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L&L JD18


PCHPottery
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I have a L&L JD 18 240-1 that I purchased new in 2002.  Shortly after purchasing, my pottery life took a long hiatus.  It's only been fired just over 30 times, a mix of bisque and glaze firings. I've recently gotten back into pottery and have been experiencing ongoing issues with reaching temperature at cones 5 and 6.  The thermocouples have been replaced and are type K with the open ceramic sleeves that extend past the metal portion. I have a TCOS of 35 degrees F to account for the ceramic tube.  I did this Based on advice from L&L tech support  after over firing based on Orton cones without the offset. I've put in new L&L  elements and replaced the Dynatrol with a Genesis 2.0 controller.  The firing information coming from the Genesis has been great. I can now see that part of  the problem is that the kiln is struggling to climb more than 65 degrees per hour during the last ramp section of a fast fire ^6 program. This cone 6 firing with the 35 degree offset produced Orton ^7 touching the shelf with glazes running.  When I adjust the TCOS to achieve an Orton ^6, the offset is 55 degrees but the glaze is underfired.  I'm assuming this is due to ^6 being achieved through heat work at the end of the firing instead of final temperature.  Will this kiln not be able to climb the 108 degrees per hour at the end? Is  it due to not having 3" brick? I've attached the firing file for a better description than I'm able to provide. Any help would be greatly appreciated!

kiln firing graph ^6 Dec 12 2020.pdf

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What does the serial plate say the max temp is? There's no reason that kiln should struggle if everything is working properly and it's a cone 10 kiln like the newer models. Make sure the elements are installed properly (series vs parallel), check your service voltage, check that the wiring is correct. Get a meter and check element resistance and amperage draw. Something's not right.

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The following are results from my husband's testing of my JD18 kiln today. Thank you in advance for any guidance you're able to give!

 

Idle configuration

Two rings, both with two elements wired in series.  Combined resistance of top ring 21.9 Ohms as measured from the plug to the controller while plug is disconnected.  Resistance of bottom ring 21.8 Ohms, again as measured at the plug to the controller.

Available voltage is 241V, measured at the main block inside the controller.

Test

Use program 12, Full Power Test, to turn the elements on full on and keep them there. 

Measurement point is now right at the block that the elements are attached to.  For each ring, test while idle, as well as when relay is not calling for power, shows 0V.  Under full draw, voltage moves from 241 down to 235 and settles there.

All measurements made with quality digital multimeter.

Ohm’s Law

Based on these measurements, full power looks like:

 

Resistance (Ohms)

Voltage (Volts)

Current (Amps)

Power (kW)

Ring 1

21.9

235

10.7

2.52

Ring 2

21.8

235

10.8

2.53

Total

 

 

21.5

5.05

 Note that the data plate for the kilns says that it is:

JD18

Serial 031802A

Voltage 240

Phase 1

Amps 23.0

Watts 5520

Max temp 2350

Questions

Is ~6V a reasonable amount of sag for a kiln switching between idle and full draw?  I did check using a voltage drop calculator and 60’ of line at 240V and 23A of 10AWG should drop something around 3V.  We are likely to replace this with bigger wire shortly, but even 4AWG drops 1V.  So is this material?

The kiln has brand-new elements, EJ18003H’s.  The only way to get to 5520W, even at full 240V, would be for each element pair to fall closer to 20.9Ohms.  Are these elements within normal parameters?

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@PCHPottery Measure the element resistance at the element ends, not through the power cord. Did you measure the actual amperage draw with the kiln on?

I see a lot of kilns that drop to 235 volts at the elements when firing. It shouldn't be a problem.

If you have the new open end type thermocouple tubes, you do not need an offset to compensate for the tube, since it's open at the end. For the old closed tubes it was 18F offset, but nothing for the open end tube. You might need an offset for temperature calibration maybe, but not for the tube. The thermocouples should be about 1/2 inch inside the tube. Make sure they're even or they won't read the same. If they're not reading the same the controller will struggle to even out temperatures and that could account for the slowdown.

