# Temperature transmitters versus thermocouple module



## Black Dog (Oct 16, 2011)

nickson said:


> my boss told me to connect three thermocouples so that he can view them at the hmi via a plc using siemens S7 300 programme
> 
> I have a thermocouple type k. I connect it to a temperature controller that has a 4-20mA as an output.can I connect this output direct to an analogue input card or a thermocouople module of a plc?
> 
> ...


I have no idea..:laughing:

But I bumped your thread. Maybe one of the controls guys will jump in


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## Safari (Jul 9, 2013)

Black Dog said:


> I have no idea..:laughing:
> 
> But I bumped your thread. Maybe one of the controls guys will jump in


hope so.am trying to learn plc and am getting confused.I know temprature transmitters amplify the signal but I dnt know if a thermocouple module does the same thing 

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## bigdan1 (Jun 16, 2013)

This might help...

http://www.technologyreview.com/sit...mperature_measurements_with_thermocouples.pdf


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## abelouellette (Nov 22, 2012)

Hello Nickson


Information about the Siemens PLC

Temperature Signal loop 



Thermocouple : signal conditioner and transmitter : Control equipment (PLC)

The temperature transmitter will amplified and condition the thermocouple voltage. 
The temperature transmitter will then convert this amplified voltage from the thermocouple and convert it to a standard 4-20ma signal. The current signal (4-20ma) can be transmitted on a single pair shielded wire over a much greater distance than the millivolts signal from the thermocouple.





The Siemens PLc (or any other plc) must be equipped with an analog input card. The input card shall be equipped with voltage input capability or current input capability.

The input card from the plc shall be (if possible) accepting the current signal. 

If the only available option for this PLc is a voltage input card, a 50ohm resistor is required to transform the 4-20ma to a voltage signal of 1 to 5 vdc


Hope this help

Abel


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## don_resqcapt19 (Jul 18, 2010)

The only thing that connects to a thermocouple module is the thermocouple wire. If you have a transmitter or controller, it will connect to an analog input module.


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## don_resqcapt19 (Jul 18, 2010)

abelouellette said:


> ...
> If the only available option for this PLc is a voltage input card, a 50ohm resistor is required to transform the 4-20ma to a voltage signal of 1 to 5 vdc ...
> Abel


A 50 ohm resistor will give you a 0.2 to 1 volt signal. You need a 250 ohm resistor to get a 1 to 5 volt signal.


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## IP-EI (Apr 7, 2014)

A transmitter should always be run to an analog input card, if available. As a previous poster said, you can add a resistor to change the 4-20ma signal to a 1-5VDC signal if all you have is a voltage input card. 

If you were connecting the thermocouples directly to the PLC, you would use the thermocouple module. I've never used one, I've always seen RTD modules, but I'm sure they are pretty much the same. It's simple. 

Good luck. Just remember, thermocouples and transducers create a millivolt signal. Transmitters condition this mv signal into a standard mA signal. mA is analog.


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## micromind (Aug 11, 2007)

If you have a thermocouple, it can be connected directly to a thermocouple input card in a PLC, but you'll need to use thermocouple wire the entire distance and very likely thermocouple terminal blocks for any splices. This wire must match the type of thermocouple; in your case (type K) it'll be red and yellow. Red is negative. 

If the thermocouple is connected to a signal conditioner that has a 4-20ma output, then it is connected to an analog input card in the PLC. From the thermocouple to the conditioner needs to be thermocouple wire, but from the conditioner to the PLC can be just about any two conductor shielded wire. 2 conductor shielded wire is a LOT less expensive than thermocouple wire. 

If the conditioner has no power input, then it is loop-powered, meaning it gets its power from the 4-20 input card in the PLC. The problem here is that some PLCs have power at the cards while most don't. So you might need to connect a 24DC power source in series with the loop. This will power the conditioner as well as the card input. 

If the conditioner has a power supply plus a 4-20 output, then the output goes directly to the analog input card, and the card cannot be internally powered, because the conditioner sends power to the PLC card. 

Confused.....welcome to the wonderful world of PLCs; it gets rough from here.....lol.

Rob


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## don_resqcapt19 (Jul 18, 2010)

micromind said:


> ... So you might need to connect a 24DC power source in series with the loop. This will power the conditioner as well as the card input. ...
> Rob


This where I have seen many electricians have problems. It is a series circuit, and just like when you put two batteries in series, there will be a point where the positive directly connects to the negative. Really a simple concept, but it trips a lot of guys up.


