Common Mode Voltage of Thermocouple Input 4

[Henneshoe]

I am specifying a 4 channel thermocouple device, and I am not sure what my requirements are for common mode range. I found some devices where each thermocouple is galvanicly isolated, so there is 500V common mode range. There are also devices where the thermocouples are not individually isolated, but have a common mode rating 10, 5, or 1.5 Volts. Each of these devices have galvanic isolation between the thermocouples and the supply voltage, but not channel-to-channel. The 1.5 Volt device is the least expensive so I would like to use it, but I do not have a good feel for what is needed.

In my application, the thermocouples will be bonded to a metal frame. They are all within a 30ft radius. Unfortunately, nothing is built yet so I cannot do any measurements. Do you think 1.5V common mode is OK for this?

 

[roydm]

My feeling is the 1.5 V will be fine, your input module needs to float because the thermocouples are grounded in the field. You might get into trouble if your thermocouples were widely seperated but you say they are close together.
Just make sure that neither the modules + or - terminals are common, channel to channel.
Roy

 

[waross]

There is a voltage gradient from end to end of each thermocouple lead and a junction voltage. If you use a common mode device you must use insulated thermocouples. Otherwise you will have a parallel connection between one extension lead of each thermocouple.
If you use grounded junction thermocouples for fast response you must use isolated input devices.
If you use common mode input devices you must use insulated junction thermocouples.
Omega Engineering used to have a free library that included a very good reference section on thermocouples. The library may be available on-line. It's well worth a look if you are doing temperature measurements.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Yes Omega still has the library. It is confusing to get to.

http://www.omega.com/temperature/Z/zsection.asp

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Henneshoe]

Thanks for the good reference. I guess I have some reading to do this weekend.

 

[danw2]

You'll gain some advantage with low common mode analog inputs by using sheathed thermocouples that are electrically isolated from the sheathing material.

Commercial thermocouple construction that isolates the hot junction from the sheath is commonly called 'ungrounded', less commonly 'isolated'.

It typically costs about $2 USD per thermocouple. Money well spent.