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I would locate where the voltage drop is first and fix it. The 3% rule for electricians (IMO) should not apply to kilns. Kilns only are designed with about 110% practical capacity brand new so 100-150 firings on a set of elements.  I advocate 1% or less in voltage drop for kiln owners. You may also have excessive drop in connections and junctions to which thermal measurement can be used to locate them and loaded measurement across contacts will reveal the voltage drop across the junction.

235 v is excessive in my view and I would solve that first. Tolerance in element winding: I would be disappointed if my element set new was off by 1ohm, but might be a connection / measurement thing as well. The voltage drop issue is big in my view and something I can remedy fairly easily.

In essence you are starting with a kiln that has about 3% less potential firings in it and actually likely less. Each firing will take more time which means more wear on the elements.

So to answer your question, nearly all voltage drop to me is material for a kiln owner. Elements arriving that far out of tolerance seems odd though as this has already increased their intended resistance value by 5% from new.

Edited by Bill Kielb
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We just measured all four elements individually end to end. All four were 10.9 ohms. The amperage measurement was taken with the kiln on.  Do you know what the elements we installed are spec'd at in terms of ohms per element? Without the Tcos the orton cones were always overfiring to cone 7 with self-supporting cone touching the shelf. With what you're advising, that must be because of heat work due to slow rise at the end of the final segment. 

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R= V^2/P    (Voltage square/ Power) 57600/5520  =10.4 ohms total. So 2*10.4 ohms = 20.8 ohms top ring in parallel with bottom ring.

By nameplate it needs to be 5520 w at 240 V

They should measure 10.4 ohms but 10.9 is fairly close and given meter calibration and manufacturing tolerance sounds more acceptable. Unfortunately series sets have this issue as each are made to be at least 10.4 so in series this does double the potential overage. If they wind them under 10.4 then there is the potential to be out of compliance with the electrical design. The proper error is to be slightly over, never under.

The final segment must be on the order of 100 degrees per hour (in about the last 200 degrees of the firing) for the control to reasonably predict the cone bend. Once it decrease to 50-60 per hour you will likely  overfire while finishing at a lower temp.

If it’s an automatic controller check the allowable relay interval timing. Generally the fastest you can go is 10 seconds for mechanical relays. If you are set to 20 seconds this will reduce the power available to keep up with top end losses.

I would definitely fix the voltage drop issue, you just don’t have that many watts to lose with this kiln.

I theory a 10% increase in element resistance equates to just under 5000 watts. Most folks would replace the elements at this point. Because of the depressed voltage and element resistance looks like you don’t have this room to start with.

Edited by Bill Kielb
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Thank you for the explanation. That makes complete sense. So if this is within tolerance, how would this kiln ever reach ^10, if it can't reach cone 6 temp without timing out for heat work in the final segment? Is it because it doesn't have 3" bricks? We believe that most of the voltage loss is due to the 60ft distance from my kiln to our panel. As noted above, we plan to change the wire to 4 gauge in the near future so that would fix the voltage issue. But would that small loss of voltage account for the difficulty the kiln is experiencing? 

 

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29 minutes ago, PCHPottery said:

e. So if this is within tolerance, how would this kiln ever reach ^10, i

Well, at cone 10 I am sure it needs every bit of the 5520 watts. More interesting would be the watts per square inch figure, which L&L does list. 3” brick would help but most kiln manufactures make very similar kilns so this is really universal across all makes. Kilns for hobby use or even small kilns are designed more for available electric, price point etc.... more insulation is definitely a plus, and more power available is as well. Just about every manufacture really has similar kilns to try and fit a variety of needs so not really a manufacture thing.

At top temperatures, losses are the most so this is where the problem shows itself first. Don’t neglect bad connections as well though, have plenty of relay examples stealing  a couple volts from an already tight design. Voltage drop across relays, wire nut connections and terminals (including breaker terminals) need to be negligible. Often tough to spot without a thermal measurement.

90A3BFF5-7E84-455D-AE52-4F55245C12C4.jpeg

Edited by Bill Kielb
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Double check all your thermocouple connections. They can come loose after a couple of firings due to the expansion and contraction of the metal during heating and cooling. If they're giving spotty readings it'll slow down the firing.

