kiln thermal sense (like a voltage divider) - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: May 09, 2018, 07:31:54 pm
content: Kilns pose unique problems to safey systems. Typically with a oven you would use a mechanical thermal switch in addition to your electrical switch that uses a thermocouple or whatever.A kiln goes to 1300C or more. The margin between failure and full power operation is small:A kanthal A1 element is rated for 1400C, but melts at 1500C. This means you only have 100C margin, so it would be good to have a thermal shutdown mechanical at 1450C, incase the electronics go haywire, meaning we need accuracy of +-25C.The time constants are a huge variable, and I don't think you would want to design a poorly insulated kiln that can survive at maximum line voltage indefinitely.Are there refractory materials that would allow you to weaken the insulation in a particular point that would allow for a high temperature thermostat to be placed there (they go up to 750F) and be repeatable over time to +-25C while being stable AND having reasonable thermal time constants? Is trying to engineer this thermal design a waste of time?are there ultra high temperature ceramic thermostats that rely on metal melting to thermally expand in existence? hard to imagine.. .but the SLAM does exist and it did some pretty incredible things.

author: daqq
author_rank: Super Contributor
timestamp: May 10, 2018, 07:07:58 am
content: Could you sense the temperature remotely? Infra cams/thermopiles, such as these fellows:https://www.micro-epsilon.com/temperature-sensors/thermoMETER_CT_laser/Pretty much no time constants involved.

author: jeduffy
author_rank: Contributor
timestamp: May 10, 2018, 05:30:26 pm
content: A C type thermocouple should be able to do the job, these for examplehttps://www.omega.com/pptst/T5R_T3R.htmlare rated up to 2760C.

author: CopperCone
author_rank: Super Contributor
timestamp: May 11, 2018, 01:16:50 am
content: No, its not that I don't know of those solutions, I just mean I wanted the most intrinsically simple thing possible.IR Sensors and redundant thermocouples are an option, but IMO they don't beat out a mechanical device in terms of reliability and systems variation (this increases safety reliability IMO, you are not placing all your eggs in one basket, i.e. monster transient, insane RF field, etc).Both thermocouples and IR sensors are complicated electrical systems, in comparison to a mechanical thermostat.I like variation and triple redundancy.The second best solution would be to use a secondary thermocouple or IR sensor or both with electronics that are isolated and heavily protected used to open a contactor in a super visor position, like you say.

author: duak
author_rank: Super Contributor
timestamp: May 11, 2018, 02:31:31 am
content: A few ideas:1.) make a series connected thermal fuse out of the same material as the main element. I think if the wire is coiled fairly tightly (but not with the turns touching) and within a cavity like the main element, it will operate at a slightly higher temperature. It should open first thus shutting things down.2.) I haven't worked with Kanthal so I don't know how physically stable it is at high temperatures. I wonder if the main element couldn't be put under a slight tension that is monitored. If the tension falls then the element may be overheating and may be creeping, perhaps irreversibly.3.) I have a little oven in the workshop that uses a metal tab a few cm long to conduct heat from the oven to the temperature control. By doing something similar you might be able to use a cheaper adjustable thermal switch that's set just slightly above the maximum anticipated temperature. Kind of like an overvoltage crowbar on a power supply.4.) Do you know about Pyrometric cones? These are little ceramic things that melt at certain precise temperatures. Perhaps one or two could be placed close to the element and a switch detects if a cone gets distorted.Cheers,

author: Cerebus
author_rank: Super Contributor
timestamp: May 11, 2018, 03:09:11 am
content: Quote from: duak on May 11, 2018, 02:31:31 amThat is exactly the method that was used in older pottery kilns to automatically shut them down once firing was complete. Gas or electric, using a valve or switch respectively on the outside with a sprung lever resting on a pyrometric cone inside the kiln.

author: Gregg
author_rank: Super Contributor
timestamp: May 11, 2018, 07:16:57 pm
content: Here is a really old school low tech solution. Doble steam cars used a flash tube boiler that was controlled by a Hastelloy tube filled with short sections of fused quartz rod. Here is patent documentationhttps://patents.google.com/patent/US1955274https://patents.google.com/patent/US1955274parts #66 through #69 on the drawing are explained in the narrative. The principle is that the Hastelloy tube expands at a known rate and the alloy is fairly impervious to corrosion in the harsh atmosphere of the boiler while the quartz rods expand much less. An adjustable lever is attached to a short push rod; it wouldn’t be too hard to adapt to a kiln. The fused quartz rods were about 10mm diameter and about 60mm long and finished square on the ends. The stack of short rods was a lot less fragile than a single long rod and served the same purpose. To get the short rods out of the long tube we used a long air nozzle and the Bernoulli effect.

author: CopperCone
author_rank: Super Contributor
timestamp: May 11, 2018, 10:03:20 pm
content: yes these are the kind of solutions i am interested in