Nichrome Resistance Wire Consistently Failing

[LotusDome]

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

 

[phinds]

...
only gets its two heating plates up to no more than 140ºF, when tested in the summer in open-air conditions

That doesn't sound like a real-world test to me since the use is in an enclosed space. Do you know what it gets up to in actual use? Even so, though, that doesn't sound like anywhere near high enough temp to cause it to break.

Does the wire break at a bend or just somewhere along the length?

 

[Bystander]

Does the wire break at a bend

It does sound like a kinked hot spot. The care described in assembly argues against such a thing not being noticed. Next time you assemble a plate, you might try smearing heat sink compound between the anodized plates, or at least along the heating element to guarantee heat transfer away from any hot spots. Never had any trouble with a "run-away" high resistance spot burn through at those temperatures with similar assemblies in grad school.

 

[Doug Huffman]

Hot spots and corrosion were my guesses. Aluminum is pretty reactive, forms an oxide layer that is probably not a good HX surface.

 

[Bystander]

oxide layer that

He said, "anodized" which actually isn't that bad, but might abrade kinks in the filament wire aggravating the hot spot problem.

 

[meBigGuy]

In the absense of any other explanations, more data is needed. I would try periodically measuring the current during operation. If the current decreases after a month or two, it might indicate that a high resistance spot is developing. At that point you can disassemble and try to determine what is happening.

Also, looking at the used plate that is still working might give you a clue. Is it heating unevenly. Has its resistance changed? Do you have access to a thermal camera or IR thermometer?

Just rambling.

 

[nsaspook]

Don't know what the problem is but here's a good checklist: https://www.watlow.com/reference/guides/0402.cfm

 

[Bystander]

here's a good checklist

What're you trying to do? Break up a "helpful" speculation party with a bunch of ugly facts?

 

[NascentOxygen]

Can you examine the break (both sides) under a low-power microscope? Looking for heat discoloration, narrowing, pitting, corrosion products.

The bare nichrome is in direct contact with the anodized layer on the aluminium? What is the appearance of the aluminium in the spot where the wire break developed?

 

[jim hardy]

One wonders if there's a short to aluminum plate shortly before failure.
First short doesn't hurt anything, but the second one completes a circuit.

Could you use a Cal-Rod heater and slightly thicker plates to make up for the wider spacing ?

Everyday RTV Silicone releases acetic acid during cure.

http://www.logwell.com/tech/servtips/RTV.html

Common RTV silicone made for automotive and general purpose use, the stuff that smells like vinegar, is corrosive to electronics, and its use is a death sentence to reliability. ........
...

RTV silicone made for non-corrosive electronics applications uses an alcohol curing system rather than an acetic acid curing system. The alcohol cure products have a slightly sweet or ethereal odor as opposed to the distinctive vinegar odor of the common acetic acid cure products. Both are one part curing systems that depend on air moisture to cure into silicone rubber. Examples of non-corrosive RTV silicone pastes are: clear or gray Dow Corning 3145; white General Electric RTV 162 or gray high strength RTV 167; and clear Loctite 5140. All these items are ridiculously expensive, no doubt the reason Oilex and others have been tempted to use the less expensive dime store variety acetic acid cure RTV silicone products. The General Cement (GC) Electronics 10-150 Silicone Rubber Adhesive Sealant is an acetic acid cure product and should not be used for electronics work, despite GC's representations to the contrary.But help has arrived! Presumably in response to consumer complaints about odor, the major manufacturers have introduced non-acetic cure silicone products for the consumer market. General Electric Silicone II is the best we have found. It comes in handy 2.8 fluid ounce tubes in clear and white (the exact same tube they now pack their high priced material in), and is available from mass merchandisers like Wal-Mart at perhaps one-tenth the cost of the above mentioned products.

Which type are you using?Can you post a picture of a failed plate's heated side with wires still in place ?

 

[Bystander]

Everyday RTV Silicone releases acetic acid during cure.

And let's give that man a cigar --- not something to have sealed between plates with a heating element.

 

[RonL]

Taking lessons from electric kiln design, might suggest a need for coiled or spring tension to keep expansion and shrinkage under control ?

