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My Elements Do Not Add Up.


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#1 High Bridge Pottery

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Posted 29 May 2014 - 10:11 AM

Disclaimer - I have no official wiring diagram so please don't ask  :(

 

One of the elements has gone on my kiln but I have decided to replace them all as the others are very much on their way out.

 

I took readings of the amps and resistance of the elements. The top circuit has 6 elements that was drawing 18.5A and the bottom with 4 elements drawing 15.5A. This was a little confusing as I was told by the electrician that a 30A fuse had been installed.

 

The resistance is shown in the picture. When I do the maths the amps comes out as 44.7 with the readings that I took for the resistance. Were my resistance readings just very badly taken as the elements are old?

 

The total resistance should be 240/30 = 8 Ohm or even with the 34amp reading it should be 7.06 Ohm

 

Right? 

So why do I get a value of 5.37 Ohm  :angry:

 

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Do elements go up in resistance as they age? The only way I can get a resistance reading near what I should have is by having the top circuit having 16 Ohm and the bottom having 15 Ohm which gives me a total resistance of 7.74 Ohm.


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#2 Chris Throws Pots

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Posted 30 May 2014 - 07:53 AM

I don't have a great electrical understanding, but I can share this. When I suspect I have an aging or shot element, using my multimeter, and with the breaker turned off, I measure the resistance. Based on conversations with Skutt techs, I know that I am supposed to look for a resistance reading within 1.5 ohms PLUS or MINUS the the specs. Anything outside of that range is the cue to replace the element. By that logic, resistance value could increase as the element wears out.


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#3 neilestrick

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Posted 30 May 2014 - 09:49 AM

Are you sure you've got that wiring diagram correct? Post some photos of the panel and the inside of the kiln. Post all the info off the serial plate. What brand of kiln is it? 10 elements is an odd setup, at least for US kilns. Can you call the manufacturer?

 

If the kiln is on a 30 amp breaker, it would not be actually pulling 30 amps. It should be no more than 24 amps, as the kiln should be on a breaker that is 25% greater than the actual draw.


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#4 High Bridge Pottery

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Posted 30 May 2014 - 11:57 AM

Yup, that is how the elements are set up. The kiln I have been told was a custom build by a guy from possibly amsterdam but these are just things I have been told. It has no plate or serial or any manufacturer that I can contact.

 

It does seem like an odd set up to have the 10 elements but when I was first buying the kiln I knew nothing about them so overlooked a lot of the problems as I had little money and needed a kiln. The guy I bought it from was just reselling and has been no help.

 

I will have to check the breaker but I do know that when I tested the amps it was between the value of 32-34 amp. Maybe it is a 40 amp breaker as surely the 30 would fuse if it was pulling over that.


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#5 High Bridge Pottery

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Posted 30 May 2014 - 12:13 PM

I wonder if I could make all the elements have a resistance of 20 ohm.

 

This gives the top circuit a resistance of 13.33... and the bottom a resistance of 20 making the total resistance 33.3/266 = 7.987 ohm which is the value I think I am looking for.

 

The top elements would have 18 amp and the bottom would have 12 amp going through which gives me a nice total of 30 amps. I just have no idea if it will keep getting up to the right temperature but it would be nice for all the elements to be the same specification. I am not sure why they don't have the same specifications to start with. Maybe at once time they did but on removing the elements the wire thickness looks the same but the coil diameters look slightly different thus implying they are meant to be different specifications.

 

Can anybody see a problem with me changing them to all be the same resistance values?


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#6 Bob Coyle

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Posted 30 May 2014 - 12:45 PM

The more resistance you have the less current and heat. If you are having problems reaching temp, then I don't think you should try for more resistance. If you are truly pulling 34 amps through a 30 amp breaker, you need to go to 40 amps. Then you need to check if the wiring from the breaker to the kiln is heavy enough to source 40 amps. Has your breaker and wiring to the kiln been getting pretty hot?



#7 High Bridge Pottery

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Posted 30 May 2014 - 12:59 PM

I had no problems reaching temperature with the old elements, it could easily climb 200 degC per hour. One of the bottom elements just burnt out in the middle, not sure if this was because something has landed on the element but I couldn't see anything. Just slightly black ends from where the break happened.