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## Safari (Jul 9, 2013)

abelouellette said:


> Hello Nickson
> 
> 
> Information about the Siemens PLC
> ...


so this is what am getting from you,if I have a temperature transmitter then I will just connect to an analogue input card of a plc not a thermocouple module right? 

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## micromind (Aug 11, 2007)

don_resqcapt19 said:


> This where I have seen many electricians have problems. It is a series circuit, and just like when you put two batteries in series, there will be a point where the positive directly connects to the negative. Really a simple concept, but it trips a lot of guys up.


True, one cannot look at polarity as absolute, it is relative. 

For example, in a series circuit with a power supply, a loop-powered device and an analog input card, the positive of the power supply connects to the positive of the device. The negative of the device connects to the positive of the card and the negative of the card connects to the negative of the power supply. 

The positive of the device is of course, positive, but the negative is actually positive relative to the input card, because the negative of the card is actually negative. 

Yep, it's simple.....until you go to hook it up......lol.


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## micromind (Aug 11, 2007)

nickson said:


> so this is what am getting from you,if I have a temperature transmitter then I will just connect to an analogue input card of a plc not a thermocouple module right?
> 
> Sent from my HUAWEI Y210-0100 using Tapatalk 2


Yes, the thermocouple connects to the transmitter (red is +, yellow is -), and the output of the transmitter connects to the PLC analog input card. 

If neither the transmitter nor the card outputs current, then you'll need to connect a 24DC source in series with the circuit.


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## Safari (Jul 9, 2013)

don_resqcapt19 said:


> The only thing that connects to a thermocouple module is the thermocouple wire. If you have a transmitter or controller, it will connect to an analog input module.


ok so if I dont have a transmitter I just connect the thermocouple wire direct to the thermocouple module of the plc right?

so a transmitter does the same thing a thermocouple module will do?

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## don_resqcapt19 (Jul 18, 2010)

micromind said:


> ...
> Yep, it's simple.....until you go to hook it up......lol.


Exactly.


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## Safari (Jul 9, 2013)

don_resqcapt19 said:


> The only thing that connects to a thermocouple module is the thermocouple wire. If you have a transmitter or controller, it will connect to an analog input module.


this is very helpful 

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## don_resqcapt19 (Jul 18, 2010)

nickson said:


> ok so if I dont have a transmitter I just connect the thermocouple wire direct to the thermocouple module of the plc right?


Yes, making sure you use thermocouple wire or thermocouple extension wire and thermocouple terminal blocks for any splices as Rob said in his post. 



> so a transmitter does the same thing a thermocouple module will do?
> 
> Sent from my HUAWEI Y210-0100 using Tapatalk 2


It does not do the same thing, but the end result is the same...the PLC knows what the temperature is.


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## Safari (Jul 9, 2013)

micromind said:


> If you have a thermocouple, it can be connected directly to a thermocouple input card in a PLC, but you'll need to use thermocouple wire the entire distance and very likely thermocouple terminal blocks for any splices. This wire must match the type of thermocouple; in your case (type K) it'll be red and yellow. Red is negative.
> 
> If the thermocouple is connected to a signal conditioner that has a 4-20ma output, then it is connected to an analog input card in the PLC. From the thermocouple to the conditioner needs to be thermocouple wire, but from the conditioner to the PLC can be just about any two conductor shielded wire. 2 conductor shielded wire is a LOT less expensive than thermocouple wire.
> 
> ...


another thing rob,a thermocouple is pair of two dissimilar metals joined together to great a junction

in an RTD resistance changes with increase or decrease in temp.

if I have a thermocouple and an RTD in my hand can you tell the difference by just looking at both of them

why I ask is that we deal with both kinds of temperature sensors and I think the two have been confused or maybe iam

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## Safari (Jul 9, 2013)

ok I get this thanks.
iam in a company where everything is Siemens.do you have a site where I can get the programming aspects of Step 7 300,400?

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## Safari (Jul 9, 2013)

right
thanks


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## micromind (Aug 11, 2007)

nickson said:


> another thing rob,a thermocouple is pair of two dissimilar metals joined together to great a junction
> 
> in an RTD resistance changes with increase or decrease in temp.
> 
> ...


The vast majority of thermocouples have a red wire and, depending on the type, another color. Type K is red/yellow, type J is red/white. Red is always negative. 

Most RTDs have 3 wires, thermocouples have two. 

Interesting item concerning thermocouples, if thermocouple wire is connected to a device that can read temperature, the you strip the insulation off the wires and twist them together, it'll read ambient temperature.