 

[The Electrician]

I bet you could find something from these people that would work:

http://www.thermo-llc.com/products/polymer-thick-film-heaters-ptf/

 

[jim hardy]

Interesting link there Electrician
http://www.thermo-llc.com/wp-content/uploads/2013/05/PTF-DETAIL-SHEET.pdf
that it's self regulating ( PTC ) is beautiful.

An old hand once told me "Always look for a way to get Mother Nature to help your design work".

 

[LotusDome]

Thank you all so much for your advice; it is very much appreciated!

I sincerely apologize for not being able to respond for the past few days, but no sunlight = no electricity = no on-line access.

To start, http://www.LotusDome.com/Heating_Plate.jpg is a photo of one of the heating plates (10" x 12"), opened up after the first set of four hours of testing. The pale-white residue around the edges and the bolt holes is the (clear) RTV silicone.

Please allow me to address the suggestions in order of posts:

from phinds:

“That doesn't sound like a real-world test to me since the use is in an enclosed space. Do you know what it gets up to in actual use?”

The open-air testing, in the summer, was specifically done to test the maximum temperature the plates could ever achieve. In use, they are buried under a few inches of compost, inside a drum that vents through a four-inch pipe straight up to open air, which, in the winter can get below -20°F.

In fact, liquids seeping through the compost and draining into the evaporation chamber—only three inches below the drum and the two heating plates bolted to the inside bottom of it—will regularly freeze in the winter, and if too deep will freeze to the bottom of the drum, while the heating plates are on!, making it unable to rotate. Regulating moisture content of the compost is critical! ;-)

The PID controller (using a teflon-encased temperature sensor set into the compost, alongside another temperature sensor for comparison) is programmed to keep the temperature of the compost at 80°F and due to the elevation, the heating plates are on almost every night of the year. For six months or more, they are on all day as well and even still, the compost can drop below 60°F during chilly nights or high winds.

Thus, as per the above (hopefully not too excessive!) details, it doesn’t make sense that temperature could cause the wire to break after, what appears to be, a consistent amount of time.

“Does the wire break at a bend or just somewhere along the length?”

The breaks have, so far, all been in the middle of straight runs. I do not know where the latest break is, as I really need some well-considered course of action before I take a few days to completely disassemble and rebuild my only toilet, high on a mountain, in the middle of winter! The past breaks have appeared to be melts, sometimes with very small balls of copper forming at one or both ends of the break. That is why I went to great effort to ensure that the 16-gauge wire was pressed tightly between the two anodized plates.

from Bystander:

“Next time you assemble a plate, you might try smearing heat sink compound between the anodized plates, or at least along the heating element to guarantee heat transfer away from any hot spots.”

Thank you for the suggestion! Again, that is why I carefully pressed the wire between the two plates, but it certainly would help!

“He said, "anodized" which actually isn't that bad, but might abrade kinks in the filament wire aggravating the hot spot problem.”

Each of the anodized aluminum plates in this latest version (repeatedly tested to ensure no conductivity between the wire and the plate), has a layer of clear packing tape taped to it to also ensure no shorts between the wire and the plate. During the hours of 140°F testing (that could never be reached during actual use), the packing tape didn’t even get singed, much less melt. The first test actually used plain scotch tape (as shown in the above-linked photo, after testing) to ensure the runs across the plate remained evenly spaced. Both the initial scotch tape and the final packing tape were completely unfazed by the heat generated by the wire. Even if the packing tape had melted, which is exceedingly unlikely, the wire would not have shorted out to the plate.

I was quite careful in unspooling the wire to ensure there were no kinks. Also, given that the plates are bolted together and maintain a steady range of (relatively) low temperatures (50-80°F) abrasion seems quite unlikely. But that’s just my opinion!

from: meBigGuy

“In the absense of any other explanations, more data is needed. I would try periodically measuring the current during operation. If the current decreases after a month or two, it might indicate that a high resistance spot is developing. At that point you can disassemble and try to determine what is happening.

Also, looking at the used plate that is still working might give you a clue. Is it heating unevenly. Has its resistance changed? Do you have access to a thermal camera or IR thermometer? ”

Thank you for the suggestions! After a few months, I stopped measuring current draw at the drum because any wide changes would be noticed by monitoring at my TM-2020 meter. The current of the still-functional plate does not appear to have changed since it was made. (Please forgive me for not having more elaborate testing equipment!)

from: Jim Hardy:

“One wonders if there's a short to aluminum plate shortly before failure. First short doesn't hurt anything, but the second one completes a circuit.”

