Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
(OP)
We are developing a tungsten heater (50-100Ω) with an integral 2-wire tungsten RTD (80-120Ω) for closed loop control, with the control being done by a custom microcontroller board to-be-developed. It has occurred to me that, just like the RTD, the heater element itself will change resistance according to its temperature.
We plan on controlling the heater with phase-angle AC power; if I can get the RMS voltage and current for the heater (which a quick internet search indicates would be pretty straightforward) it seems pretty easy to take their ratio to get resistance, and derive the temperature based on a defined temperature coefficient of resistance.
Of course the heat element will be the hottest part of the heater (when on anyways), but accuracy is not a concern, just precision (still the precision tolerance is fairly wide, ±10°C).
One tricky part is that we'll also need to monitor temperature while the heater is off, so there would need to be a solution for that as well, possibly a more typical RTD circuit. There would need to be a reliable means of switching between the two circuits to avoid frying the RTD circuit.
This heaters need to be fast - removing the integral RTD will reduce its mass which will speed response. It will also reduce the wire count from 5 to 3 (the extra being for ground).
Has anyone ever done this? Does this seem feasible? Is this even wise? What am I not considering that I should be?
We plan on controlling the heater with phase-angle AC power; if I can get the RMS voltage and current for the heater (which a quick internet search indicates would be pretty straightforward) it seems pretty easy to take their ratio to get resistance, and derive the temperature based on a defined temperature coefficient of resistance.
Of course the heat element will be the hottest part of the heater (when on anyways), but accuracy is not a concern, just precision (still the precision tolerance is fairly wide, ±10°C).
One tricky part is that we'll also need to monitor temperature while the heater is off, so there would need to be a solution for that as well, possibly a more typical RTD circuit. There would need to be a reliable means of switching between the two circuits to avoid frying the RTD circuit.
This heaters need to be fast - removing the integral RTD will reduce its mass which will speed response. It will also reduce the wire count from 5 to 3 (the extra being for ground).
Has anyone ever done this? Does this seem feasible? Is this even wise? What am I not considering that I should be?





RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
Well, not quite, depending on how hot your heater is supposed to get - the resistance curve is pretty nonlinear. But you can curve-fit the Temperature vs. Resistance curve pretty easily (the data for tungsten is in the CRC Handbook of Chemistry and Physics), and from that infer the mean heater temperature.
"This heaters need to be fast - removing the integral RTD will reduce its mass which will speed response. It will also reduce the wire count from 5 to 3 (the extra being for ground)."
Ok, but you will still have the external wiring if you want to do low-power temperature sensing, and the switch speed (from power to low temp sensing) will likely be the speed limiter.
You could just control the voltage to the heater to control power output pretty directly, and not bother with on/off cycling.
"Has anyone ever done this? Does this seem feasible? Is this even wise? What am I not considering that I should be?"
Yes, as a method of finding/controlling wire temperature it works, given you take the non-linearity into account. But you also have to worry about the wire diameter; the tungsten wire will disappear over time, due to oxidation. How fast will depend on the temperature it runs at, and what atmosphere it runs in, and what means you use to mitigate oxidation (research water cycle oxidation of tungsten, and halogen lamps). Even in a perfect vacuum, the tungsten will slowly evaporate away if the temperature is high enough.
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
> exposure - bolometer is unbiased and collecting scene energy
> readout - bolometer is biased in a Wheatstone bridge to detect the change in resistance due to scene thermal energy
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
Still, it's an interesting approach. I hope that you'll keep us informed of how it goes.
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
If you can feed the heater un-switched power it would work well. Perhaps rectify the power into filtered DC and send that to the element. That would also make the measurement almost trivial.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
Control the output voltage of the DC power supply with a PID controller. Don't turn the heater completely off, drop down to a low voltage level and continue to monitor voltage and current.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
It does seem like the pulsed power control will present a significant challenge. But maybe if I can get away with a slower response time, I could use a circuit like below to get a steady RMS value. I'm not sure how slow this circuit is though, maybe too slow?
http://www.eetimes.com/document.asp?doc_id=1280109
I would like to avoid DC power control - although, if its just rectified AC, depending on the amount of filtering needed that may not be too bad... I wonder if there's a switching frequency or sampling rate that would help mitigate the 50/60 Hz noise?
The tungsten erosion is an aspect I hadn't considered. This would require periodic ambient temperature calibrations, with a known/independently measured ambient temperature. We need to do this at least once regardless, but this would require more frequent checks.
Those bolometers are pretty impressive old technology! "By 1880, Langley's bolometer was refined enough to detect thermal radiation from a cow a quarter of a mile away."
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
After patenting it of course...
" We are all here on earth to help others; what on earth the others are here for I don't know.
" -- W. H. Auden
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
It's responding to almost exactly the same question as was asked here, except motor vs heater.
Practicality still remains debatable, compared to other approaches.
I've lined up the following YouTube link to save your time, you can rewind if you like.
https://www.youtube.com/watch?v=2N6cjGS7lUE&fe...
* Yes I actually do watch these sorts of videos.
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
Keith Cress
kcress - http://www.flaminsystems.com
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
I once sent him a letter (this before email was common) on some technical detail in his 'Pease Porridge' column, and he wrote back to me using the same very paper. But he cut off the edges of my letter to eliminate all the extra whitespace (save weight? save postage?). So his note back to me was scrawled on my own quite-randomly-heavily-trimmed letter.
Pleasingly eccentric.
I still have it somewhere.
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
This actually got me thinking about what "instruments" TI actually made before the company got subsumed by the hungry beast of integrated circuits. Supposedly, there was a brief period in history when TI was a sonar company.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
Keith Cress
kcress - http://www.flaminsystems.com
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio
Semiconductor -- TexasInstrumentsTTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm
RE: Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio