I demagnetized my thermometer

[DiscoInferno]

Loaded the stove for on overnight burn last night. Started with the still-burning remains of several earlier logs, so the stove was already 450 F and I could only fit three more logs in. (With apologies to George Thorogood, I think it was one hickory, one oak, and one dogwood.) Opened the air up, and left for a few minutes while it heated up. Five minutes later I hear a clunk; the fire is raging and the thermometer (which sits on the little stub of black pipe behind the top louvres in my Ultima ZC) has dropped onto the stovetop. I have no idea how hot it was, but I hit 800 once before, and it didn't fall. The package it came in says this will happen around 1000. (Yikes!) Nothing was glowing or anything, and it calmed down fine when I cut the air, but lesson learned. Curiously, the instructions for the Ultima say that air wide open is an acceptable (if inefficient) operating mode; perhaps that is for chimneys shorter than my 25'.

Oh, and today it's a magnet again.

 

[Andre B.]

Oh, and today it's a magnet again.

Permanent magnets available today are awesome compared to what they had 40 years ago. Likely it was not the magnet that demagnetized but the steel pipe it was on. At somewhere around 1400 or 1500 deg. F steel becomes non magnetic and a magnet will not hold same as a magnet will not hold to aluminum.
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Andre' B.

 

[Corey]

The temp where the magnetic properties disappear is called the Curie temperature. For common ferrite magnets and ceramic magnets it's about 860F, some are higher, some lower. Most low carbon steels ie stove pipe are around 1400F - which is also starting to get into the "cherry red glow" stage. Lets hope it was the magnet, not the steel!!

Corey

 

[DiscoInferno]

I can believe it hit 860, since I've seen it hit 800 once before. (It looks like a ferrite magnet.) 1400 would be a stretch in that short a time, and nothing was glowing (except the fire itself, of course).

 

[Andre B.]

The temp where the magnetic properties disappear is called the Curie temperature. For common ferrite magnets and ceramic magnets it's about 860F, some are higher, some lower. Most low carbon steels ie stove pipe are around 1400F - which is also starting to get into the "cherry red glow" stage. Lets hope it was the magnet, not the steel!!

Corey

I thought some of the magnets were a lot higher then that, I guess even if so that does not mean they use them in stove pipe thermometers.
__________
Andre' B.

 

[MrGriz]

I just bought one of the Rutland magnetic thermometers. There was a warning that you needed to wire it to the pipe (or whatever surface you are mounting it to) in any vertical application. They indicate that the magnet can let go over a certain temp and drop this hot bomb. I forgot what temp they listed and I threw away the package.

 

[Corey]

The temp where the magnetic properties disappear is called the Curie temperature. For common ferrite magnets and ceramic magnets it's about 860F, some are higher, some lower. Most low carbon steels ie stove pipe are around 1400F - which is also starting to get into the "cherry red glow" stage. Lets hope it was the magnet, not the steel!!

Corey

I thought some of the magnets were a lot higher then that, I guess even if so that does not mean they use them in stove pipe thermometers.
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Andre' B.

They can be...that is why I threw in the phrase "some are higher, some lower" Although I suspect the stove thermometer has a simple ferrite or ceramic magnet. I doubt the mfr went to the trouble to specify some exotic high Tc material. Although anything is possible

Corey

 

[DiscoInferno]

The Rutland is what I have, it looks like a standard cheap ferrite magnet, and the packaging says 1000 F is the demag temp (although I doubt my stove was quite that hot). This is a pretty common thermometer; am I really the only one to get it to drop, or just the only one who didn't use the wire? (My short stub of pipe is behind a metal louvre, so I didn't bother.) How high up the pipe do most people put their thermometer?

 

[Corey]

I don't know about the wires. Never heard of anyone else loosing a thermometer!

The 1000F may be what the manufacturer believes the Curie point of that specific magnetic alloy is. Although the trick here is that the Curie temp is the point where the magnet looses all its ferromagnetic properties. For it to slip off a vertical pipe, you don't necessarily have to reach the Curie temp...the magnet just has to be hot enough that it can't hold it's weight to the pipe. This could be a point well below the Curie temp.

Corey

 

[MrGriz]

I am burning an insert so I don't have a place to attach it to the pipe (liner). I have the thermometer laying flat on the cooktop of the insert.

