Hello all,
I am an applications engineer and a customer has recently requested a validation of why we cannot install a Type K TC extension cable wire run of 150M length for the purpose of monitoring surface metal temperature on a steam turbine as feedback for a MI heater PID loop control. I have explained to said customer several times that the ET200 SP PLC cards we are using specify a maximum distance of 50M for TC cable. However they are wanting to test this scenario as they believe the "error" created could be acceptable. I am wanted to find a way to validate this for them theoretically and show them (without physically testing it) that the error is too much (my control loop specs shouldn't allow more than 10F measurement error between actual and measured/reported temperature). I appreciate any insight or advice you can offer to the subject. attached is the specification for the plc input card we are using. Thank you!

 

Siemens has to answer as to what the effect of higher lead wire resistance is on the input circuit because that depends on the input circuit and its biasing current and that kind of information isn't covered in the marketing spec sheet. The lead wire resistance would have to be known: the total distance, type of thermocouple and the wire gauge or thickness.

But neither Siemens nor you can predict or know what the effect of noise pickup on the longer lead wires is. And, if the thermocouple is grounded, there's also the possibility of a ground loop error.

Search the Texas Instrument app note for 'resistive leads' to get an idea of the effect of biasing current.

https://www.ti.com/lit/an/sbaa274a/sbaa274a.pdf?ts=1720497042688&ref_url=https%3A%2F%2Fduckduckgo.com%2F

The comment below on the TI forum has an example of the use of thermocouple lead wire, although you have to wonder who would use 30AWG wire (pretty thin and delicate) for 100 feet of thermocouple extension wire.

For precision temperature measurements it is critical that the voltmeter has very low input bias current. The resistance of the thermocouple depends naturally on length but also on both type and temperature and the ratio of the resistance in the 2 individual thermocouple wires may be as high as 25:1. 100 feet of 30AWG type K has a resistance of ~600ohms (by my rough calculations) so if the input offset current was 50nA the voltage error is ~30uV. Type K sensitivity is ~40uV/degC (it varies with temperature as well) so this equates to ~0.75degC.

https://e2e.ti.com/blogs_/archives/...-stuff-that-every-analog-designer-should-know