kiln heating twice as fast as is programmed

[sine]

Hi I have a Nabertherm eco 45 electric kiln, bought in 2007. Its a small kiln with only 4 elements ( ie 2 elements joined x 2)

and a pretty basic controller which can store 2 programmes.

Works fine usually, although mostly used for low bisque firing around 960' Celsius.

Replaced elements in Feb. 2020, as was needed and did maybe ten 1260' Celsius glaze firings.

All working fine as normal, during Covid lock down..

Recently, I put a heavy saggar to bisque  at a slow 10 hr climb to 600' and then rising to 1020' normal bisque

but it exploded about 250' ish. I put it down to an air bubble...not a common occurrence

Gone to put on another 1260'c glaze firing but keeps failing to heat, reaching 300's or 400's, the programmer shows ' heat' 

and the kiln does 'click' a bit but not the usual strong clicking sound as it heats.

Eventually the temperatures drops totally as it s cooling yet there is no error showing up on the controller at all

I have checked the 4 elements with strips of paper and all are working that I can see.

Usually if elements need to be changed, it would get up to about 900'c and then limp along so I know its time to change them.

This is very different.

Just tried again a normal bisque firing to 960'c, at 6hrs to 600' c, and after only 3 hours it was hitting 530'c so nearly twice as fast

but the controller is not showing any fault errors.

The thermocouple inside kiln looks fine, even after the exploded saggar...

Any thoughts would be great- is it an electrical voltage issue at my workshop?

Would that explain the sudden increase of the rate of temperature?

Relays were mentioned to me, although I was told they tend to either work or not- I've never had to replace before

p.s. the weights on the lid is to counter the warping, creeping along each yr

With thanks

[Bill Kielb]

You may have a relay that is stuck on for the mechanical types and even if the newer solid state will sometimes fail in the on position. I believe this kiln has a lid switch which If true  makes diagnosing A bit tougher. My thoughts check for a relay that is stuck on or in the event of solid state stuck on or partially on. Clever kiln lid lock BTW!

[neilestrick]

It could be a sticky relay like Bill said. Sticking relays are difficult to diagnose unless you catch them in the act or have a laser thermometer to check if they're running hot. Often they only stick for a couple of firings before failing completely, though. It could also be a thermocouple problem, despite it looking okay. What type of thermocouple does it have? Type K, S, R...? I would also check the thermocouple wire connections, in case one came loose or the wire broke under the set screw (I see that a lot).

[sine]

Thanks a lot Bill and Neil, 

Will check the relay, the lid does have a spring that attaches to a mechanism on the relay. Although when bought new, within 4 months the lid has warped so much had to send back to Germany to be replaced. Lid has slowly warped again, I stuff the crack with ceramic fibre and seems to be ok..Thinking that the exploded saggar force may have done more damage to lid (even with 2 x 10kg weights on it (am firm believer in  recycle, reuse Bill!) and perhaps messed with that spring on the relay. Opening and closing lid, can see mechanism working...

The thermocouple is an S, Neil, jiggled the wires carefully and can see screws a bit loose so will take it all apart and see if anything jumps out at me

Thanks again for the suggestions 

All the best

Sinéad 

[neilestrick]

@sine Be careful about using fiber blanket as a lid seal. Every time it is touched or moved it releases fibers into the air that are dangerous to breathe.

[Bill Kielb]

19 hours ago, sine said:

I stuff the crack with ceramic fibre and seems to be ok..

Ceramic fiber rope is probably the best idea I have seen that stays intact and can be simply glued to the lid or shoulder around the kiln. I suppose it could be gently tucked in around the kiln after closing. Less potential fibers, not zero though. Comes in round or square.

 

[sine]

Thanks Bill. I've never seen this product before. Will check it out with the suppliers 

[sine]

Update- released the catch on the kiln lid so no longer an issue with the warped lid, stopping the flow of electricity

and took apart the thermocouple wiring- there was loose screws so tightened all back up

Have that glaze firing on again, and so far am at 900'c so result!

Fingers crossed will continue with no more problems

Thanks again for the suggestions