Good day,

I hope im posting this in the right area.. it my first time making a post on the forum!

I was wondering someone can enlighten me a bit on the following subject .

I'm thinking of making a tube furnace for some lab work, as these things are way way to expensive to buy, and it should not be all that difficult to building one.
I have seen a few ppl make tube furnaces using nichrome or kanthal wire (resistive heating), this is super simple, but this wire will need to be replaced every so often, so i was thinking a more permanent solution would be to use a induction coil to heat a graphite tube (used as heating element), with a inner ceramic tube that would contain the samples to be heated..

I was just wondering if a thermal couple would still work inside a induction furnace..? as its hes a conductive/light magnetic casing,  would grounding the thermal couple casing be enough to prevent the direct heating of the outer casing or ferrous material where the thermal couple wire is made from..?
or could a high enough freq be enough to not allow the current to penetrate deep enough into the sample area (skin effect), so that this would not be problem at all.


kind regards

 

 

 

 

We use induction heaters at my work to convert liquid white/yellow phosphorus to red phosphorus at 270^o C.

The vessel is not enclosed by the induction heaters, as an agitator provides temperature and red phosphorus distribution, and thermocouples seem to work quite well in areas away from the heaters.

Sorry I cannot ffer any help for your particular situation.

On 8/2/2022 at 12:44 AM, MigL said:

We use induction heaters at my work to convert liquid white/yellow phosphorus to red phosphorus at 270^o C.

The vessel is not enclosed by the induction heaters, as an agitator provides temperature and red phosphorus distribution, and thermocouples seem to work quite well in areas away from the heaters.

Sorry I cannot ffer any help for your particular situation.

Olah! Thank you for your response!

Is see. im not sure if i can keep the themocouple away from the element, as its all quite small. 
what i did read is higher freq do help, so might just go to 500khz and try and see if it heats the thermocouple in any meaningful way,
I could also get a pyro sensor,but these are expensive as hell in the range i would need it to be. and will cost me more then the build of the actual oven, although still cheap compared to buying one.

i will do some experiments, and try and figure out if there is a way to use a themocouple inside the coils field itself.

thx!

 

 

Your project sounds really interesting—building a tube furnace is definitely doable, but there are a few things to keep in mind, especially if you go the induction heating route.

A few points:

1. Induction vs resistive heating:
   Nichrome or Kanthal wire is simple, but they wear out over time. Induction heating a graphite tube can be more “permanent,” but you’ll need to think about coil design, power supply, and cooling. Graphite heats via eddy currents induced in it by the coil.

2. Thermocouple use:
   Induction fields can induce currents in metallic thermocouples, leading to inaccurate readings or even heating the thermocouple itself. Some tips:

   • Use a ceramic or quartz sheath to electrically isolate the thermocouple from the graphite tube.

   • Grounding the thermocouple casing helps, but might not fully eliminate induced currents.

   • Non-magnetic thermocouples (like Type C or S) are less affected than ferrous ones.

   • At high frequencies, the skin effect keeps currents near the tube surface, which may reduce interference—but exact frequency depends on tube size and conductivity.

3. Practical setup:
   Many builders place the thermocouple in a small ceramic sheath inside the graphite tube to isolate it. This usually minimizes interference and allows accurate temperature measurement.

In short, it can work, but careful design of thermocouple placement, shielding, and grounding is key. Experimenting with lower frequencies first can help gauge interference.