Kiln ohms what the heck!

[Chalkie76]

Help I’m so confuddled with this and my brain is fried.  I am based in the UK. 

i have a second hand very small (17 litre) 40 year old klln that I have fired 20 times to bisque/cone 6.  I have noticed the kiln taking longer each time to reach cone 6 so suspected I needed new elements. 

it’s a Cromartie CTL-17 2kw 240v hard wired to a 13amp fused switch.  I measured the resistance at 46.6 ohms.  I believe the 2 elements are linked by a jumper cable so does that mean I have to divide this number by 2? 
 

if I need to divide by 2 then the power I’ve calculated on these worn elements is actually higher than the rating for the kiln.  
 

Therefor I’m thinking it is possible that the previous owners upgraded the elements to greater than 2kw? Maybe 2.5kw? Would this be dangerous? 
 

if I don’t divide by 2 then I get a ridiculously low power value which I don’t think is right either!
 

I don’t want to buy new 2kw elements and find that they perform worse than my current ‘worn’ elements.

any advice would be really appreciated thank you 

[Bill Kielb]

The diagram below may help. Your 2 kw kiln ought to measure: 240^2/2000w = 28.8 ohms.  So we dont know if these are series (additive) or parallels which would generally half the measurement but what we can say is if the resistance measured at the cord end (where it plugs into the wall) should be about 28.8 ohms to produce 2000 watts of load. Higher measurement, means wear and less heating watts. So measured in total or at the cord end that plugs into the wall or where power is applied. if you are approx. 10% or more above 28.8 ohms (28.8 + 2.88  = 31.68) then they are worn. Just make sure all measurements are done safely, (no power) and both elements do glow when they are powered. Sounds likely they are worn.

You may find formula circle below handy

Edited June 22, 2023 by Bill Kielb

[Chalkie76]

So the kiln is hard wired so has no plug.  As I’m replacing the ceramic insulators anyway I disconnected the top element and tested the end to end resistance of this element which goes round the kiln twice (to make 2 rows of elements inside the chamber).  Resistance is 52 ohms.  What am I doing wrong.  stance 

[PeterH]

Can you post a picture of the kiln plate?

PS
I haven't yet found anything on CTL-17s on the net, but many of the CTLs seem to exist in 220v & 240v versions.

The CTL28 is listed as a 3KW kiln for 220v & 240v operation in https://www.cromartiehobbycraft.co.uk/files/Kiln Downloads/CTL Range Leaflet (1st).pdf
But both 3.0kw and 3.3kw versions of the elements are offered here https://tinyurl.com/58knhuwy
... other models seem to have an optional N suffix, for which a higher wattage element is sold.

Might be worth asking  Cromartie what they think the resistance of your element(s) should be.

[Chalkie76]

So the kiln is hard wired so has no plug.  As I’m replacing the ceramic insulators anyway I disconnected the top element and tested the end to end resistance of this element which goes round the kiln twice (to make 2 rows of elements inside the chamber).  Resistance is 52 ohms.  What am I doing wrong.  stance 

[Chalkie76]

I have already asked Cromartie and when I mentioned my readings to them they said the resistance when cold isn’t really useful to determine if they are worn or not and to just buy replacement’s if firings are taking longer.  But I’m concerned that the new 2kw elements will perform worse than the existing ones so I have gone back to them again.  (They have asked for some pictures so waiting to hear back from them).  The more things I test the more confident I am becoming that the elements currently fitted are  NOT 2kw.  I just don’t know how to prove it for sure. 
 

my real question is: is it dangerous to have higher rated elements in a kiln? 
 

Voltage supply tested at 244v.

13amp fused switch connected 

20amp breaker 

Thanks again

 

 

[Chalkie76]

[High Bridge Pottery]

17 hours ago, Chalkie76 said:

Therefor I’m thinking it is possible that the previous owners upgraded the elements to greater than 2kw? Maybe 2.5kw? Would this be dangerous? 

 

You can run up to 3kw through a 13 amp fuse no problem. 