A pair of shorts would result in the length of resistance wire being reduced and thus would result in significant changes in both temperatures and current. I saw no such results. (But that doesn’t mean it might not have suddenly occurred, long after being assembled, and right before breaking!)

“Everyday RTV Silicone releases acetic acid during cure. ”

I have been using DAP 100% Silicone Caulking; I must assume it is an RTV silicone. I have used the same in the past to secure 20-gauge resistance wire to aluminum sheets, to avoid shorts. I never saw any degradation of the wire in any of the earlier assemblies, though they also broke after about 18 months.

Could the RTV silicone possibly be the cause of 16-gauge wire consistently breaking/melting after 17-18 months of contact? Practically speaking, how likely is that, given that the heating plate assemblies have consistently failed, at relatively low temperatures?

from: The Electrician:

“I bet you could find something from these people that would work:

http://www.thermo-llc.com/products/polymer-thick-film-heaters-ptf/”

Thanks for the link! When I was refining the design, I tried to find such heating element films with no luck, thus, my home-made solution. They probably aren’t cheap, but they may be necessary!

One critical issue that haunts me: Is there some consistent degradation over time of the nichrome resistance wire itself, simply due to current flowing through it, regardless of the temperature generated or any RTV silicone? The wire in each heating plate assembly is carefully protected from any liquids in the compost, and being pressed between two plates was specifically done to avoid any temperature fluctuations within the wire that might result from the wire pulling away from a plate.

Thank you all very much, yet again, for any insights you might be willing to offer!,

Richard Fairbanks

 

[jim hardy]

You are certainly a thorough fellow !

Do you know what type of Nichrome you have?

One possibility to rule out is chloride stress corrosion,
It makes microcracks that grow along grain boundaries surprisingly fast
especially under tensile stress.

go to the 3M site and see if your tape is made from PolyvinylChloride

and inquire of wire supplier about Chloride Stress Corrosion resistance for the alloy you have
the ones with significant Fe are likely susceptible

Search engines are toying with me tonight. Can't find a good paper on chloride stress corrosion in Nichrome. Too many different Nichromes, i guess..

Abstract

Chloride stress corrosion cracking of stainless steel under insulation can occur because of concentration of chlorides at the metal surface. This paper discusses our company's current recommended practices for controlling this type of corrosion and results of a research program to improve our recommended practices.

Our current recommended practice for controlling corrosion of stainless steel under insulation is to select insulation materials low in extractable chlorides, jacket or coat insulation to exclude water and chlorides from the environment, and to coat the stainless steel equipment where applicable. Coating stainless steel to prevent contact with chlorides is recommended with some reservation because of generally poor experience in coating stainless steel equipment.

http://www.astm.org/SUBSCRIPTION/DIGITAL_LIBRARY/STP/PAGES/STP39166S.htm

might be as simple as a different wire or tape.

See bottom of page 9 here , the table showing chloride stress corrosion inception times for a few different alloys.
http://www.haynesintl.com/pdf/h3179.pdf
their engineering department would likely suggest an alloy ...

 

[jim hardy]

Aluminum expands more with temperature than most steels.
http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

if clamped tightly your wire will be under tension when the assembly heats.Monel 400 looks interesting...
http://www.niwire.com/product/corrosion-resistant-alloy/monel-400/

good luck !

old jim

 

[sophiecentaur]

I have only just come to this thread but the first thing that struck me is that ordinary Electrical Immersion Heaters work fine for years and years - in hot water tanks. Why not use a simple immersion heater in a small s/s water tank and use that as an intermediate heating element? That way, someone else has taken care of the mechanical / electrical problems of the electrical heater and all you need to do is make a heat exchanger. The heater could even be remote from the composter and you could use convection to get the heat inside it. Car Coolant would allow you to operate right up to 100C without boiling / bursting.

 

[Windadct]

Hello Lotus -

Note on the silicone - in addition to the noted acidic curing issue - most RTV Silicones are Hydroscopic - meaning they absorb water - add to this thermal cycling and you accelerate the effect... so my first guess is you are getting the nichrome wet (or just moist) with an acidic solution... not so good.

