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measuring temp with 200 J-type thermocouples

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(@fastrunner08)
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Hello Everyone,  

I'm looking for some advice or recommendations on taking measurements with a large quantity of thermocouples.  I am aware there are some off the shelf J-thermocouple break out boards, but most of them or for 1 or 4 thermocouples and the application requires 200 of them.  I'm trying to look for the most efficient way to do this without breaking the bank as the one channel board are about $10 to $14 each.  I was hoping to run them on a mega that is tied to a sd card/RTC shield from adafruit.  The design contains I have to work within are below. 

Temp readings need to be taken every 2-4 minutes and recorded to the SD card. 

Minimum amount of thermocouples needed are 200, they are taking temp readings across the surface of a wall in 2 inch squares.  This cannot change as it is required for the research i'm trying to do. 

They must be J-type thermocouples 

Data recording for the test will last between 2 and 5 hours. 

The mega board and data logger can be something different than what i mentioned above. 

I have been reading into multiplexers, but would something like that work between the thermocouples and the amp?  How would that be wired? 

Any ideas or advice would be greatly appreciated. 


   
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Inq
 Inq
(@inq)
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Sounds like you're pretty knowledgably... so I'll be quick (I tend to bore people to death with my diatribes)... check out "multiplexers".  Here is a good tutorial I keep referenced.  http://www.gammon.com.au/forum/?id=11976http://www.gammon.com.au/forum/?id=11976

I don't know anything about your specific devices and whether this is appropriate, basically you use several pins to switch the Mux and it send the data from a different sensor through your one-channel board.  Even an Uno can handle hundreds of channels if you use enough Mux's.  I know they make 16 channel Mux's, if you do a search you may even find ones that do 256 channels.  

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Ron
 Ron
(@zander)
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@fastrunner08 In your first post you said

Most of my projects I can't share due to them being for my engineering job.  

So what rate are you paying us to do your job for you?

 

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
My personal scorecard is now 1 PC hardware fix (circa 1982), 1 open source fix (at age 82), and 2 zero day bugs in a major OS.


   
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Ron
 Ron
(@zander)
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@inq Yup, on each MUX you drop another mux and so on so 16x16 is 256. If he needs more than that then some sort of ethernet and a class 1 private network will allow lots.

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
My personal scorecard is now 1 PC hardware fix (circa 1982), 1 open source fix (at age 82), and 2 zero day bugs in a major OS.


   
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Ron
 Ron
(@zander)
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@fastrunner08 Check this out

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
My personal scorecard is now 1 PC hardware fix (circa 1982), 1 open source fix (at age 82), and 2 zero day bugs in a major OS.


   
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(@fastrunner08)
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Joined: 3 years ago
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Posted by: @zander

@fastrunner08 In your first post you said

Most of my projects I can't share due to them being for my engineering job.  

So what rate are you paying us to do your job for you?

 

 

Not for my job at all.  I'm playing with some heating/insulation in my pole barn/shop for the farm.  Probably over engineered, but I'm wanting to gage heat lost from a wood stove, vrs the heated floor which is powered by an out door pellet stove, and then planning to build a waste oil burner as well to see how efficient i can get it.  When not farming, i tinker in the winter time.  I have a ton of J-type thermocouples already on hand that i took off of some old trash equipment, and want to use them rather than buying something else.  

I'm hoping to use Arduino to make it affordable on my tinker's budget. 

 


   
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Ron
 Ron
(@zander)
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@fastrunner08 Ok, good to know, be sure to let us know how it goes now that you have the answer.

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
My personal scorecard is now 1 PC hardware fix (circa 1982), 1 open source fix (at age 82), and 2 zero day bugs in a major OS.


   
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(@fastrunner08)
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Joined: 3 years ago
Posts: 22
Topic starter  

Posted by: @inq

Sounds like you're pretty knowledgably... so I'll be quick (I tend to bore people to death with my diatribes)... check out "multiplexers".  Here is a good tutorial I keep referenced.  http://www.gammon.com.au/forum/?id=11976http://www.gammon.com.au/forum/?id=11976

I don't know anything about your specific devices and whether this is appropriate, basically you use several pins to switch the Mux and it send the data from a different sensor through your one-channel board.  Even an Uno can handle hundreds of channels if you use enough Mux's.  I know they make 16 channel Mux's, if you do a search you may even find ones that do 256 channels.  

This is what I was thinking the solution would be.  I haven't quite tried it yet, as the parts are still on their way from amazon.  What I am trying to wrap my head around is how to do the wiring.  In this case the mux would be be between he thermocouple and the amplifier, instead of the typical set up which would be between the Arduino and a sensor?  So as far as wiring it then to I put the positive end of the thermocouples into the switch pins on the mux and just tie all the grounds together then run them directly to the amplifier, or do i need a switched mux pin for both the ground and the positive between the amplifier and the thermocouples?