 

Those elements are wired in parallel so you need to use 1/RTotal = 1/R1 + 1/R2 

If I assume the bottom element has the same ohms as the top 52 ohms you measured it comes out to 26 ohms (2.2kw) for RTotal so it seems like they may have upgraded the elements to draw 2.5kw.

 

You will be better off switching from parallel to series elements as then they can use thicker wire and the elements should last longer. I do agree if you order a 2kw set they will probably be worse than what you have now.

[PeterH]

42 minutes ago, High Bridge Pottery said:

so it seems like they may have upgraded the elements to draw 2.5kw.

I've  been thinking along similar lines, but wondering if they are 220v elements run at 240v.

As power is proportional to V^2, this would give (240/220)^2*2000  ~2380w at 240v.

... probably not good for element life, so your series comment seems even more appropriate.

PS
220v would give 2000w with a resistance of 220^2/2000 = 24.6 ohms. So if the current resistance is 26 ohms that's about 100*(26/24.6-1) ~ 5.6% gain over time.

Edited June 22, 2023 by PeterH

[Bill Kielb]

I still am Interested in the whole kiln measurement it could give us an idea what condition ALL the connections are in. Having said that, it appears the kiln at 2.5 kw has trouble making cone 6.  So, measure both elements, then measure where the power comes in at - for total kiln resistance (No Power applied)  to assess just how good all those connections are. My sense is the connections may be removing some available power but in the end the elements were changed to get this to go to cone 6 and even at their current worn state wattage, they are not really able to power effectively anymore.

Which leaves you with what wattage works? Sometimes it’s easy to look at similar models and note what wattages are offered to achieve a temperature / cone. My sense is this thing was upgraded to 3000w and now has dropped to 2500 w. due to wear and tear and has trouble making cone 6.

I would rule out the bad connections first though, just in case we find out they were decreasing the potential power by a few hundred watts. Finally to know the actual firing wattage measure the amperage if possible, and yes knowing this we can derive the heated resistance.

Edited June 22, 2023 by Bill Kielb

[Chalkie76]

Thanks Bill I will try to do this. I’ve turned off the power at the fuse box and removed the power cable from the switch, next steps:

1. reconnect elements

2. set to high 

3. push the plunger on kiln sitter 

Select resistance on multimeter and contact red  to “live” and black to “neutral” on the kiln cord? 

Is this correct? 
 

thanks for your help 

[PeterH]

To measure the resistance you could measure it at the switch (turned off, maybe power off as well), or remove the connections from the switch and measure via the leads.

You can measure current with a clamp ammeter, although you might need to jury-rig a connection to do so. Just one of the power leads (live or neutral) has to be inside the clamp.

eg https://www.ebay.co.uk/sch/i.html?_from=R40&_nkw=clamp+ammeter&_sacat=0&_sop=15

[Dick White]

Can you tell us the dimensions of this kiln - outside diameter and height, and the thickness of the brick walls. That will help us estimate whether the wattage of the elements will make cone 6 with reasonable ease.

 

[Chalkie76]

In reply to bill the total resistance at the cord end is 26.5. 
 

Kiln interior is 17 litres and dimensions are 28cm accross by 22cm tall, the kiln brick is 3.5 inches thick.  The lid had the fibre replaced when I purchased it and it seems to compress into the cavity quite well. 
 

I did test fire on full last week and here is the rate of climb for an empty kiln.