You may want to look into a marine store - for a true water proof and harsh environment type sealant. -- Found this http://www.dxengineering.com/parts/umi-82180

Looking at your plates - is is possible to get someone with a router to make channels for the wire? It looks like when you clamp the plates together - they are clamping against the wire - while good for thermal transfer - not so good for the wire - remember as this heats and cools it is moving back and forth- since it is very unlikely (impossible) the plates are clamped uniformly across all the wire there will be a couple locations that take all of the mechanical stress. ( with the above permatex - you may want to use a bead along all of the wire - to set the wire in place and probably have a better thermal conductivity than the air around the wire.

I know you want to keep this as a DIY - and an electrical heater seems to be a simple enough system - making them work reliably in a real world application is really not trivial- and the compost is actually a pretty harsh environment.. So -- if the heaters are critical - you may want to consider some silicone strip heaters - you can get them for 12 V

Side note : You seem to be doing excellent considering what you have been through and where you are! -- keep in touch.

 

[sophiecentaur]

@Lotus
I am surprised that you can source Resistance Wire as cheaply as a commercial heater, actually (the way the market works in these matters). Seriously, though, you want this to be a long-term working system, presumably, so it would be in your interest to treat it as a serious Engineering Project. That implies using available technology when it is not much more expensive than DIY. Engineering tends only to be 'fun' when the results work and survive for a reasonable time. I have to agree that it would be good to make it all for yourself, from scratch, but don't you also want a working composter?
At the very least, I would suggest isolating the wires from the 'grot' with an intermediate layer of corrosion inhibiting coolant, in a tank.

 

[jim hardy]

Sophie's thermosiphon idea is intriguing
If heat source is lower than heat sink, natural circulation will commence.
a flat plate solar collector mounted lower than the composter would give direct solar heat by day at probably 10X efficiency of photovoltaic
RV antifreeze makes an environmentally friendly working fluid. It's the orange stuff at Walmart, used to winterize potable water systems in RV's.

But you'd need a south sloping hillside for that geometry.

 

[sophiecentaur]

Sophie's thermosiphon idea is intriguing
If heat source is lower than heat sink, natural circulation will commence.
a flat plate solar collector mounted lower than the composter would give direct solar heat by day at probably 10X efficiency of photovoltaic
RV antifreeze makes an environmentally friendly working fluid. It's the orange stuff at Walmart, used to winterize potable water systems in RV's.

But you'd need a south sloping hillside for that geometry.

A thermosyphon doesn't actually need the source to be lower than the sink - just a thermal gradient to get it started. (Old motor cars used thermosyphon for cooling the engine). Not very efficient, true, but a tiny circulating pump (solar electric) could achieve better heat transfer. A couple of square meters of collecting panel would give hundreds of Watts of mean power. Thermal storage, rather than electrical storage would make sense. I know this would be turning things completely upside down - but that's the price you pay when you switch on the eager young (?) minds in this forum. The do say "if it ain't broke then don't fix it"but this system appears to be broken despite an impressive run of 17 months (well done for perseverance).
I feel at this stage, we should bear in mind what sort of power we are discussing (50W?). Composting may well be exothermic - just needing a nudge to achieve a starting temperature. That plus good insulation (and being prepared to wait for the Sun to be strong, to get things going) could reduce the mean power needed.

 

[Baluncore]

This post was composed off-line, it duplicates others ideas.

The melt beads on the end of the wires at the break strongly indicate fusing.

1. Prior to fusing, is the wire cross-section reduced?
1a. By corrosion? Is any other wire corrosion visible?
1b. By physical damage?

2. If not, where does the over-voltage and high current come from?
2a. Controller transient failure?
2b. Momentary battery switching, from parallel through series?
2c. The inductive flyback from a starter motor connected to the same battery cables?
2d. An externally induced current such as a lightning strike?

3. Is a short circuit between the wire and plate able to reduce the circuit resistance?
What is the plate electrically connected to? Can you install a 1 amp “tell-tale” fuse in the ground bonding to identify if a plate current has flowed.

