j/k thermocouples

mosquito56

·Does anyone have any experience with j/k thermcouple wire?

Does the size of the sensor make a difference?
Should the sensor be made of copper?
Do I hook it to 5v and into adc pin?

Any help would be appreciated as this is the major part of my project.

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James Long

Mosquito,

Thermocouple wire "is" the sensor. If you twist the bare wire together for about 1 inch (25.4mm) the wire will output a voltage when heated (the end twisted together). This is because of the dissimilar metals that each strand is made of.

You have to amplify that signal to be able to read the voltage. It is very small. A chip like the Maxim 6675 will read a K type thermocouple. It will also read other types, but you will need to scale it's output.(The sensor is calibrated to K type thermocouples).

I know this is probably not what you wanted to hear (the amplification part) but I have a few thermocouples in my current project, and they can be complicated to work with (if your going to design your own amplifier......just more chips to deal with).

I hope this helps,

James L

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James L

Partner/Designer
Lil Brother LLC (SMT Assembly Services)

mosquito56

Exactly what I wanted to hear, some information about using the darn things. They show pictures of putting it in ice water and alot of theoretical stuff but never a schematic or a real number.
Any idea where I can find a schematic? I have a max1270 allready for these wires. I am using the 0831 also for voltage sensing.
Thanx

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······· "What do you mean, it doesn't have any tubes?"

······· "No such thing as a dumb question" unless it's on the internet

Technologically challenged individual, Please have pity.

James Long

mosquito56 said...
Exactly what I wanted to hear, some information about using the darn things. They show pictures of putting it in ice water and alot of theoretical stuff but never a schematic or a real number.
Any idea where I can find a schematic? I have a max1270 allready for these wires. I am using the 0831 also for voltage sensing.
Thanx

I have some sites that have schematics for using the thermcouples, but someone here would probably have better schematics. With the Max 6675 it is a direct connect situation (the reason I used them).· So If no one else chimes in, I'll give you some links.

James L

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James L

Partner/Designer
Lil Brother LLC (SMT Assembly Services)

mosquito56

·Thanx, i found one. It uses the 6674/6675 with a pic. I don't understand why i would need one specific for the thermcouple. I need 12 of them and I don't want to tie up all my pins. ·The 1270 has software input select with 8 inputs. Only need to use·3 pins for 8 inputs.

·Just thinking out loud.


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······· "What do you mean, it doesn't have any tubes?"

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Technologically challenged individual, Please have pity.

Tracy Allen

I have some inforrmation posted at
www.emesys.com/OL2therm.htm

Every thermocouple is made out of two kinds of wire, not just one type. It is the dissimilarity (as James pointed out) that generates the voltage. Many of the wires are alloys that have been developed specifically for use in thermocouples. The different types vary in how much voltage they produce, the operating temperature range, aging, and a lot of other things. Pure copper is one type that is used. It is found in the type T thermocouple, along with its opposing wirre made of constantan, which is an alloy of copper and nickel. The type K is a chromel wire and an alumel wire.

If you have a multimeter with a 200 millivolt range, connect your pair up to the input. You will measure a voltage in the millivolt range, and you will see it change when you heat up the end of the thermocouple. You can also see it change if you leave the thermocouple at a constant temperature and heat up the multimeter instead. The purpose of "cold junction compensation" is to eliminate the effect of heating up the multimeter (or your measurement circuit, of whatever form). It is no longer necessary to use an ice bath.

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Tracy Allen
www.emesystems.com

James Long

mosquito56 said...

·Thanx, i found one. It uses the 6674/6675 with a pic. I don't understand why i would need one specific for the thermcouple. I need 12 of them and I don't want to tie up all my pins. ·The 1270 has software input select with 8 inputs. Only need to use·3 pins for 8 inputs.

·Just thinking out loud.

Mosquito,

The Max 6675 is expensive. But if you don't mind the price, you can save a lot of I/O by combining the comm pin (output of the max's...the "SO" pins) , and the clock. You would·need to have separate CS pins though (Chip Select). With this format (if you can afford the wait time to query all the chips) you would only have 14 I/O pins tied up.

The Max 1270 will not likely read the output voltage of the thermocouple (this should be expanded on....it will read it, but not with good enough resolution to be useful). If you want to use the 1270 you will have to add amplifiers for each thermocouple, which will increase your chip count by 12 fold. You could use a multi opamp chip (which would be much easier), I was just to lazy to do all the circuit design necessary to use a regular ADC.

It is important to remember, (like Tracy stated) the voltage from a thermocouple is in the mV range. An adc or similar chip is usually designed to read 0-5 volts.

James L


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James L

Partner/Designer
Lil Brother LLC (SMT Assembly Services)

Post Edited (James Long) : 1/19/2008 7:10:06 PM GMT

mosquito56

I hooked up the k to the input of a 0831. But with the resistence changing such small values, I don't see any change in the output. I was wondering if I put the red wire on input and the white on vref if that would work?

·Since I only have a vauge idea how vref works, any input is appreciated.

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······· "What do you mean, it doesn't have any tubes?"

······· "No such thing as a dumb question" unless it's on the internet

Technologically challenged individual, Please have pity.

James Long

mosquito56 said...

I hooked up the k to the input of a 0831. But with the resistence changing such small values, I don't see any change in the output. I was wondering if I put the red wire on input and the white on vref if that would work?

·Since I only have a vauge idea how vref works, any input is appreciated.

Because thermocouples are self exciting, adding a voltage to it will not increase it's amplification.

Thermocouples require amplification of some sort for the ADC to read the voltage. The Max 6675 has a amplifier and single channel adc inside of it.

Depending on you project, you could change to thermisters, Maxim make a chip that will read 4 (or 8....can't remember) in one chip.

James L

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James L

Partner/Designer
Lil Brother LLC (SMT Assembly Services)

Post Edited (James Long) : 1/19/2008 7:42:22 PM GMT

mosquito56

ahh, the light bulb goes on.·Amp inside the chip. I was wondering if I could hook the 12 adcs to a i/o register and try to save pins.

··· You said tie up 14 pins but didn't specify how many adc's that would be? If it is all 12 then we good to go. I have plenty of room for 12chips and even 14 pins is not a problem but if I could cut it in half it would be much better.

· I am trying to read temps from an engine. j's for cylinder head and k's for exaust gas. Most apps will require 8(4cyl) or 12(6cyl) sensors. I doubt there is a chip that would last long sitting on an exaust pipe of an engine.

BTW: The last time I replace a cht sensor it costs me $170 for a screw in type.
Thanx for the quick replies.


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······· "What do you mean, it doesn't have any tubes?"

······· "No such thing as a dumb question" unless it's on the internet

Technologically challenged individual, Please have pity.

James Long

mosquito56 said...

ahh, the light bulb goes on.·Amp inside the chip. I was wondering if I could hook the 12 adcs to a i/o register and try to save pins.

··· You said tie up 14 pins but didn't specify how many adc's that would be? If it is all 12 then we good to go. I have plenty of room for 12chips and even 14 pins is not a problem but if I could cut it in half it would be much better.

· I am trying to read temps from an engine. j's for cylinder head and k's for exaust gas. Most apps will require 8(4cyl) or 12(6cyl) sensors. I doubt there is a chip that would last long sitting on an exaust pipe of an engine.

BTW: The last time I replace a cht sensor it costs me $170 for a screw in type.
Thanx for the quick replies.

That would be 12 max 6675s that would take 14 pins (if you can spare the time.....you could only talk to one chip at a time) But for your application, would probably work ok. I use them for a very similar purpose.
You could expand your I/O somewhere else, but would probably want to talk to the Max 6675's directly.
Before you jump in too far, these chips are not on the cheap side. Digikey does carry them, I think.


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James L

Partner/Designer
Lil Brother LLC (SMT Assembly Services)

CableGuy67

James Long said...
That would be 12 max 6675s that would take 14 pins (if you can spare the time.....you could only talk to one chip at a time) But for your application, would probably work ok. I use them for a very similar purpose.
You could expand your I/O somewhere else, but would probably want to talk to the Max 6675's directly.
Before you jump in too far, these chips are not on the cheap side. Digikey does carry them, I think.

Also check Maxim direct
https://shop.maxim-ic.com/storefront/priceavailable.do?Partnumber=MAX6675ISA+&event=PartSearch&menuitem=PriceAndAvailability

and SparkFun
http://www.sparkfun.com/commerce/product_info.php?products_id=307

Mark

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mirk handwrote
admire nth work

Harrison.

You could also take a look at Parallax's thermocouple board/kit. It includes three types of thermocouples and a 1-wire interface board. It is a bit more complex to work with since it requires a lookup table and 1-wire, but it is cheaper.

Schematics and BS2 code examples are on the product page. It shouldn't be too difficult to convert the code for use with the Propeller.

parallax.com/Store/Microcontrollers/BASICStampModules/tabid/134/txtSearch/thermocouple/List/1/ProductID/96/Default.aspx?SortField=ProductName%2cProductName

mosquito56

·Thanx looks like a winner.

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······· "What do you mean, it doesn't have any tubes?"

······· "No such thing as a dumb question" unless it's on the internet

Technologically challenged individual, Please have pity.

KatyBri

The number of wires connected between the thermocouple and the sensing electronics can make a huge diffeence on stability and absolute accuracy.

I always used four-wire thermocouples with electronics made to interface with them. They extra two wires allowed the electronics to compensate for long-lead resistance.

mosquito56

Yeah, the more I read about them the more I see they are going to be very complicated to work with. It looks like noise is going to be an issure with 6 ft runs and 12 wires. Also mixing j and k TC's.

·· ·I was hoping to find a chip with 8 inputs to multiply the voltages and hook it into a max1270 to the prop.· Now I just need to find the right volt amplifier.

·I was thinking of a differtial to single ended op amp but still open to ideas.

·Part numbers always appreciated.

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······· "What do you mean, it doesn't have any tubes?"

······· "No such thing as a dumb question" unless it's on the internet

Technologically challenged individual, Please have pity.

mosquito56

I finally got the k thermcouple working with an lm354 op-amp - lm0831ADC- propchip. Seems to work well.·I will be switching to the·MAX1270·soon.


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······· "No such thing as a dumb question" unless it's on the internet

Technologically challenged individual, Please have pity.