 

 


   
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Inst-Tech
(@inst-tech)
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Posts: 554
 

@fastrunner08,

Hi, while your project sound most interesting, there are some things you probably need to know about thermocouples in general.. one of which, the is that unless you plan on using a thermocouple input converter like the Max6675, at the site close to the fire, you'll need type-J extension wire as you can't just use any wire as they will form a new thermocouple at every junction and then the temperature signal will be averaged..you'll also need voltage followers( like a hex buffer 74HC04) after  the multiplexers like the CD4051, or AG608,to compensate for the slight temperature offset do to the R(on) resistance of the multiplexers.

Please see this link; https://forums.adafruit.com/viewtopic.php?t=59103 for more information on this subject matter, and how to program your micro-processor. Hope this helps you with your project..good luck..

regards,

LouisR

LouisR


   
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Ron
 Ron
(@zander)
Father of a miniature Wookie
Joined: 4 years ago
Posts: 7283
 

@fastrunner08 No need to 'switch' the grounds, but see the post above from LouisR, for absolute clarity.

First computer 1959. Retired from my own computer company 2004.
Hardware - Expert in 1401, and 360, fairly knowledge in PC plus numerous MPU's and MCU's
Major Languages - Machine language, 360 Macro Assembler, Intel Assembler, PL/I and PL1, Pascal, Basic, C plus numerous job control and scripting languages.
My personal scorecard is now 1 PC hardware fix (circa 1982), 1 open source fix (at age 82), and 2 zero day bugs in a major OS.


   
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frogandtoad
(@frogandtoad)
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Posted by: @zander

@fastrunner08 In your first post you said

Most of my projects I can't share due to them being for my engineering job.  

So what rate are you paying us to do your job for you?

Take it easy fella!

😉

 


   
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(@fastrunner08)
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Joined: 3 years ago
Posts: 22
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Posted by: @inst-tech

@fastrunner08,

Hi, while your project sound most interesting, there are some things you probably need to know about thermocouples in general.. one of which, the is that unless you plan on using a thermocouple input converter like the Max6675, at the site close to the fire, you'll need type-J extension wire as you can't just use any wire as they will form a new thermocouple at every junction and then the temperature signal will be averaged..you'll also need voltage followers( like a hex buffer 74HC04) after  the multiplexers like the CD4051, or AG608,to compensate for the slight temperature offset do to the R(on) resistance of the multiplexers.

Please see this link; https://forums.adafruit.com/viewtopic.php?t=59103 for more information on this subject matter, and how to program your micro-processor. Hope this helps you with your project..good luck..

regards,

LouisR

 

Thank you for the info! that is very helpful.  I'm not very use to programing with thermocouples.  I have mostly used thermistors in the past since the the 10k versions can directly plug into an analog pin with a pull up resistor. 

With the quantity I have/need and the fact these were free convinced me to expand my horizon and learn more through the process.   They have fairly long leads, most of them are 6ft of cable and look like they were welded vrs just being twisted together. 

 

 

 

 


   
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Inst-Tech
(@inst-tech)
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Posts: 554
 

Posted by: @fastrunner08

Posted by: @inst-tech

@fastrunner08,

Hi, while your project sound most interesting, there are some things you probably need to know about thermocouples in general.. one of which, the is that unless you plan on using a thermocouple input converter like the Max6675, at the site close to the fire, you'll need type-J extension wire as you can't just use any wire as they will form a new thermocouple at every junction and then the temperature signal will be averaged..you'll also need voltage followers( like a hex buffer 74HC04) after  the multiplexers like the CD4051, or AG608,to compensate for the slight temperature offset do to the R(on) resistance of the multiplexers.

Please see this link; https://forums.adafruit.com/viewtopic.php?t=59103 for more information on this subject matter, and how to program your micro-processor. Hope this helps you with your project..good luck..

regards,

LouisR

 

Thank you for the info! that is very helpful.  I'm not very use to programing with thermocouples.  I have mostly used thermistors in the past since the the 10k versions can directly plug into an analog pin with a pull up resistor. 

With the quantity I have/need and the fact these were free convinced me to expand my horizon and learn more through the process.   They have fairly long leads, most of them are 6ft of cable and look like they were welded vrs just being twisted together. 

@fastrunner08, Indeed..thermocouples are a very much different technology than thermistors. Since they operate in the very low mV range.. Also, you might find that a type J may not be suitable for the task you are currently using them for.. I have provided a link to info about the differences between the two types:

https://www.infra-view.com/j-type-vs-k-type-thermocouple-the-difference-explained/

I do have a question for you though.. do you know what temperature your furnace will be operating at the burner level.. and where are you placing the thermocouples?

Hope this helps with your understanding of what type to use and how they work..

Good luck on your project

LouisR

 

 

 

 

 

LouisR


   
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