Stainless steel is only stainless in the presence of oxygen, I would expect similar behaviour from nichrome alloys. RTV is acidic so it will remove oxygen from the wire environment and push the corrosion chemistry towards metal migration. What does the inside face of the element plate look like after a failure. Does the wire or plate show corrosion or a darkening of the surface?

Self resetting thermal breakers as used in trucks are available. Does the heating element have a fast fuse or low voltage breaker in the circuit?What is the speed and rated current?

A 10A fuse and an SCR crowbar circuit would detect an over-voltage and protect the element from the supply. It could also help identify the source of the problem. For crowbar circuits see; http://axotron.se/index_en.php?page=26

 

[LotusDome]

Greetings, folks!

Thank you all again for all your help!

For starters ;-) :

from: sophiecentaur

“Engineering tends only to be 'fun' when the results work and survive for a reasonable time.”

<rant> Dear friend, I pray that you NEVER have ANY understanding about severe PTSD. (*VERY* FEW DO, and NONE are doctors. We will not debate this in this forum!) The living nightmare that has been my life for the past decade (as of Sunday . . . or so I’m told!) is something I would never inflict upon anyone for any reason. I am not homeless, camped out in the remote mountains of Utah, fighting for survival, for “fun.” If I had been able to find ANY other heating solution for what I NEED to accomplish, I certainly would not have done this myself. </rant>

“Why not use a simple immersion heater in a small s/s water tank and use that as an intermediate heating element?”

“I would suggest isolating the wires from the 'grot' with an intermediate layer of corrosion inhibiting coolant, in a tank.”

The suggestion of using a water tank heating element inside the drum was the obvious first consideration, and extensively explored. I did not find any such heating element that would be able to meet the electrical and space constraints. A separate tank, within the composting drum is certainly a creative!, and reasonable idea, but it would have to fit through a 4" x 9" opening into the drum and be less than ~10" long, and thus, as per the two heating plates which were thin enough to (barely) be able to go through the opening at an angle into the drum, would need to be multiplied a few times, as two may well not be enough.

“The heater could even be remote from the composter”

Please review the information already provided; a thermosyphon is wonderful in theory, but a composting drum needs to be rotated every couple of days and I have already explained why the heat source must be within the drum. As has long been noted: “In theory, theory and practice are the same.” (Please forgive my blunt reply, but when I take a mandatory shower at night, right before climbing into an often-below-freezing bed in the winter, the water is scalding—as I stand on a sheet of ice.)

I do very much appreciate the suggestions, however!; please enlighten me, as I am ignorant about fabricating such liquid-based heating element containers! As previously alluded to, but not specifically stated, I have a limit on the amount of current I can continually draw from the battery bank overnight. That limit is about 5A, 12VDC. (During extended storms, like this past weekend, I must turn off the heating plates.)

I spent two months researching possible solutions, to avoid having to make the heating plates myself. I found no other possible solutions. (Can you imagine trying to anodize aluminum, outdoors in the mountains during monsoon season, using only solar panels as a power source? <chuckle, chuckle> )

from: Jim Hardy

“Do you know what type of Nichrome you have?”

It is the NiCr80/20. Given that I could only afford one spool, I do not have any left over. Thus, I need to first figure out why the NiCr80/20 failed, and then explore how best to avoid any such failure in the future. Your (and everyone’s!) assistance is deeply appreciated!

“go to the 3M site and see if your tape is made from PolyvinylChloride”

The tape is http://www.duckbrand.com/products/packaging-tapes/hd-clear-packaging-tape and the manufacturer only states in their Q & A: “The HD Clear packaging tape is not acid free.”

“might be as simple as a different wire or tape.”

“Perchance to dream . . . ”

;-)

That would certainly be a relief!

“Aluminum expands more with temperature than most steels.”

I am not a fan of aluminum, but I respect its capabilities. I taught myself to gas-weld aluminum (out of necessity!) to fabricate the prototype Lotus Dome. The expansion, and especially contraction when welded, is certainly a significant drawback. I used aluminum for the compost drum’s heating plates because I had sheet stock available after cutting all the Lotus Dome curves, and when properly anodized, it becomes non-conductive. Making another metal sheet non-conductive would, I believe!, have created a layer of insulation that would have restricted the thermal conductivity between the wire and the metal sheet, reducing its effectiveness as a heating plate.

“if clamped tightly your wire will be under tension when the assembly heats.”

