Nichrome Resistance Wire Consistently Failing

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jim hardy
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I took the risk that the temperature fluctuation would be relatively small, given the PID controller, and the relatively small size of the heating plates.
I was thinking of Monel for your heating wire not the plates
alloy 400 looks at first glance to have coefficient of expansion very close to aluminum's
and good chloride resistance
and it comes in wire

http://www.dragonalloy.com/CuNi_Alloys_213.html
Coefficient of thermal expansion @20-100C ( x10-6 K-1 ) 13.9-14.1

im having difficulty locating chloride resistance of 80-20 Nichrome
here's a tech brief that might have some search terms for a long winter nigh
thttp://materion.com/~/media/Files/PDFs/Alloy/Tech%20Briefs/AT0024-0311%20-%20Tech%20Briefs%20-%20Chloride%20Stress%20Corrosion%20Cracking%20Resistance.pdf [Broken]

just musing
if tension is a culprit , matching expansion rates should help
if it's chlorides , the right alloy should help

old jim

get those broken ends under a magnifier
can you see grains or did it melt and erase the evidence ?
 
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  • #28
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Greetings, folks!

I just wanted to thank all of you for your very-much appreciated assistance! For the past eight months, I have been using a pair of BriskHeat 24V, etched-foil heating pads (http://www.briskheat.com/pdf/silicone/briskheat-silicone-rubber-heaters-catalog.pdf [Broken]), running on 12V (nominal), mounted to the underside of two sheets of 12-gauge stainless steel plates, and “so far so good.”

It will be at least another year before I can determine if this combination of “pad and plate” is any better that the resistance wire assemblies that consistently failed every 18-24 months, but I have also re-worked the mounting frame such that the plates can be quickly replaced.


Having also been able (finally!) to get a spare/backup composting drum is a big relief for my severe PTSD as well!

;-)

I will update this thread when I have more news to report. Thank you, so much, again!

Blessings!,

Richard Fairbanks
 
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  • #29
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Greetings, folks!

I have spent the past six years developing a matching pair of 13"-square heating plates to mount to the inside bottom (front and rear) of a Sun-Mar Excel NE composting toilet drum. The heating plates are necessary to keep the compost in the composting drum between 70º and 100ºF.

I have been using Chromel-A nickel chromium resistance wire (see http://www.amazonsupply.com/dp/B000FMW8X0/ref=sp_dp_g2c_asin) mounted to aluminum plates and it works very well! The challenge is that after an average of about eighteen months, the nichrome wire breaks at some point in the circuit, thus requiring replacing both heating plates. (If one has failed, prudence requires that both be replaced.)

In the latest incarnation of each of the two assemblies, I used fifteen feet of 16-gauge nichrome wire, carefully laid out in a grid pattern, pressed between two anodized, 1/8" thick aluminum plates, bolted together with stainless steel bolts, lock nuts, and neoprene rubber washers. Nine bolts are used for each heating plate assembly to ensure that there is no point where the resistance wire is not in direct contact with both of the two plates.

The finished plates are sealed shut with a large bead of 100% silicone sealant, inside the edges of the two plates. The resistance wires are connected to 12-gauge THHN solid-core wire at the point where they leave the heating plate assembly, and the splices are sealed and secured to the plate assembly with epoxy. A PID controller is used to maintain the temperature of the compost.

It really does work quite well! Each assembly has a resistance of about 3.7 ohms and draws about 2.7A at a nominal 12-15V DC, fed by Trojan T-105 batteries, charged by solar panels. (Details HERE.)

I am offering all this (perhaps excessive!) information because, once again, after seventeen months, one of the heating plate assemblies has stopped working. I knew as soon as it happened, because its reed-switch-controlled LED indicator light ( thank you! ) went out, and after careful inspection, the cause can only be that the resistance wire has, once again (for the fourth time!), broken inside the heating plate assembly.

The two plates were tested extensively before mounting them inside the composting drum; it is a multi-day process to replace them, after the replacements have been assembled, and I am a monk, camped out in the wilderness, in the mountains of Utah. This is a really bad time for one of the heating plates to fail!!

Can anyone offer any reason why such nichrome resistance wire, especially 16-gauge!, would fail so consistently, when it is in a sealed environment, and only gets its two heating plates up to no more than 140ºF, when tested in the summer in open-air conditions.

Is there any better solution for heating these two heating plate assemblies? Any heat source outside of the composting drum is not possible, as it does get “a bit chilly” high up in the mountains of Utah! :-)

Blessings and thanks in advance!,

Richard Fairbanks
Have you performed an autopsy on the deceased heater plate? You should be able to locate the point where the circuit lost continuity. If not by simple visual inspection, then using a cheap multimeter will definitely get you there.
You can also check to make sure both ends of the wire present high resistance to both individual plates. That is important information.
.
From the information presented so far, I suspect your heater plates may be largely intact and easy to fix.
.
With less than 50 watts heat being transferred through a set ot reasonably large aluminum plates to a barrel and compost, corrossions heat cycle stress is not working very quickly to the detriment of the nichrome.
.
Your inital explanation has a curiosity which may point to the eventual mode of failure.
The resistance for that length of 16 gauge nichrome 80 is exactly where it should be.... you measure that before wiring to the 12 gauge THHN, right?
The curiosity it your amperage. 12 to 15 volts should be pushing 3.2 to 4.3 Amps through 3.7 ohms.
To get down to 2.7 amps the resistance needs to be 5 ohms for 13.5 volts (halfway).
That means the is another ohm of resistance and while the other 3.7 ohms are spread over 15 feet, this additional resistance is likely to occur of a very short distance.
To make matters more dire, that extra resistance is not in good contact with the heat sink as it is enveloped in epoxy.
The final kicker is that when the epoxy heats up, it likely produces corrosive agents which attack the copper and/or nichrome....eventually leading to failure.
.
So tell us how the autopsy goes. Might end up being a relatively simple fix,
.
BTW, how about insulating the barrel better and perhaps placing a greenhouse type structure over it to get some help from the sun?
 

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