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Thermocouple

pavanmys

Mar 31, 2021
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Hi,I am very new to this hardware design field and i have a very basic doubt regarding thermocouples and i.e I have a standard J type thermocouple temperature sensor and as of my basic knowledge i know that it has 2 wires which are joined together and they are of different materials and there outputs are of two wire +ve and -ve,but in my case i have 3 wires(2 white in color and 1 in red color),so i have a doubt for what the 3rd wire is for and is indicate the ouput??and then it becomes RTD or still it is a standard j type thermocouple???,plz let me know ASAP

Thanks.
 

bertus

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Hello,

Is the third wire connected to the others?
If so you have an RTD and not a thermocouple.
If it is not connected the third wire is a shielding wire.

Bertus
 

pavanmys

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Hey Bertus thanks for the quick response and i have uploadin a image of my product,plz have a look and guide me regarding this,then any type of thermocouple has only two wires right?
 

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bertus

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Hello,

Can you measure the resistance between the wires?
When you measure 100 (PT100) or 1000 (PT1000) Ohms between the red and one of the white wires and 0 ohms between the two white wires, it is a RTD.
Then the third wire is a sense wire.

Bertus
 

pavanmys

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Thank you very much Bertus,i got to know that its RTD,as you suggested i founded a resistance of 115 ohms between red and white wires and 1 ohms between 2 white wires.But i am interested in knowing that whether thermocouple comes only in 2 wire or is ther possibility of 3 wire?
thanks for your reply that too for newbies like me
 

Harald Kapp

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i am interested in knowing that whether thermocouple comes only in 2 wire or is ther possibility of 3 wire?
Thermocouples are afaik 2 wires only. That is based on their principle of operation: there is no need for additional cables.
 

bertus

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Hello,

If there is a third wire that is not connected to the other wires, it is for shielding:
If you are using a shielded thermocouple, connect the COM terminal of your device to the shield and the shield to a common-mode voltage reference of the thermocouple. A common-mode voltage reference is a voltage that is within ±1.2 V of the common-mode voltage of the thermocouple. If you are using a floating thermocouple or a thermocouple within ±1.2 V of earth ground, connect COM and the shield to earth ground. The shield grounding methodology can vary depending on the application. Refer to Figure 6 for an illustration of a typical shielding configuration.

sensor_81.gif

Figure 6. Connecting a Shielded Thermocouple

https://www.ni.com/getting-started/set-up-hardware/data-acquisition/thermocouples

Bertus
 

pavanmys

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Hello,

If there is a third wire that is not connected to the other wires, it is for shielding:
If you are using a shielded thermocouple, connect the COM terminal of your device to the shield and the shield to a common-mode voltage reference of the thermocouple. A common-mode voltage reference is a voltage that is within ±1.2 V of the common-mode voltage of the thermocouple. If you are using a floating thermocouple or a thermocouple within ±1.2 V of earth ground, connect COM and the shield to earth ground. The shield grounding methodology can vary depending on the application. Refer to Figure 6 for an illustration of a typical shielding configuration.

View attachment 51429

Figure 6. Connecting a Shielded Thermocouple

https://www.ni.com/getting-started/set-up-hardware/data-acquisition/thermocouples

Bertus
then the picture which i have posted is a thermocouple without shield is it right?? or RTD
 

bertus

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Hello,

As you said there is about 1 Ohms between the 2 white wires, it is most likely a PT100 RTD.
That should be connected as followed, as given in the link of post #6 :
rtd-21.jpg

As you also said that you measured 115 Ohms between the red and white wire implies that the tempareture at the sensor was about 40 degrees celcius.
See the attached table.

Bertus
 

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Harald Kapp

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then the picture which i have posted is a thermocouple without shield is it right??
No, your picture shows 3 wires, that would be either a thermocouple with shield or an RTD.
More likely - almost certainly - an RTD as your measurements show.
 

pavanmys

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Hello,

As you said there is about 1 Ohms between the 2 white wires, it is most likely a PT100 RTD.
That should be connected as followed, as given in the link of post #6 :
View attachment 51430

As you also said that you measured 115 Ohms between the red and white wire implies that the tempareture at the sensor was about 40 degrees celcius.
See the attached table.

Bertus
Thanks Bertus
 

pavanmys

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Thanks Bertus
Is there a way to connect this RTD to PLC directly ? OR is there a requirement of any driver circuit for this? as we are not interested to make the circuit which includes SPI,I2C etc,as we are aiming to extract the raw data(range is 0 to 10V) from the RTD,if you have worked on this then let me learn from you.
thanks
 

Harald Kapp

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You need a dedicated RTD input. not simply an analog channel. An analog channel is used for measuring analog values (e.g. voltages between 0 V and 10 V). An RTD doesn't give you this voltage, it is a temperature dependent resistor. To measure its resistance it needs to be excited by a current source. A dedicated RTD for a PLC input delivers the current and also provides the linearization required to convert the reading of resistance to an equivalent temperature. See e.g. this manual.
 

pavanmys

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You need a dedicated RTD input. not simply an analog channel. An analog channel is used for measuring analog values (e.g. voltages between 0 V and 10 V). An RTD doesn't give you this voltage, it is a temperature dependent resistor. To measure its resistance it needs to be excited by a current source. A dedicated RTD for a PLC input delivers the current and also provides the linearization required to convert the reading of resistance to an equivalent temperature. See e.g. this manual.
thanks for the reply and for the document
 
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