NI 9213 Can I use one thermpcouple to drive two inputs.

[Mavam]

Any reason one thermocouple can't or shouldn't be connected to two inputs on a NI9213?

More info: I want to add an additional layer of over temp shut down ability.

N9213 is monitoring two heaters, via thermocouple. Heaters are individually controlled with Relays.

 I want to have a third relay and fourth relay with normally closed contacts in series with the AC power to the heater.

These 3rd and 4th relays will open if the heaters go over temperature, in such case the heaters relays weld shut.

 the desire is to have the 3rd and 4th relays controlled by the same thermocouples that do the temperature control.

This would require that one thermocouple be connected to more than one input of the NI9213.

Can that be done?

Go ahead and tell me there is an easier way to do this.

But there is an interoffice argument about whether you can connect one thermocouple to two inputs of a NI9213.

I think you can, but if you can't, I'd like to know why.

           Thanks all, Mavam

[santo_13]

The easier way is to reuse the data from those existing thermocouples in your code instead of reading them through another channel.

[Mavam]

"The easier way is to reuse the data from those existing thermocouples in your code instead of reading them through another channel."

OK, so the temp data of the two thermocouple can be accessed in software, they could be queried, If this, OR this, is above X value, change the value of the Y output to change the state of a relay shutting off power to heaters.

  I'll pass this along, actually my son's project (He's a chemist). So, learning all he can about control systems.

       Thanks for the input, Mavam

[AnalogKid2DigitalMan]

A basic tenant of such a control system in order to prevent a ruined experiment/production run/equipment/lab/building/loss of life...

Use a separate TC connected to a dedicated over-temperature safety relay that will remove power to the heaters when OT is reached.

Depending on software that could lock up/crash or control relays that could fail in a closed position can be a risky proposition.

Mavam- may your Son's endeavors be fruitful and Kudo's for supporting him!

-AK2DM

[SteveD123]

I really need to support what AnalogKid2DigitalMan says. Standards like AMS2750 specify an independant over-temperture monitor for good reason.

However, there is ambiguity in the meaning of "overtemperature". Do you mean simply so hot that whatever you are heating will be ruined if it gets any hotter (i.e. setpoint +10°C), or do you mean so hot that damage to the equipment may result (e.g. 1000°C) ? In practice you may need both. The deviation from setpoint alarm can probably done in software as discussed above, but the ultimate shutoff really should be a seperate thermocouple and alarm system completely independant of the control system.

Also you need to have a routine that explicitly tests that all these work on a regular basis.

Couple of questions:

1. What range of temperatures will the system need to work over?
2. Why is your son reinventing the wheel? Eurotherm and Yokogowa have many off-the-shelf systems for this exact task.