How EXACTLY do immesion heater tank thermostats work ?

[RogerJ]

My first post after extensive Googling. I'm qualified as an elctronics engineer (retired) Posting this question here in hopes that an enquiring electrcian will know the answer. The last time I changed a tank stat, I disasembled the old one and kept the 18" brass tube and central steel thin rod.. ( I do things like that a lot) I can see how it works but wonder if anyone here knows if that central thin rod is plain steel or Invar..
I have a use for a thin Invar rod in a DIY clock.. Thanks..

 

[Lister1987]

My first post after extensive Googling. I'm qualified as an elctronics engineer (retired) Posting this question here in hopes that an enquiring electrcian will know the answer. The last time I changed a tank stat, I disasembled the old one and kept the 18" brass tube and central steel thin rod.. ( I do things like that a lot) I can see how it works but wonder if anyone here knows if that central thin rod is plain steel or Invar..
I have a use for a thin Invar rod in a DIY clock.. Thanks..

Best bet is to ask a manufacturer?

 

[Julie.]

Very unlikely to be invar as invar has a very low expansion rate to keep the change in length of the pendulum at a minimum, in the case of a thermostat, you want the expansion to be quite high such that the increase in length is sufficient to operate the contacts.

It's the same whether the expansion comes from the expansion of the whole rod, or from a sensor at the remote end transmitted by the rod, you wouldn't need to maintain a low rod length expansion rate.

 

[Avo Mk8]

Very unlikely to be invar as invar has a very low expansion rate to keep the change in length of the pendulum at a minimum, in the case of a thermostat, you want the expansion to be quite high such that the increase in length is sufficient to operate the contacts.

It's the same whether the expansion comes from the expansion of the whole rod, or from a sensor at the remote end transmitted by the rod, you wouldn't need to maintain a low rod length expansion rate.

Hi Julie
I did a little research on this yesterday for my own interest. and found a patent application that specified Invar for the central rod.
The principle of operation, at least for that stat, was that the outer brass tube lengthens with temperature increase, and since you want to get that movement back to operate the switch, you don't want to cancel it out with a material that also expands.
So with the brass outer and Invar centre rod, you get maximum 'gain'. if you use steel, you would get a lot less movement, and hence less 'sensitivity'.

 

[Julie.]

Hi Julie
I did a little research on this yesterday for my own interest. and found a patent application that specified Invar for the central rod.
The principle of operation, at least for that stat, was that the outer brass tube lengthens with temperature increase, and since you want to get that movement back to operate the switch, you don't want to cancel it out with a material that also expands.
So with the brass outer and Invar centre rod, you get maximum 'gain'. if you use steel, you would get a lot less movement, and hence less 'sensitivity'.

Ah OK.

I assumed it would be the central rod that would expand, but yes it makes perfect sense.

 

[brianmoooore]

It's the outer tube that provides the expansion. The inner rod is just to couple that expansion back to the switch mechanism.
Any expansion of the inner rod is undesirable, but as long as it's less than the outer tube, the 'stat will still work, but will be less sensitive the closer the expansion rates of the two materials are.
You can make a 'better' 'stat with an invar inner rod, but an 'adequate ' one with steel, and since steel is cheaper than invar, I expect that it's steel that is used.

 

[RogerJ]

It's the outer tube that provides the expansion. The inner rod is just to couple that expansion back to the switch mechanism.
Any expansion of the inner rod is undesirable, but as long as it's less than the outer tube, the 'stat will still work, but will be less sensitive the closer the expansion rates of the two materials are.
You can make a 'better' 'stat with an invar inner rod, but an 'adequate ' one with steel, and since steel is cheaper than invar, I expect that it's steel that is used.

Thanks Brian and Julie...I think you may now have guessed I'd got that far and was hoping to find if the central rod was the best answer (Invar) - or the cheaper, less sensitive option, steel.. I've no way here to tell the difference myself so thought there was a chance someone would know. I suppose it's possible that it may even differ between manufacturers.
So unless there is someone out there who knows EXACTLY we'll leave it there.. Roger

 

[RogerJ]

As it's New Years Eve I hope you won't mind me going off topic and say why I hoped it might be Invar ;-)
I've built an accurate electricaly powered pendulum clock, with a 1 metre (1 second) Invar pendulum rod. The bottom supported 15lb steel bob however expands upwards with heat causing the clock to go slightly faster with increase in temp. If I could transmit that slight rise of the bob top to a suitable point up the main rod... with a thin Invar rod...and add a small weight to the top of it, I could cancel the effect out.. happy New Year all..Roger

 

[brianmoooore]

Couldn't you get the same effect with a small additional weight attached to a brass rod, under the main bob? That weight moves down to slow the pendulum to counteract the expansion of the main bob speeding it up.
Alternatively, couldn't you attach the pendulum arm to the bob near its centre of mass, so that part f it expands upwards, and part down?

 

[RogerJ]

Hi Brian.. The second option is the solution used in high class work.. It involves drilling the bob with a shelf half way up.. and using an Invar tube back down to the rating nut.. Beyond my workshop capabilities.. But a small weight on a brass weight below is possible..food for thought there ! Thanks..