Jump to content
Sign in to follow this  
Sputty

Is This Normal? Kiln Suddenly Firing V-E-R-Y Sloo-O-O-Wly.

Recommended Posts

I have a small 3KW electric kiln I use for tests, or when I need something quickly.

Normally, it will fire to cone 6 in 8 - 8 1/2 hours. Over the course of the last 3 firings, the firing time has stretched to 12 - 12 1/2 hours. The final temperature for these last few firings has been way below the nominal temp for cone 6, but fortunately I always rely on actual cones, which have eventually gone over...

Is this normal? I've always assumed that elements gradually lose their mojo, slowly building up resistance. But this increase in firing time seems rather sudden to me - over the space of only 3 firings.

The voltage to the kiln is good, and the voltage drop across each of the three elements is even, so presumably it's not that one has just gone a bit duff in its old age. All connections are solid. The elements themselves look a little aged, but don't yet have an abundance of that grey-pinky oxide that normally signifies retirement is close.

I don't remember this happening prior to the last time I changed the elements, but that was a few years ago, so I could be wrong.

I don't know what the resistance of the elements should be from new, so I've no way of knowing how much they've aged. (How much of an increase in resistance is considered terminal, anyway? Is there a figure?)

 

Any comments?

 

EDIT - the firing that is (was) going on as I type this has just returned an error F1 - the kiln cannot heat at a rate of 1 degree over 20 minutes. It got to 1140 deg C (2065 deg F), and plateaued.

 

I've ordered some new elements!

Share this post


Link to post
Share on other sites

Have you checked your relays? 

 

Roberta

 

There's only the one, and as far as I can tell it's fine. The kiln is on full from early (-ish) in the firing, so the relay stays closed from that point onwards. I assume that if a relay fails, it does so catastrophically, and simply goes permanently open-circuit (or, I suppose possibly permanently closed circuit). Also, the controller (where the relay lives) does not get unduly warm, which I think it would if (say) corroded relay contacts were adding unwanted resistance to the circuit.

I could be wrong - I often am!

Share this post


Link to post
Share on other sites

 I've always assumed that elements gradually lose their mojo, slowly building up resistance. But this increase in firing time seems rather sudden to me - over the space of only 3 firings.

 

Only guessing here, (I've not yet worn out any elements) but as you have less element material than in a larger kiln wouldn't the effect of deteriorating elements tend to show up much quicker?

glazenerd likes this

Share this post


Link to post
Share on other sites

 

 I've always assumed that elements gradually lose their mojo, slowly building up resistance. But this increase in firing time seems rather sudden to me - over the space of only 3 firings.

 

Only guessing here, (I've not yet worn out any elements) but as you have less element material than in a larger kiln wouldn't the effect of deteriorating elements tend to show up much quicker?

 

You may be right - but it has been quite dramatic: a lengthening of 4 - 41/2 hours firing time over a 3 firing period. It just seems a little odd that any deterioration would be quite that sudden!

Share this post


Link to post
Share on other sites

In my experience, the drawing out of the firing time is more gradual as the elements age gracefully. The sudden decline is worrisome, but I don't what else to test other than ohms of resistance. The conventional wisdom of resistance testing is that a measured resistance of more than 10% over specification indicates worn elements. However, you don't have a factory specification (yet) so no reference point to determine failure. When the new elements are installed, take a resistance measurement and write that down in your kiln manual so you have it for future reference.

Share this post


Link to post
Share on other sites
Guest

I'm not the resident expert on this but is it possible that the elements need to be replaced?

Share this post


Link to post
Share on other sites

How many elements are in this kiln? When my elements are getting old, the firing time gets longer slowly. When one of my elements has failed (element breaks and melts into a little puddle at the break) the firing time gets dramatically longer all of a sudden. If there is more than one element, losing one will cause this behavior.

