Test resistance, or trust current reading on KM series kiln?

[NoArtist 7]

Hi, I am not an artist, but am helping my daughter set up a new studio in her garage. Or a garagio as I believe it is sometimes called here.  She has worked in a community studio for some time, and now wants more control over her work.  The plan is to slowly add to the studio and eventually transition to it over the next year or two.  She recently bought a used Skutt KM 1027 kiln which we wired up while I was visiting. This kiln has built in diagnostics, so I was wondering if regularly testing the element resistance is really needed most of the time. From the diagnostics screen you can have the kiln measure full load amps, voltage under load and unloaded, and the current through each set of element pairs individually. It does not seem to have an automatic resistance measurement, but these built in readings seem adequate for most day to day monitoring.

I checked the service manual, and it states that  an increase in resistance of more than 1.5 ohms indicated elements that need to be replaced.  For the elements in this kiln, that is about a 5 or 6 percent increase. Using ohms law, this would mean that a decrease in current of 5 or 6 percent would also indicate worn elements. If you have a kiln with built in diagnostics, shouldn't it be fine to just monitor any drop in current until you see a significant change?  My daughter certainly has a good quality multi-meter, and regularly checks the elements in her smaller Paragon kiln using it.  But the wiring on the small kiln is less complicated and easier to access. Unplugging the larger kiln and opening up the access panel to individually check the element resistance seems like overkill unless there is an indication of issue.  Particularly given the built in diagnostics. What do you folks do?

Now I know that if a problem occurs, nothing beats directly reading the element resistance, because you might have other problems. A relay might be going bad, or there could be a poor connection somewhere.  There might also have been a drop in line voltage under load, although the kiln does measure that as well.  But on a regular basis, it would seem fine to trust the current readings, and not break out the multi-meter until there was more of an indication that something was going wrong.  How often do you folks break out the ohm meter and test elements at the source?   And do you open up the access panel and test the resistance at the connectors as Skutt shows in their manual, or just probe the elements inside the kiln?  Also, would it depend on weather the kiln has any built in diagnostics?  I will probably call Skutt to get the official story, but sometimes the folks using the equipment on a day to day basis have more practical advice.

By the way, the kiln is supposed to be capable of reaching cone 10, but she only plans to fire to cone 6. Does that have any effect of how carefully she needs to adhere to the recommended replacement resistance? The elements right now are showing less than 1 ohm of resistance change,and are not warped or sagging.  But I was wondering in the future what advice to give her.  

[Bill Kielb 1,114]

I think ok to create a benchmark with your diagnostics and use that. As the elements age the firing times will increase and you will have a decent indication of their need for replacement. As a practical mattter we get 150 - 200 firings on our studio kilns before we absolutely have to change the elements.

We fast glaze which takes about 5-6 hours, when our elements are 10-15% worn this time will creep up to 10, 11,12 hours or more. If we measure the resistance cold, anything in the 10% change is enough for me. Interesting to note when the element is warmer it will gain some resistance. Same with your benchmarks of current, if you watch the element in startup the current will be higher and will sink as it warms up.

it really sounds like you both have a good command with respect to checking and servicing so I believe you will notice the longer firing times and simply measure the element  resistance to confirm the need for replacement.

Eventually the elements will run out of the necessary power (rate of heating) to drive the kiln to top temperature at the rate specified in the firing schedule. This is a function of the wear and tear on the element and duty cycle of the relays which often is limited to 10s or higher. Solid state relays allow a duty cycle of 2s and generally fire at zero crossover (0v) so they tend to extend the useful life of elements.

Just  a thought, in our studio I thermal scan the relays and connections on occasion just so I can change out a relay prior to failure as this tends to back up a bunch of wares when one kiln goes down mid firing. We also prediagnose the thermocouples and change them  prior to failure. I think this preventative approach might fit well into everything you have said thus far.

Here  are a couple videos of some simple diagnostics. It might give you some ideas. Pretty simple stuff actually but you guys seem to be very well versed.

Hope that  helps!

 

Edited April 11, 2019 by Bill Kielb

[neilestrick 4,592]

I would measure the element resistance. Elements should be changed when they are 10% off from original. This isn't something you need to do every week or month, though. If firing a combination of bisque and cone 6, you should get around 150 firings out of your elements. You test them inside the control panel where the feeder wires connect to the element ends.

[NoArtist 7]

Wow. Thanks for all the great advice. Particularly the videos. I will definitely forward that to my daughter. She is new to doing her own firing and kiln maintenance, so the advice should go a long way to getting her on the right track.  BTW. I did end up calling Skutt. It took quite a while to get through to a technician. But once I did, they were quite helpful. Here is a summary of what they told me. 

 

If you are having trouble with the kiln not reaching temperature, or having long firing times, there is nothing better than reading the resistance directly with an ohm meter to better understand what is happening. However, for typical operation of a kiln with built in diagnostics, you should not need you to do that.  Just using the diagnostics screen to read the current is probably all you need to do.  For this kiln they said the current should read about 18 amps for the top and bottom elements when they are new, and 15 amps for the center elements, also if new.  Once they drop to 16 amps or lower for the top and bottom elements, or 13 amps or lower for the center elements, they probably need to be replaced.  

 

I asked if reading the current was less reliable than reading the resistance. But they said that directly reading the resistance has no advantage as long as you have a good reading. If the current reads good, that is a reliable indication that everything is working properly. If the current is low there might be more reasons for it other than worn elements, such as low line voltage, a loose connection, or a failing relay. That is when you need to do more testing. But if the current reads good, that is the best indicator that you should get a good firing.  

 

They also said that if you are ever worried that a firing is taking a bit longer than expected, or something just does not seem right on an SK series kiln, you can reassure yourself by just hitting the 7 key on the front panel at any point during a firing. Hitting the 7 key while the kiln is firing will cause it to report the current in amps that each element set is pulling.  You can compare that to the rated current for your kiln, and verify that the kiln is still operating properly.  If one of the elements reads a really low or zero current, an element or relay is probably worn or damaged, and you will probably need to abort the firing. But otherwise it just helps to reassure you that everything is OK, and the firing is proceeding properly.  

 

I also asked if you had a bit more leeway in when you replace the elements, since my daughter is only firing to cone 6 rather than cone 10. But he said not really. Cone 6 does give you a little extra margin, but not a lot.  If she was doing low fire work to say cone 06, you might be able to wait longer to replace the elements. But cone 6 is close enough to the temperature of cone 10 that you probably would not get a lot of extra life out of an element that had dropped below the rated current.  

 

At least that is what they said. 

 

Thanks again.

[Bill Kielb 1,114]

I totally agree and would add that measuring all these things in a dynamic situation. ( kiln running) can cause difficulty because of the cycling action of the relays and  the temperature  the elements currently are at. I  am glad they said what they did and included time, or how long it is now taking to fire. Many potters get to know their kilns so well that when their firing times start to climb significantly and everything else is fine (relays) they confirm the need for new elements by measuring. I think with your technical propensity it really will become simple as you operate that kiln.

Best of success in her garage studio!