All values are centigrade 

1hr 760

2hrs 970

3hrs 1100

4hrs 1160

5hrs 1210

5hrs 40 minutes cone 6 bent at 1228

 

so it is capable of reaching cone 6 it just struggles for the last 50 degrees or so

 

[Dick White]

Some rough technical calculations, for me in dumb English units as that's what we in the colonies still use despite the mother country having gone modern on us with the superior metric units... Your kiln has a surface skin area of about 650 square inches. If it is producing its rated 2kW, that is about 3 watts per square inch of surface area. The issue with kiln size and power is that as the kiln interior heats, the surface of the kiln radiates some of that heat to the air rather than keeping it to the ware. Looking at the range of US kilns made by Skutt and L&L and their advertised maximum cones, most of the models rated to cone 10 are generating 3.5 or more watts per square inch of exterior surface. Ones that are rated to cone 8 (i.e., typically large kilns operating at lower voltages) generally are 3 or less watts per square inch. We have a general rule of thumb that the maximum manufacturer's ratings only apply when the elements are in perfect shape, and for regular usage over the long term, one should downrate the kiln maximum by 2 cones, e.g., one should expect a cone 8 kiln to remain serviceable in the long term only to cone 6. So, your kiln should be able to go to cone 6 when everything is in good shape.

That's where we make judgements about "in good shape." The Orton cone table specifies that cone 6 should bend at around 1220℃ when the kiln is heating at a constant rate of 60℃ per hour for the last 90-120 minutes of the firing. During the last 2 3/4 hours of your firing, it was averaging about 45℃ per hour, and in the last 40 minutes, it was only making 24℃ per hour. So, yes, it is struggling with an empty kiln. It will be worse when you put some ware in there.

And regarding the resistance reading, Ohm's Law calculations suggest the kiln is producing around 2,175 watts, pulling 9 amps when on high.

[neilestrick]

Your kiln is about 0.6 cubic feet in volume. L&L makes a 0.5cf model, so just slightly smaller,  with 3" brick, and at 2160 watts they rate it at cone 6. Their cone 10 model is 2,800 watts. That said, I've got a 0.6cf Evenheat with 2.5" walls and a 2.25" lid that I put 2800 watt elements in and it's got plenty of power to spare getting to cone 6. It could hit cone 10 without struggling in the least. It works about 20% less hard than my larger kilns. So I would think that somewhere around 2400-2500 watts you'd be fine getting to cone 6 and have good element life.

[Bill Kielb]

Ok, looking at the numbers your current old element calculate to 2215 watts ( presumed well worn) if you can measure everything and just confirm then new these were likely 2500 watts which fits Neil’s comparison well. We are just not sure in the end how well this kiln is insulated so 3000w may still be the reasonable goal for longevity of use at cone 6. Especially if you plan to pack it with lots of wares at some point. From there since it is a retrofit you will need to make sure wiring and fuse size are correct.

This type of load that can run for hours is often classified in a special way as it tends to heat up the breaker and fuse. In North America (and many other places) protecting a kiln requires a breaker 125% of the load, or put another way, we are not allowed to load a rated breaker greater than 80% of its rating.

Fuses have always been treated the same, so if you are my age you would remember the slow blow fuses for fuses that were too tightly sized. At 3000 watts and 125% that puts this just over 15 amps. So in the end, maybe easier to have Euclids wind you new elements at say 2850 watts giving you plenty of fire power and in reasonable order for your present fusing using typical load protocols for most fuses and wire.

If you go for the 3000 watt option then wire and fusing will need to be checked to be in compliance with code. 16 amps actually works for 125% @ 3000 w so not sure what your next available fuse size is from 15 amps. Fuse and wire typically would be sized for the new 125% load.

Whatever you do, I suggest when you are done, remark the label of the kiln as to what this thing wattage wise is with something permanent. What would I do? I think I would splurge and upgrade everything to 3000w if possible so I had plenty of fire power and could make cone 6 for lots of firings to come. 
 

edit: Just realized you have a 13 amp circuit, all the info below is great -I would suggest size it and the wiring to your current codes and current available products.

Edited June 23, 2023 by Bill Kielb

[High Bridge Pottery]

You can run 20 amps through a 13 amp (British Standard 1362) fuse without it blowing out. My Cromartie HobbyTech 40 litre draws about 2.9kw/12 amps through a 13 amp plug fuse without any problems or excessive heat generation. For a 2.5kw kiln I would still change from parallel elements (46 ohms per element) to series (11.5 ohms per element) as they will use a thicker wire giving better element life. I know Cromartie advise that on all their 40 litre kilns and it seems sensible to do it here too.