True, but 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 may well have been in error—I certainly mis-calculated somewhere! (Besides falling down a mountain, head-first into an ice wall! ;-) )

“Monel 400 looks interesting...”

It does! The question becomes (should it be both affordable and available), again, how to make it non-conductive.

from: Windadct

“so my first guess is you are getting the nichrome wet (or just moist) with an acidic solution... not so good.”

Point taken, thank you. I have used RTV silicone for so many uses, for so many years, I was ignorant of its detrimental qualities. I still use it to protect temperature sensor probes (among many other uses) and it works quite well.

“Found this PERMATEX”

Excellent lead! Thank you! (https://www.amazon.com/dp/B000HBNU9U/?tag=pfamazon01-20 it is on Amazon)

“is is possible to get someone with a router to make channels for the wire? . . . with the above permatex - you may want to use a bead along all of the wire - to set the wire in place and probably have a better thermal conductivity than the air around the wire.”

I do have a router, and could cut a grove for the wire. It would not be a simple procedure, but with a wide grove on both plates, combined with the Permatex, could alleviate both electrical conductivity and friction issues. Hmmmm! (Again, not knowing the exact cause of the breaks, I still wonder about the long-term integrity of the NiCr80/20.)

“I know you want to keep this as a DIY . . . ”

I do not (as noted above <blush> ); my ego is not that fragile, but my wallet is. (I only have to contend with the consciousness of the physical body, i.e. the PTSD.)

;-)

“the compost is actually a pretty harsh environment”

In the initial assembly, with NiCr80/20 RTV siliconed to bare aluminum plates, the aluminum had dissolved significantly around the edges after eighteen months. Thus, I chose to enclose the NiCr80/20 between two anodized plates, etc. The anodized plates have held up quite well (as reasonably expected).

“you may want to consider some silicone strip heaters - you can get them for 12 V”

My very first trial was with a Kat's 22500 60 Watt 8.5"x 11.5" Battery Pad Heater (see https://www.amazon.com/gp/product/B000I8TQ9A/?tag=pfamazon01-20, running off a small inverter). It failed after 18 months. The 22-24 gauge resistance wire inside the rubber pad burned out, and someone told me that such failures were common. Thus, I first thought a larger gauge wire would be better, sealed inside a silicone bead against an aluminum plate as a heat sink . . . and here we are.

(Thank you, especially, for the Side note—it is most appreciated, even more than all of the other replies. “I don’t want to have to take this class again!”)

Folks, please allow me to again state how much I appreciate all the suggestions and advice! It is DEEPLY appreciated! I just have to be cautious in how I proceed, as I do not want to have to go through all this again in eighteen months. Please forgive me for my “compromised” conditions (on all levels).

It appears that another reply has come in while I type this. The sun has set and it is now 47°F at my desk, please forgive any typos as I must get off-line.

Blessings to you all!,

Richard Fairbanks

 

[jim hardy]

I taught myself to gas-weld aluminum (out of necessity!) to fabricate the prototype Lotus Dome.

That's impressive ----- it doesn't change color before it melts !

 

[jim hardy]

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

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 ?

 

[jim hardy]

Industrial electronics uses a lot of this Kapton tape and sheet for insulation

http://www.dupont.com/products-and-...imide-films/brands/kapton-polyimide-film.html

it's pricier than your Duck Tape
but the life cycle cost has to include the aggravation of maintenance...
click where to buyI envy you,,,,,, living out "Paint your Wagon", eh ?

 

[LotusDome]

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 ), 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

 

[Benbenben]

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.
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From the information presented so far, I suspect your heater plates may be largely intact and easy to fix.
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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.
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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.
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So tell us how the autopsy goes. Might end up being a relatively simple fix,
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BTW, how about insulating the barrel better and perhaps placing a greenhouse type structure over it to get some help from the sun?

 

[Max Chooch]

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.

Your problem is stated above. Your attempt to ensure good contact has prevented the wire from expanding when its heated. This expansion from the heated wire pushes the plates apart just enough for that pressure to relieve itself by moving a short section of wire quite a lot within the plates. This repeated cycling eventually creates a weak spot of high resistance and the wire burns itself out.
Let the wire move unobstructed and the life of your device will increase.