44 minutes ago, PCHPottery said:

But would that small loss of voltage account for the difficulty the kiln is experiencing? 

With the voltage drop to 235 you're losing less than 1/2 amp of power, which isn't enough to make the difference between reaching cone 6 or not. I would also check your voltage at various times of day and see if you get different readings. I've been at two schools in the past month where their 208 volt systems were giving about 228 volts for a short period, and I've been at houses where the 240 volts dropped to 230 at random times. If you're getting variation I would talk to your electricity provider.

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33 minutes ago, PCHPottery said:

Another issue is that without the current 35F offset, the kiln will be tasked with reaching 35 degrees higher. This will take a minimum of another 30 minutes. We did check the TCs and they are evenly placed into the kiln, not knocked out of place, or unevenly placed within the tubes.

So this kiln needs as much help as practical to get near that 100 degree per hour mark in the last 200 degrees of firing so guessing based in current performance will likely not be accurate. Not sure what timeframe you measured this in or at what temperature. Minimize the electrical losses, if you have a counterflow vent top of cycle is not necessary, no big gaps or plugs out, definitely check the relay cycle time and try dropping it to 10 seconds. If it is set too long this will be an easy way to get instant more power for this kiln. Other tricks would be to use good jumpers between elements, don’t use the element tale itself it is resistive. All good solid connections with no losses and in a pinch  about 40% of the kiln loses are from the top and bottom so you can try coating the inside of the top and bottom with a refractory coating such as ITC to get a few more percent back in the kiln.

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2 minutes ago, Bill Kielb said:

So this kiln needs as much help as practical to get near that 100 degree per hour mark in the last 200 degrees of firing so guessing based in current performance will likely not be accurate. Not sure what timeframe you measured this in or at what temperature. Minimize the electrical losses, if you have a counterflow vent top of cycle is not necessary, no big gaps or plugs out, definitely check the relay cycle time and try dropping it to 10 seconds. If it is set too long this will be an easy way to get instant more power for this kiln. Other tricks would be to use good jumpers between elements, don’t use the element tale itself it is resistive. All good solid connections with no losses and in a pinch  about 40% of the kiln loses are from the top and bottom so you can try coating the inside of the top and bottom with a refractory coating such as ITC to get a few more percent back in the kiln.

I get what you're saying here, but tricks shouldn't be necessary. That kiln will work just fine once the problem is found.

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56 minutes ago, PCHPottery said:

Do you have an infrared thermometer you find most accurate and would recommend? Thanks so much for your help! 

I do not, the flir you see is old, maybe 1-2k when I bought it. For low temps nowadays there are phone adapters for hundreds. Unless you are measuring above 600 degrees most are fine as well as even non contact with a laser for spot.

have a video here, I think they have some real inexpensive non contact these days https://youtu.be/c7HZlAn4nZw

At some point in the vid I did an internet search and came up with $20.00 I believe for a non contact.

 

Edited by Bill Kielb
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Good question on the Dynatrol. Yes. The firing times for them are almost identical. We got the Genesis because there was not enough transparency with the Dynatrol. The Genesis has confirmed what I'd seen happening with the Dynatrol by sitting next to it for 90 minutes taking minute readings.  I'd done this with the Dynatrol for various firing cycles in an attempt to understand what was occurring.

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9 minutes ago, neilestrick said:

With the voltage drop to 235 you're losing less than 1/2 amp of power,

Looks like between elements and voltage he is at 5000 watts. If his elements wear ten percent (time for replacement) he is at 5000  watts. Easiest thing he can do is fix any voltage issues for now. If he has installed a new genesis controller then check your default  relay cycle timing for sure.

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14 minutes ago, neilestrick said:

I get what you're saying here, but tricks shouldn't be necessary. That kiln will work just fine once the problem is found.

I agree, but we know he has a voltage issue, his elements are slightly over in resistance and this is not an over powered kiln so fix the obvious stuff first is the suggestion.

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