RTD

[dknguyen]

I have an RTD that lists the maximum temperature as being 600C. The maximum current is listed as being 1mA. However, I suspect this is just the maximum current to alleviate self-heating excitation currents and not the actual maximum current that the RTD can handle before burning out.

The reason is that I want to use this as a hot-wire anemometer and calculations using the dissipation constant provided estimate that passing a current of 25mA through the RTD would raise the RTD about 300C above ambient. So logic would seem to indicate that the RTD would be unharmed if I passed 25mA through it. Can anyone confirm this?

 

[chemelec]

I Really Doubt that 25ma would be acceptable.
Especially when there is LOW Movement of Air.

 

[dknguyen]

Is there any particular reasoning for that? If it's significantly below 600C should it not burn out? Or is there some difference between external heat being applied equally across the RTD versus the platinum traces directly heating up on the ceramic substrate?

 

[chemelec]

You can try it if you want, but I think there is a Big Difference between Applied External Heat and Internal Self Generating Heat with applied Current.
The Junction is made to Withstand the Temperature, Not Necessarily that Current.

 

[KeepItSimpleStupid]

The relative difference in thermal expansion will be greater when the element itself is heated and not the substrate+element.

 

[dknguyen]

The relative difference in thermal expansion will be greater when the element itself is heated and not the substrate+element.

Hmmm. Yes that had occurred to me. Couldn't you just circumvent that though by ramping up the temperature when it's turned on rather than just going full throttle right off the bat?

It's starting to seem like I am going to be limited to around 138C anyways due to the RTD resistance values available for use in the ambient vs velocity legs of the wheatstone bridge.

 

[MikeMl]

Does the data sheet list a thermal resistance, or temperature rise vs power dissipated in free air? I have seen such ratings for low-mass glass bead thermistors that are used for this purpose...

 

[dknguyen]

It does. That's how I "know" what temperature it will heat up to, but am unsure if it is meant to imply that it is applicable all the way up to the top of the temperature range of the RTD.

https://www.mouser.com/ds/2/719/PPG102A6-769771.pdf