 

The wire going from the breaker to the kiln is only ever slightly warm to the touch even when drawing power constantly for over an hour.


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#8 neilestrick

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Posted 30 May 2014 - 01:29 PM

Heat the 2 ends of the element break to red heat with a torch and straighten out about an inch of each end. Heat them again and twist them together. Easiest to do with someone holding the torch and someone using 2 pairs of needle nose pliers. Not a permanent fix, but will usually hold for quite awhile, at least until you get this figured out.

 

Often the very top and very bottom element are made to run hotter than the others to compensate for heat loss through the lid and floor.


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#9 High Bridge Pottery

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Posted 30 May 2014 - 01:45 PM

Thanks for the advice :D I would give that a go but I have already cut out a big chunk of the element out. My bad.

 

This is me just thinking out loud on the forum but please think along with me. 240v supply with 30amp gives me 7.2kw. 

The total surface area of the kiln is roughly 2(π0.25^2) + π0.5*0.6 = 1.178m2 

The wall thickness is 0.06m with I assume k23 IFB with a thermal conductivity of 0.24 at 1093degC. There is an extra cm of insulating fiber blanket round the bricks but I will ignore that for the moment.

 

 

Theoretical max temperature

 

C = Watts * Thickness / (Area * K)

C = 7200 * 0.06 / (1.178 * 0.24)

C = 1528

 

So in theory changing the elements so they are all 20 Ohm gets the circuit to use 30 amps and I should be able to reach 1260 degC. The whole kiln is just a bit of a miss fit. Little did I know when I bought it I would end up redesigning the circuit in some way. My only worry is that the bottom elements will have too much resistance and won't heat up enough.

 

How do I know what thickness I want the element wire to be? Is it just a balance between the length your element needs to be and the total resistance you need it to have?


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#10 High Bridge Pottery

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Posted 30 May 2014 - 04:21 PM

I just noticed that equation doesn't seem to take volume into account. Am I missing something?

 

Edit - I have been thinking too much. Surface area must define the volume  -_-


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#11 neilestrick

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Posted 30 May 2014 - 04:35 PM

There's a lot that goes into element design- resistance, wire thickness, coil diameter, coil density, etc. It's not something I'm versed in, though. I always buy factory parts. Call the guys at Euclids in Canada. They can design coils for your kiln.


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#12 High Bridge Pottery

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Posted 30 May 2014 - 05:23 PM

I would have thought coil diameter, coil density and wire thickness are all related to the length of space you have to span and how much resistance you need.

 

My thoughts are that a bigger coil diameter gives you more resistance over a shorter distance. Same with the coil density, tighter packed loops giving more wire over a set distance. Probably the same for wire diameter but where thicker means less resistance over a set length.

 

I do have some kanthal a1 wire so might have a go at coiling an element and see what resistance it ends up being.


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#13 neilestrick

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Posted 31 May 2014 - 09:15 AM

I would have thought coil diameter, coil density and wire thickness are all related to the length of space you have to span and how much resistance you need.

 

My thoughts are that a bigger coil diameter gives you more resistance over a shorter distance. Same with the coil density, tighter packed loops giving more wire over a set distance. Probably the same for wire diameter but where thicker means less resistance over a set length.

 

I do have some kanthal a1 wire so might have a go at coiling an element and see what resistance it ends up being.

 

Yes, they are related to the span and resistance, but each of those factors is altered by the others. There is an optimal number for each, which have to work with the optimal heat output for the size of kiln.


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#14 High Bridge Pottery

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Posted 31 May 2014 - 09:21 PM

One thing I do know is the coil diameter on my old elements. I can see that between the top and bottom elements there is 1 or 2 mm difference between them. Both wires diameter and coil density looks close if not the same, they are old and have bunched up/stretched out in places.

 

I am still stumped as to why the kiln has even been working with the resistance values I have. I think they are just wrong. 

