Author Topic: Talking Thermodynamics  (Read 133792 times)

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1335 on: July 07, 2020, 03:01:35 AM »
Hi MJM, just a quick question ...wondering what the central bar of the thermostat is made of ?? I usually saw these apart to use the brass tube ??!!  and  don't worry Per  about the toaster as I count to 100 and then switch it off. I am sure one could do a calculation here ...Colour = time ,,volts. area.. waste heat.. bread density .. etc etc    :lolb: :lolb: :lolb:  ok I know we are in lockdown but I'm sure we have better things to do !!!  also carboot find ...a part Boley staking tool set !!

thanks

Willy.

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1336 on: July 07, 2020, 10:35:32 AM »
Hi Willy, great to have you back, I was starting to think that you were missing in action.

I don’t really know the answer to your question.  It depends on just what is in that junction box at the end of the metal tube.

Some have a thermocouple in that outer sheath, so the junction box is simply for convenient connection.

Others have a micro switch and operate by a difference in thermal expansion coefficient between the metal of the outer sheath and the centre rod which would be welded to the sheath at the end dearest from the junction box.  Just what metal is used would depend on the required switch actuation temperature.

I found a table with a few values. 

Brass    18.7
Stainless steel. 17.3
Aluminium   23.9
Carbon steel 10.8
Tungsten 4.3

To calculate the expansion for a change in temperature, divide those values by a million, and multiply by the number of degrees C change.  Then each metre expands or contracts by that rather small number of meters.  If the outer tube is brass then carbon steel or, better still tungsten would give you some differential expansion to operate a switch.  But these are just a few standard materials, I believe there are other alloys which might have a much bigger difference that would be used in that application, but not for building bridges.

It is also possible to amplify that rather small difference using levers to increase the movement.  You can’t stop thermal expansion with any reasonable force, so there is plenty of force available to operate levers and a switch.


MJM460

The more I learn, the more I find that I still have to learn!

Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1337 on: July 07, 2020, 11:56:11 AM »
Willy - the safety is there for when you forget or are suddenly otherwise called away.

Weller used another solution in their old soldering irons. The tip had a magnet on the backside, and it lost it's magnatism when it reached a certain temperature. An iron rod 'connected' the magnet to a magnetic switch inside the handle, that switched the heating element On and Off.

Offline derekwarner

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Re: Talking Thermodynamics
« Reply #1338 on: July 07, 2020, 01:15:01 PM »
mmm... Chromium is put to good use in such thermocouple type devices with it's curious characteristic...... :happyreader:

'Chromium has unique magnetic properties in the sense that chromium is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, its magnetic ordering changes to paramagnetic'

[I stumbled on this obscure fact when trying to understand the apparent difference in the reported thickness of the electrolytic Nickle deposit [under Chromium] on 380mm diameter hydraulic cylinder piston rods as scientifically measured by a NATA accredited body in Abou Daubi , then the same cylinder rods checked for Nickle depth as measured in Denmark by a DNV registered facility....

Abou Daubi certificates confirmed 42 degrees C, Danish certificates confirmed 3 degrees C  :hammerbash:]

Derek

« Last Edit: July 07, 2020, 01:34:39 PM by derekwarner »
Derek Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1339 on: July 08, 2020, 02:29:40 AM »
Hi MJM, et al ,   I have taken the thermostat apart and the central rod is magnetic there is a makers plate with it...I have not done a spark test yet , or attempted to turn it ..yet ?
 Derek.. interesting about Nickel plating..between steel and chrome  this is the correct way to do it chemically !!! Nowadays on bikes the chrome goes strait on the steel. so in a few years it starts to peel off...after the guarantee ,of course..

willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1340 on: July 08, 2020, 12:52:47 PM »
Hi Willy, there you two more possible explanations of the mechanism.  From the picture, I am not sure that it can be identified, but with it in your hand and turning it around, you might be able to work it out.

As for the rod material, your original question, your spark test and a test cut should tell you something useful, but I am no expert on metallurgy.  And worth testing it for corrosion before putting a lot of work into it.

MJM460
The more I learn, the more I find that I still have to learn!

Offline steam guy willy

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Re: Talking Thermodynamics
« Reply #1341 on: July 10, 2020, 02:31:57 AM »
Hi MJM, ok...the spark test is a deep red dusty looking a bit like the tipped tool steel but more red than orange   and it turns nicely with curly swarfe it also files well and docent want to go rusty ???? also a pic of the thermo contact breaker...

Willy

Offline MJM460

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Re: Talking Thermodynamics
« Reply #1342 on: July 10, 2020, 10:50:53 AM »
Hi Willy,

It sounds like that rod will be useful for something.  Can’t beat material that is good to machine and also resists corrosion. You will have to tell me what material the spark test colour is indicating.  You might also rescue some points for an engine, though the spring material might not be good for a high number of cycles.  Perhaps more suitable as a boiler cut off switch.

As to the switch, is there a chance that you have removed a part that allows the movement of that part with the green tape to open and close the contact as it moves?  It certainly looks like a simple device, that would have been quite reliable.

MJM460
The more I learn, the more I find that I still have to learn!

Offline Admiral_dk

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Re: Talking Thermodynamics
« Reply #1343 on: July 10, 2020, 12:12:15 PM »
The picture of the switch imidiately made me think Bi-Metal when I first saw it and these can be adjusted simply with a screw pressing on the Bi-Metal part ....
If this is the case, there should be a "Heat transport system" - so is the yellow rod by any chance copper ?... Certain brass types are ok (not great) in this aplication too ....


Offline MJM460

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Re: Talking Thermodynamics
« Reply #1344 on: July 10, 2020, 12:38:12 PM »
Hi Admiral, it’s certainly a puzzle.

In Willy’s first picture, and implied by the original question, the operation seems to depend on a rod within a brass tube, and perhaps differential expansion, but how that connects to the switch is still mysterious.  I am not so familiar with these things.  The oil industry changed to electronic controllers long ago.  I did not have much to do with the early switches that were used in some applications for lower cost, but did not have a good reputation for reliability.  Hence the change to controllers

MJM460

The more I learn, the more I find that I still have to learn!