Share this post


Link to post
Share on other sites

How many elements are in this kiln? When my elements are getting old, the firing time gets longer slowly. When one of my elements has failed (element breaks and melts into a little puddle at the break) the firing time gets dramatically longer all of a sudden. If there is more than one element, losing one will cause this behavior.

 

There are 3 elements, although each one sort of doubles back on itself, so a quick glance into the kiln looks like there are 6.

They are wired in series, so if one fails completely, there's no circuit at all.

However, during the last (half a) firing, I tested the voltages across each segment, and they were even across the 3 elements. If one element had gone 'a bit' bad (but not catastrophically), there would be a clue in an uneven voltage drop across one segment. No such luck.

It's the suddenness of the lengthening of firing time that puzzles me. To go from 8 hours to 12 hours in the course of 3 firings seems odd.

I had been hoping that it was a line voltage problem - I live rurally in France, where we are prone to... erm... variation. But it's fine - I checked all the way through the last firing.

I even racked my brains to try to remember if I'd included anything unusual in the kiln, which might have fumed and done mysterious instantaneous damage to the elements, but I really haven't.

When the new ones arrive, I'll check the resistance of one, and put that against the resistance of one coming out of the kiln. That should tell something, at least.

Share this post


Link to post
Share on other sites

 

How many elements are in this kiln? When my elements are getting old, the firing time gets longer slowly. When one of my elements has failed (element breaks and melts into a little puddle at the break) the firing time gets dramatically longer all of a sudden. If there is more than one element, losing one will cause this behavior.

 

 

There are 3 elements, although each one sort of doubles back on itself, so a quick glance into the kiln looks like there are 6.

They are wired in series, so if one fails completely, there's no circuit at all.

However, during the last (half a) firing, I tested the voltages across each segment, and they were even across the 3 elements. If one element had gone 'a bit' bad (but not catastrophically), there would be a clue in an uneven voltage drop across one segment. No such luck.

It's the suddenness of the lengthening of firing time that puzzles me. To go from 8 hours to 12 hours in the course of 3 firings seems odd.

I had been hoping that it was a line voltage problem - I live rurally in France, where we are prone to... erm... variation. But it's fine - I checked all the way through the last firing.

I even racked my brains to try to remember if I'd included anything unusual in the kiln, which might have fumed and done mysterious instantaneous damage to the elements, but I really haven't.

When the new ones arrive, I'll check the resistance of one, and put that against the resistance of one coming out of the kiln. That should tell something, at least.

Gotcha. Yup you're right that's a different configuration from my elements. I have six elements, they are wired together in pairs, so three different circuits. Curious to hear the resistance comparison between new elements. Let us know!

Share this post


Link to post
Share on other sites

 

How many elements are in this kiln? When my elements are getting old, the firing time gets longer slowly. When one of my elements has failed (element breaks and melts into a little puddle at the break) the firing time gets dramatically longer all of a sudden. If there is more than one element, losing one will cause this behavior.

 

 

However, during the last (half a) firing, I tested the voltages across each segment, and they were even across the 3 elements. If one element had gone 'a bit' bad (but not catastrophically), there would be a clue in an uneven voltage drop across one segment. No such luck.

 

A change in element resistance won't affect the voltage. It'll still measure the same voltage regardless of the resistance change. The difference will be in how hot the element gets. You need to measure the element resistance.

 

If the relay stays on for most of the firing, that tells me that this kiln may under-powered for the temp you're firing to, and so a little change in element resistance will have a big effect on its ability to get to temperature.

 

Have you changed the thermocouple?

Share this post


Link to post
Share on other sites

 

 

How many elements are in this kiln? When my elements are getting old, the firing time gets longer slowly. When one of my elements has failed (element breaks and melts into a little puddle at the break) the firing time gets dramatically longer all of a sudden. If there is more than one element, losing one will cause this behavior.

 

 

However, during the last (half a) firing, I tested the voltages across each segment, and they were even across the 3 elements. If one element had gone 'a bit' bad (but not catastrophically), there would be a clue in an uneven voltage drop across one segment. No such luck.