 

Using the amp reading that I took I think is more reliable. The total resistance should be 240/34= 7.059 Ohm

If I have 6 top elements with 20 ohm and 4 bottom with 15 ohm that gives me 1/(40/3) + 1/(30/2) = (15/200) + (13.33/200) = 7.06 Ohm total resistance. This is ignoring the resistance of relays and wires but does that add up to much?

 

I need to go and check the breaker to find out what fuse it has but haven't been able to yet. I guess I have two choices, try and keep it operating on the same amps that it was when I know that works and gets up to temp easily or try and change it so it operates on 30amp and risk it not being able to get to temperature.

 

Once I know what resistances I should be working with I can start figuring out wire diameter and so on.


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#15 Bob Coyle

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Posted 01 June 2014 - 11:08 AM

I think you need to find the real total resistance of the circuit, unplug from the wall, turn all of the switches to full on and measure the resistance across the male plug. There may be quite variable resistance from element to element in this strange Kirchhoff loop of kiln wiring, but the reading at the plug is what the circuit sees going in.

 

Also, at this low a resistance value, your multimeter may have some slop in the reading.I find it hard to get good readings by using the probes on the oxidized elements. Taking one good reading at the plug is better than 10 difficult ones across each element.

 

If the kiln was working, giving consistent ramps and the wires to the kiln are not heating up, I would stick with the original (theoretical) resistance values of each element.

I think you would be pretty disappointed if you spent lots of time and money re-wiring it to try to get 30 amps  and couldn't hit temperature fast enough. If your  breaker is marked 30 amps and you are pulling 34 then you need to change it out for a 40 amp because a breaker that fails to trip at the correct current max is dangerous.



#16 High Bridge Pottery

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Posted 01 June 2014 - 11:28 AM

Is there an easy way to apply the correct voltage to the relays so I can get them to switch on? I am not sure what voltage it is as there doesn't seem to be a transformer anywhere. Maybe it is in the programer.

 

One problem is I have taken a chunk of element out but could do a messy fix like neil was saying and heat and twist it back into place. I do have the amp reading which I feel is much more reliable as I managed to take it from a few different places and they all match.

 

If I was trying to get it down to 30 amps the top elements would have the same resistance as they would with 34 amps so it wouldn't be a complete waste. I would just need 4 new bottom elements and if I could get them out without breaking I would then have 4 spare for the top.


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#17 High Bridge Pottery

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Posted 01 June 2014 - 12:15 PM

Another question, I was trying to work out the volume of my kiln and it has radius 26cm and height 55cm.

 

It gives me out the value 0.12m3 which just seems totally wrong. 12cm3 just sounds so small. When I convert it to feet3 it makes 4.23feet3. That sounds a lot bigger. Why is this number confusing me so much.  :angry: a box 4x4x4ft sounds so much bigger than a box 0.12x0.12x0.12m.

 

My brain is doing something wrong. I just can't figure out what.

 

Turns out 0.12m3 is 120000cm3. I still don't understand as 1m3 is 1x1x1 and my kiln just seems a lot bigger than 10% of that. Oh well..


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#18 Chilly

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Posted 01 June 2014 - 04:29 PM

It's not 4x4x4 ft.  That would be 4ft cubed.  It is 4 cubic feet.  1x1x4 ft


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#19 PeterH

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Posted 01 June 2014 - 04:43 PM

Sounds like you're confusing a volume of 4 cubic-feet with an object that is a 4-foot cube.

 

For the record:

0.12m3 is the volume of a cube with sides of cuberoot(0.12) m ~ 0.49m.

4.12feet3 is the volume of a cube with sides of cuberoot(4.2) feet ~ 1.6feet.

 

Regards, Peter

 

Putting pi*0.26*0.26*0.55 into google gives the volume as  0.11680441486 [m3]

Putting pi*0.26*0.26*0.55 cubic meters in cubic feet into google gives 4.12490898 cubic feet
 



#20 High Bridge Pottery

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Posted 01 June 2014 - 05:12 PM

Thank you  :D now it makes a lot more sense. For some reason I had the idea volume=length when imagining my cubes of space.

 

I have no idea why I didn't realise that before..


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