 

[Gooserider]

Our stove has a horizontal flue that goes into a T fitting that connects to the chimney liner The thermometer is sitting vertically in the curve of the T at the junction with the liner. I've pegged it a time or two, and at least once had a serious glow (not intended, but while I was just getting started with learning this stove) but have never had the thermometer fall off. (yet)

Gooserider

 

[hardwood715]

They indicate that the magnet can let go over a certain temp and drop this hot bomb. I forgot what temp they listed and I threw away the package.

Thank God you were there and it landed on the stove, it may have turned out differently. at that temp it could become a rolling fire starter!

 

[babalu87]

I just bought one of the Rutland magnetic thermometers. There was a warning that you needed to wire it to the pipe (or whatever surface you are mounting it to) in any vertical application.

I screwed mine to the stove pipe with the screw they provided in the package, hasnt moved since
Its about 30" over the top of the stove on the single wall.

 

[Mike Wilson]

I did the same thing to the same thermometer last year, stove was at 800-850 when it happened... personally I think the Rutland thermometer is a piece of replaceable junk... which is why I went out and bought another one for a whopping 8 bucks.
Anyone have any suggestions on better quality magnetic thermometers?

-- Mike

 

[Gooserider]

I did the same thing to the same thermometer last year, stove was at 800-850 when it happened... personally I think the Rutland thermometer is a piece of replaceable junk... which is why I went out and bought another one for a whopping 8 bucks.
Anyone have any suggestions on better quality magnetic thermometers?

-- Mike

I think the terms "good quality" and "magnetic bimetal thermometer" are contradictory and mutually exclusive... I haven't seen one yet that I'd really trust. I'm still looking for a good digital unit, found some stuff that's close, but not quite.

Gooserider

 

[Andre B.]

I haven't seen one yet that I'd really trust. I'm still looking for a good digital unit, found some stuff that's close, but not quite.

Gooserider

Something like this.
http://www.turbobits.co.uk/acatalog/digital_exhaust_gas_temperature_egt_gauge.html

Or this, two probes put one at the stove end of the pipe and one where it goes into the chimney. See how much it drops over the 4 to 6 feet of single wall, more then you think I am betting.
(broken link removed to http://www.hoytmeter.com/pyro2001/pyro2001.html)
With this one you could swap out the boost pressure sensor for a sensor that would read the vacuum(draft) in the fire box.
(broken link removed to http://www.hoytmeter.com/pyroboostfx/boostfx.html)

Now ya got me thinking, just how much instrumentation can you get on a stove. We can have bragging rights for the longest "PreFire Checklist".
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Andre' B.

 

[Andre B.]

Aircraft.
(broken link removed to http://www.sportflyingshop.com/Instr/Stratomaster/SmartSingles/tc1.pdf)

There are 4 channels individually configurable for EGT or CHT, digital and bar display, and a programmable alarm for each channel.

Maybe over the top.
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Andre' B.

 

[Gooserider]

Good stuff for the "cool factor", but I'm on a reasonable budget!

THIS is more the sort of thing I had in mind, and we have gotten similar items for even less - our local hardware store is currently selling one with a remote for $10.00! Why can't someone make an equivalent form factor device for monitoring stoves?

It would presumably need a different thermocouple, and a different calibration for it. Also there would be a need for slightly different programming and button functions, but that would probably be relatively trivial as the requirements are less than what one of these meat thermometers does right now.

(BTW, we use one regularly for cooking, and they are fantastic)

Gooserider

 

[Andre B.]

Why can't someone make an equivalent form factor device for monitoring stoves?
Gooserider

I would guess it is just simple volume economics. Not anywhere near as many people running wood stoves (and also care to monitor them that closely) as there are people cooking.

The auto stuff exists because when you paid 20 or 30 thousand dollars for an engine that you are going to be pushing as close to the limits as you can get a few more dollars for an instrument gets lost in the round off errors.
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Andre' B.

 

[rover47]

Speaking of the wire that came with the rutland, were you actually able to use it? I couldn't get it untangled enough to use. The thermometer is on the pipe about 12'' above the stove and hasn't moved yet. I may look into the screw thing. I was toying with the idea of using a gas grill thermometer in the pipe and just putting the magnet on the stove surface. Was 7 here this am.