[PeterH]

For non-brits, the UK 13A ring main is described in
http://wiki.diyfaq.org.uk/index.php/Ring_circuit

The ring circuit and BS 1363 plug and socket system were developed in Britiain during 1942-1947.(ref 1) They are commonly used in the United Kingdom, and to a lesser extent in the Republic of Ireland.

The ring circuit came about because Britain had to embark on a massive rebuilding programme following World War II.(ref 2). There was an acute shortage of copper, and it was necessary to come up with a scheme that used less copper than would normally be the case. The scheme was specified to use socket outlets with 13 Amp fused plugs.

...
The ring circuit was devised during a time of copper shortage to allow two 3kW heaters to be used in any two locations and to allow some power to small appliances, and to keep total copper use low.

[Chalkie76]

In my case the kiln is not on a ring main and is actually on a radial so maximum load would be on just one cable and Im pretty sure this is just a standard 2.5mm copper core cable so I think that restricts things to 16amps.  I don’t think I should go up as high as 3kw for the elements as there are other things (small things like phone charger, Alexa, lighting and a security camera on the same circuit). 

using the logic of watts per square inch in one of the posts above if I change to 2.5kw elements the heat potentials would be 3.8 watts per square inch which is line with current cone 10 kilns so allowing for degrading over time my kiln should continue to reach cone 6. 

I think reading all the excellent advice and help on here that I will ask Cromartie to custom wind either 2.5kw or 2.8kw elements.  I have no idea how to change the wiring from parallel to serial or if that’s even possible for my little kiln but again will ask Cromartie.
 

I will change the rating on the plate to show the new Kw to avoid any confusion further down the line. 
 

Again thanks to everyone for helping me understand what I have and also what I need. 

[High Bridge Pottery]

7 hours ago, Chalkie76 said:

I have no idea how to change the wiring from parallel to serial

Instead of having a live and neutral running to the start/end of each element with jumper wires you will run the live wire to the top connector and neutral to the very bottom connector and then just use the element tail from one element to join the middle connectors. You could use a wire in the middle but the element tail will probably be long enough.

[Bill Kielb]

9 hours ago, Chalkie76 said:

using the logic of watts per square inch in one of the posts above if I change to 2.5kw elements the heat potentials would be 3.8 watts per square inch which is line with current cone 10 kilns so allowing for degrading over time my kiln should continue to reach cone 6. 

Just an add - really agree with everything thus far and your approach as well. Just an FYI - 3.5 watts and above is probably desirable, table below might help with what’s out there commercially. Those that approach 4 watts tend to exhibit significant firing longevity at cone 6. Second add, for series or parallel (within reason) you ought to be able to get heavy gauge wire. When any break happens in a series circuit, the entire kiln stops working, so parallel operation has an advantage in that respect.If you are stuck, Euclids generally can wind them.

Edited June 23, 2023 by Bill Kielb

[High Bridge Pottery]

1 hour ago, Bill Kielb said:

Second add, for series or parallel (within reason) you ought to be able to get heavy gauge wire. When any break happens in a series circuit, the entire kiln stops working, so parallel operation has an advantage in that respect.If you are stuck, Euclids generally can wind them.

It's a lot easier to use a thicker gauge wire for two elements in series as the resistance for each element is about 4x lower. I also think if you lose 1 out of 2 elements you may as well have the kiln turn off because it probably won't finish firing with 50% power.

[Bill Kielb]

46 minutes ago, High Bridge Pottery said:

It's a lot easier to use a thicker gauge wire for two elements in series as the resistance for each element is about 4x lower

No worries, his wire size needs to fit the existing kiln. Element loading is partially dependent on element length so the more the merrier I suspect, It’s likely a reason why many kilns are parallel as well as the ability to zone. I think either likely would be a lasting solution. I have no preference to serial or parallel really, just that he gets a workable wattage and it is safe.

Edited June 23, 2023 by Bill Kielb