intrinsically safe thermometer

[seventh son]

I need to add a couple temperature probes into the vapor path in my distillation column. I'm looking for a simple stand-alone thermometer to plug the probes into so that I don't have to rebuild my control panel. I'm having trouble finding an intrinsically safe thermometer. The question is...does it have to be intrinsically safe, or can it be a standard thermometer?

[Silk City Distillers]

RTD probes are considered intrinsically safe if they are connected to an intrinsically safe meter or controller.  If your meter is not, you can use an intrinsic Safety Barrier for RTD, which is a stand-alone module that both will connect to, such as:

https://www.pepperl-fuchs.com/usa/en/classid_14.htm?view=productdetails&prodid=870#functions

This is getting into fairly sophisticated electronics design and safety engineering. What I am saying is that if your AHJ is indicating this is an issue, the necessary certifications are likely more important than the individual circuit components.

This kind of stuff is far more complex than a typical inspector would understand.  You would likely lose them at Intrinsically Safe.

Keep in mind what you are protecting against.  The temperature probe is not the problem, it’s that if something goes very wrong in the controller, and the rtd is now exposed to enough voltage/current - that short circuit could create an ignition source.  The goal of that barrier is to protect the probe and wiring from anything that might go wrong in that box.

[richard1]

I have recently gone through this exercise.  I have 7 temperature probes in my boiler and column and had initially designed on standard PT100's with 4-20mA PUKS and this all feeding into my PLC.

 

With peer review it was suggested to not bugger around and rather do it properly with Ex requirements.  The cost of this is obviously huge especially if you consider additional level switches and pressure transmitters also both with Ex requirements.

 

So for temperature, I have used Ex rated 4-20mA PUKS as well as Ex rated 4-20mA isolation barriers.  In hind sight I wish I had gone the route as Silk has mentioned, RTD's with Ex isolation barriers.

 

But what gets me is that not everyone does this.  Some still suppliers offer this as an optional extra.

[seventh son]

1 hour ago, Silk City Distillers said:

This is getting into fairly sophisticated electronics design and safety engineering. What I am saying is that if your AHJ is indicating this is an issue, the necessary certifications are likely more important than the individual circuit components.

 

Thanks for the reply. It's not an AHJ issue, I just want to be sure that my still is operating safely. 

Someone suggested to me that simply adding a thermowell would solve the problem, but I am still skeptical. If a short in the controller leads to the thermocouple, I don't see how the thermowell would provide a barrier.