
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If this was just a single temperature point, the DS would be adequate, but when trying to measure flows, levels, temperatures, and feed the information to multiple PID loops, time becomes important. The constraint of the analog input chip is that it requires individual channel polling, group data requests are not possible, upside is you can poll the channels in any order you want.
Having built and operated a 2 vessel brewing system with a flash boiler, precision control and fast response was required to make it operate. The current project is working toward a lower hardware cost control system with greater accuracy and flexibility than currently available.

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