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Feasibility and wisdom of measuring heater temperature via voltage-to-current ratio

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RyreInc

RyreInc

Electrical
Apr 7, 2011
205
US
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?
 
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" derive the temperature based on a defined temperature coefficient of resistance."

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.
 
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infrared bolometers do something like that.
> 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! faq731-376 forum1529 Entire Forum list
 
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Many applications actually want to know and regulate the temperature of the target (the thing being heated), not the heater element. That's probably why this technique is not used (or perhaps, not commonly used).

Still, it's an interesting approach. I hope that you'll keep us informed of how it goes.

 
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I can't see it working at all. Too many non-linearities to make it fly. The biggest problem is that you're cycling the heater element with varying pulses so its temp will be a mass of non linear swings that you're trying to understand the instant temperature of.

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 -
 
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To expand on Keith's suggestion:
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
 
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Thanks everyone for your responses.

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?

[URL unfurl="true"]http://www.eetimes.com/document.asp?doc_id=1280109[/url]

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."
 
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I wasn't expecting to read the phrase "thermal radiation from a cow" this morning. :)

 
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I recall seeing a time-division technique (not in the same application as this) where the power (which caused some sort of interference to the measurement) was cut for a very small fraction of a second while the measurement circuit took its reading in peace and quiet. The timing was so quick that nobody was bothered by the very short power cut. I don't recall any further details.

 
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Erosion is an issue over time. A friend with a pipe fabricating shop had an argon/co2 mixer control. It had two cells with elements in a bridge. One measuring heat loss of the mixture and the other of the argon to determine the mix percentage. The bridge could no longer be balanced due to erosion of the mix element. Factory said the unit could no longer be repaired. Rather than change resistances in the bridge, I opted to change the plumbing so the argon cell was now the mix cell. This insured the functional life pretty much doubled and saved my friend $3K.
 
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I'm with Keith on this, there are too many non-linearities (is that a word?) to make it feasible. But hey, if we are wrong and it works out, be sure to post back on it!

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
 
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I just happened to find * an old video of the late/famous Bob Pease explaining 'The Seeley Box'.

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.


[sub]* Yes I actually do watch these sorts of videos. :) [/sub]
 
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Good old Bob Pease. As a young engineer I once wrote to the help desk at National Instruments regarding the planned obsolescence of one of their IC's (which was used by a vendor of ours), and that started a fairly long email chain between myself and a really helpful person at NI. He later called me on the phone to discuss my options with me, including having the circuits produced by an offshore foundry, or duplicating the circuits with other available parts. I thanked him for his help, and remembered the name, Robert Pease. I didn't realize until later how much esteem others held for the lowly "help desk tech" at NI, but I know exactly why they do. Bob was the inventor of the IC in question, and I had the impression he would drop everything and drive his VW up the coast to Mukilteo, WA to help me wire up some circuits if I'd have asked.
 
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"Good old Bob Pease. ..."

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.

 
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Perhaps he was conflating NS+TI --> NI

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! faq731-376 forum1529 Entire Forum list
 
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