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## ScooterMcGavin (Jan 24, 2011)

Download this book and read the chapter on continuous temperature measurement.  It should help clarify things for you. Warning its a pretty big file.

http://www.ibiblio.org/kuphaldt/socratic/sinst/book/liii_2v02.pdf


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## Safari (Jul 9, 2013)

ok

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## Safari (Jul 9, 2013)

micromind said:


> The vast majority of thermocouples have a red wire and, depending on the type, another color. Type K is red/yellow, type J is red/white. Red is always negative.
> 
> Most RTDs have 3 wires, thermocouples have two.
> 
> Interesting item concerning thermocouples, if thermocouple wire is connected to a device that can read temperature, the you strip the insulation off the wires and twist them together, it'll read ambient temperature.


am in the company at the moment
and yes I have conformed that.thanks

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## Safari (Jul 9, 2013)

scameron81 said:


> Download this book and read the chapter on continuous temperature measurement. It should help clarify things for you. Warning its a pretty big file.
> 
> http://www.ibiblio.org/kuphaldt/socratic/sinst/book/liii_2v02.pdf


thanks I will download it in my PC when I getting home.am currently using my phone 
it's 54MB
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## gesparky221 (Nov 30, 2007)

I normally go the transmitter route. Less problems getting thermocouple wire into the plc card. I usually get the transmitter that fits inside the thermocouple head unless T/C is in a high heat area such as our ovens. I also have had more issues with T/C cards failing and very seldom have an analog input card fail. Using an analog input card also opens up other uses rather than a staright T/C card, 4-20ma loops, etc.


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## Big John (May 23, 2010)

I'd always use signal conditioners if given the choice. They can make calibration adjustment a lot easier and many times I've seen them save a card from a loop fault.


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## Semi-Ret Electrician (Nov 10, 2011)

When one heats a TC the output leads produce a voltage, and RTD produces a change in resistance

A bunch of TCs tied together make a pelter pile. Send DC volts to it and you get heat, reverse the polarity and you get cold.

Outputs from TC's and RTDs are non-linear and have to be "characterized" to get a useful temperature reading. 

Transmitters make these measurements much more accurate.


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## glen1971 (Oct 10, 2012)

micromind said:


> Yes, the thermocouple connects to the transmitter (red is +, yellow is -), and the output of the transmitter connects to the PLC analog input card.
> 
> If neither the transmitter nor the card outputs current, then you'll need to connect a 24DC source in series with the circuit.


Red is "-" and yellow is "+".. You had 'er right once and I'm guessin just a typo...


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## micromind (Aug 11, 2007)

glen1971 said:


> Red is "-" and yellow is "+".. You had 'er right once and I'm guessin just a typo...


Yes, dangit!!!! Red is negative, not +........don't know what I was thinking.....lol.


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## David Pailley (Oct 5, 2011)

*Using Data Acquisition Module*

If he wants just to get the data to the HMI you might consider a thermocouple or 4-20 MA data acquisition module. If logic is required you then will have to go to a PLC or HMI with a control unit.
Look at ET-7018Z http://www.icpdas-usa.com/et_7018z.html

David


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## dronai (Apr 11, 2011)

You guys know the reason the preferred method is 4-20- ma vs Volts


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## abelouellette (Nov 22, 2012)

Hello dronai

Advantage of current loop over voltage
· The current loops have the advantage to allow longer distance between devices than voltage loop. The maximum distance for a voltage loop is a few meter before the electrical interference and magnetic interference cause communication problem. 
· Distance: The current loop can stretches hundreds of meter or even kilometer without repeater.
· The current loop utilizes a decaled zero: the control systems can deference between the process values been at zero and a problem or fault in the loop. 
· The current loop is less affected with the resistance (impedance) of the wire. 
· The current loop can have digital signal supersede to the current signal, with frequency modulation. Hart protocol or other. 
4ma = process value at zero (0%) of span 
20ma = process value at 100% of span

0ma = problem with the loop, broken wire short circuit etc 

You can refer to google for more information and diagram 

Hope this help


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## dronai (Apr 11, 2011)

abelouellette said:


> Hello dronai
> 
> Advantage of current loop over voltage
> · The current loops have the advantage to allow longer distance between devices than voltage loop. The maximum distance for a voltage loop is a few meter before the electrical interference and magnetic interference cause communication problem.
> ...


Wow ! I was taught only one thing. That the voltage can vary with impedance while amperage is stable. Thx !


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## abelouellette (Nov 22, 2012)

As promised 
This is interesting paper I found on the internet about instrument control loop
All right reserved to National Instrument and YouTube producer
http://www.ni.com/white-paper/6940/en/


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