connecting a thermocouple directly to SM331(siemens)

[Sandor-CRX]

Hello guys,

I have a thermocouple with a green wire which measures a temperature 0-1100 degrees. I dont have any information, only the color of the wire. no data or manufacturere
This thermocouple was connected to a digital microcontroller which converted the mV signal to mA and this was connected to the PLC. The controller was defect and now I want to connect the thermocouple directly to the analog input of a SM331 card.

Question1: Is this possible?
Question2: Looking at the temperaturerange and color of the wire, I believe it must be a Type K thermocouple. What is the measuring type I must choose for the analog input--> TC-I, TC-E, TC-IL, TC-EL or just E (voltage)...

I really can't find any good info about this matter.

Thankss

 

[jimfun71]

If it is a type K thermocouple then it will put out about 40 micro volts per deg. C. Also, that will be at just above 0 current. Honestly I don't know for sure but I would be willing to bet my pay check that that a thermocouple hooked directly to a plc will not work. In 25 years of industry I have never seen a hookup like that. The plc inputs are just too insensitive to read a thermocouple. Thats why the ampilifier was there in the first place.
If you do get that to work I would love to hear about it though.
Jim

 

[bornwild]

If it is a type K thermocouple then it will put out about 40 micro volts per deg. C. Also, that will be at just above 0 current. Honestly I don't know for sure but I would be willing to bet my pay check that that a thermocouple hooked directly to a plc will not work. In 25 years of industry I have never seen a hookup like that. The plc inputs are just too insensitive to read a thermocouple. Thats why the ampilifier was there in the first place.
If you do get that to work I would love to hear about it though.
Jim

what about plcs which have thermocouple inputs card? aren't they connected directly.please clear my doubt

 

[hanziou]

Have you looked at the s7300_module_data_manual? link to US version http://tinyurl.com/23kb5cj (from support.siemans...)
There is a section about using/connecting thermocouples and a bit of theory as how TCs work. (4.6)

It looks like the difference between the -I and -E is where the Cold Junction compensation (CJC) sensor is. If you have a CJC device wired, then you use the -E. If you want to use the internal compensation short the CJC terminals and use -I.

As to the L, setting I didn't read too much, but I imagine that has some thing to do with the type of compensation you have .or else the representation of analog values.

 

[jimfun71]

Clarification

what about plcs which have thermocouple inputs card? aren't they connected directly.please clear my doubt

I know in recent times that some manufactures are making cards specifically for certain devices. ie Thermocouples, pressure sensors, etc. etc. Thats not the impression I got from Sandors post. Forgive me if I'm wrong but I thought the SM 331 was your basic run of the mill 4-20 or 0-10 input card. If it is not then I stand corrected and retract my previous reply.
If you purchase specific hardware for your plc like a thermocouple card. Which by the way is not by any strech of the immagination a standard input card but rather a thermocouple amplifier that has been modded to fit in a plc chasis. Then of corse you can hook a thermocouple to the PLC. I'm talking about hooking a thermocouple to a standard input card as I thought Sandor was also talking about.
Also type K thermocouples dont function on resistance they are bi-metal. They actually generate a micro voltage. So unless you have a micro voltage reader then just pumping 12 VDC into a thermocouple isnt going to work.
Bornwild, I hope this clarifies my previous post.

 

[bornwild]

thanks jim . very transparent now