 

A change in element resistance won't affect the voltage. It'll still measure the same voltage regardless of the resistance change. The difference will be in how hot the element gets. You need to measure the element resistance.

 

If the relay stays on for most of the firing, that tells me that this kiln may under-powered for the temp you're firing to, and so a little change in element resistance will have a big effect on its ability to get to temperature.

 

Have you changed the thermocouple?

 

 

 

This is the way I was looking at it: if you imagine the elements as three resistors in series (which they are), then a change in the resistance of one of them (i.e. if one element is damaged, but not totally broken) will alter the voltage drop across that resistor.

Put another way, if you wire three equal resistors in series, and arrange a potential difference across the series, the voltage drop across each of the resistors in the series will be equal. If you change the value of one of those resistors, then the voltage drop across that resistor will change in relation to the other two resistors (although of course the voltage drop across the entire series will remain the same).

So, with my elements, the fact that the voltage drop across each was virtually identical allows me to know that each element has pretty much the same resistance, and thus all are worn to the same extent. This matters because I initially thought that one element might have suddenly gone a bit duff (not completely), given the speed with which the firings suddenly extended - that would have made sense to me.

But that's not the case. Agreed - it doesn't tell me how much the elements have worn, but it does tell me that they are all in the same state (without taking them out and measuring their resistance).

 

The controller is a basic electronic one - you can program one ramp, a final temp, and a soak period. That's it! So for a cone 6 glaze firing I choose to fire to 400 deg C (750 deg F) in 2 hours, and then let it go to full temp as quick as it likes, then soak for the cone. Crude, but effective. The whole process normally takes about 8 hours - so 6 hours from 400 deg C (750 deg F) to 1220 deg C (2230 deg F), on full power. Obviously, the rate of change of temp tails off as it gets up there, but the graph is smooth. Is that under-powered? I honestly don't know - it's the only electric kiln I have! It is supposedly rated for 1260 deg C (2300 deg F), but I've never tried it.

When that 8 hours suddenly stretched to 12 hours (and more) over a mere three firings, it seemed odd to me. The kiln had been behaving consistently since whenever, and I don't remember this very sudden deterioration happening before I last changed the elements - I'm sure it was a gradual process.

 

I haven't changed the thermocouple. I have borrowed the one from my gas kiln and introduced it through the spyhole every so often, to see if they agree. They do, within about 2 degrees. I was impressed.

 

Anyway, I can only imagine that it is the elements, and have ordered another set accordingly. It just seemed odd that the firing time extended so far, so rapidly!

 

Do elements vary in their manufactured quality to the extent that they might (all of them at once) suddenly and rapidly deteriorate? I've no real way of knowing what I'm buying, of course. This is probably the third set of elements the kiln has got through in 13 years, but it is only a test (or occasional emergency!) kiln, so it doesn't get that much use most of the time.

Share this post


Link to post
Share on other sites

No need to take elements out to measure resistance. I did hear you can take off the neutral connection if you are getting strange readings but I just put the probes where they entered/exited the kiln to measure when they are installed.

 

Have you opened the controller to see if any components are looking suspect?

Share this post


Link to post
Share on other sites

No need to take elements out to measure resistance. I did hear you can take off the neutral connection if you are getting strange readings but I just put the probes where they entered/exited the kiln to measure when they are installed.

 

Have you opened the controller to see if any components are looking suspect?

 

I come from a (youthful, i.e. mostly forgotten) background of hobby electronics and amateur radio, where testing components generally required their isolation from the rest of the circuit. It actually didn't occur to me that the element array is probably isolated from other circuits when the kiln is powered off - my automatic thought was that it was necessary to decouple them! My approach still works though, to see if one element is behaving strangely...

 

I have opened the controller up, and all looks good - no obviously broken, discoloured or swollen components, nothing weeping, no dry joints. It just has to be the elements.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×

Important Information

By using this site, you agree to our Terms of Use.