Resistance change in elements

[Dick White]

I have another brain twister (or at least for my brain) for the hive mind of the electrical geniuses here. I am looking at a kiln that is getting FTL (on an Orton controller) at less than 1000℉. Normally, I would suspect the elements are worn. The factory spec resistance for elements in this kiln (Cress B23H) is 9.8 ohms, and normally the measured resistance of a worn element would be higher than spec, and if higher by 10% or more the element is considered worn enough to replace. In this kiln, we are getting resistance measurements of 9.3 ohms, or 5% less than spec. Is this logical? Does the resistance go up for a time as the element wears and then go the other way if the usage continues?

(one of these days I'll be a smart as you guys...)

dw

[Bill Kielb]

Technically as the element heats its resistance generally raises slightly as the energy within  it is in a higher state (Random electrons bouncing around)  resistance is also generally  a function of cross section as well so as elements oxidize, their core decreases and hence their resistance goes up.

The resistance drift with temperature is generally never really an issue, it is small. I would verify the total wattage of the kiln with heated elements to be sure my ohmmeter was reasonably accurate and to locate any places where voltage drop is excessive  such as bad connections. For relays  and junctions really need zero volts across them when closed and powered.

My understanding of FTL is less than 27 degrees per hour for two hours so that would be easy to verify. I can’t find a diagram on the Cress site for the B23 H  240 v  so I would work backwards from the nameplate wattage and amperage R= V^2 /P to confirm what the measured overall element resistance of the kiln should be new.. Also would check for voltage drop everywhere I could. It would not be the first time one leg of a breaker was smoking it’s way to death or a loose wire nut.

If I use the 208 V schematic and assume the same wattage I get 6038 watts which at 240 volts would be 9.5 ohms total load for all elements in parallel. But the 208v model has two sets in series and those two sets in paralel.

If I assume that the 240 volt model is just 13% more than the 208v  design but the wattage remained the same  I get 9.9 ohms top and bottom and 7.5 for two middles. Not necessarily how they built the 240v model so getting the right figures for these is probably gonna solve this for you.

Element chart below, temp/resistance/cross section.

 

[liambesaw]

Maybe it's not your elements and instead is a relay.  And brand new elements may test a higher resistance if you measure the ohms before you cut the pigtails off.  So if you measured them straight from the bag or before cutting the pigtails off.  Probably nowhere near 5% loss of resistance though.  Could be the meter? Weird

[Dick White]

Thanks Bill, we did finally find a diagram for the 240V model somewhere else on the Cress site. Interestingly, while the 240V circuit layout is the same as the 208 version (an upper and lower pair of elements where each pair is in series and the two series pairs are in parallel, the 240V model has all four elements at the same 9.8 ohms compared to the 208V model with higher ohm elements top and bottom and lower ohm elements in the middle.  That 208V arrangement surprised me as it is the opposite of the Skutt (and others) method that the top and bottom are hotter (lower ohms) than the middle to account for heat loss through the lid and base. But what do I know? My question, though, is not about resistivity changes vs. temperature, but resistivity changes over time/usage/wear. Conventional wisdom has resistance increasing ever so slightly each firing as some of the kanthal burns off leaving a smaller wire. By 100 firings +/-, enough wear has occurred that the resistance has increased and thus wattage has decreased, leaving the kiln unable to reach temperature. 10% increase in cold resistance is the rule of thumb. As such, a 9.8 ohm coil would be marginal when it degrades to 10.8 ohms. But these elements are measuring in at 9.3 ohms? (I am working remotely with her, so I think she is doing it right). Doing the math, as the resistance goes down 1/2 an ohm, her kiln should be overheating, not crapping out. Or maybe she is measuring incorrectly, or her meter is crap? Just trying to eliminate the theoretical possibility that extremely extended wear could cause resistance to swing down rather than up. Or maybe there is something else wrong with the voltage situation. I built the external controller on this old kiln for her when she lived here in VA and it worked fine, but she recently moved to TX and the kiln stopped working the first time she plugged it in. We'll figure it out one piece at a time.

[Bill Kielb]

1 hour ago, Bill Kielb said:

Technically as the element heats its resistance generally raises slightly as the energy within  it is in a higher state (Random electrons bouncing around)  resistance is also generally  a function of cross section as well so as elements oxidize, their core decreases and hence their resistance goes up.

The resistance drift with temperature is generally never really an issue, it is small. I would 

 

10 minutes ago, Dick White said:

Thanks Bill, we did .........My question, though, is not about resistivity changes vs. temperature, but resistivity changes over time/usage/wear. Conventional wisdom has resistance increasing ever so slightly each firing as some of the kanthal burns off leaving a smaller wire. By 100 firings +/-, enough wear has occurred that the resistance has increased and thus wattage has decreased, leaving the kiln unable to reach temperature. 10% increase in cold resistance is the rule of thumb. As such, a 9.8 ohm coil would be marginal when it degrades to 10.8 ohms.

I think you.are all good, I was answering both temp and cross sectional area questions. There are compositions I am aware of that decrease with heat, but none  I am aware of that decrease with cross section for kiln use. . I think I would have her unplug the kiln and measure it at the cord end if there is some way to turn all the knobs on if you will with no power.. Measure all four in parallel might be more goof proof, but no way to tell remotely if the meter is right, etc.... interesting to see what you discover.

[Min]

9 hours ago, Dick White said:

Or maybe she is measuring incorrectly, or her meter is crap?

 This seems the most logical, unless you want to rewrite the laws of physics. 

[Dick White]

24 minutes ago, Min said:

 This seems the most logical, unless you want to rewrite the laws of physics. 

Yup, been trying to rewrite the laws of physics for almost my entire life; still haven't succeeded. But when I do, I'm outta here, gonna get rich quick. ;-) Late night new development... I had her pull up her big girl panties and take a deep breath while testing the interior voltages at the element connections while firing it with the case open. Not something one should do without thinking it all the way through. Only 120V inside. She messed up reinstalling the power cable after moving to TX. Of course, if only half power, it will only fire to half temperature. Duh. But she learned a lot in the last 2 days.

[neilestrick]

I have run into many kilns where the elements are worn out and actually measure lower resistance. I don't know why it happens, but it does. 

[Bill Kielb]

45 minutes ago, neilestrick said:

actually measure lower resistance.

Interesting! Did they start to fire too quickly?

[neilestrick]

57 minutes ago, Bill Kielb said:

Interesting! Did they start to fire too quickly?

No, it's always the same issue as worn elements- the kiln not getting to temp- and the elements show all the other visual signs of being worn out. I don't think the resistance actually was lower, but there was something happening that messed with the reading. It could be something to do with the crusty/worn surface layer of the element causing a bad reading or something. I see it about once a year.

[liambesaw]

2 hours ago, neilestrick said:

No, it's always the same issue as worn elements- the kiln not getting to temp- and the elements show all the other visual signs of being worn out. I don't think the resistance actually was lower, but there was something happening that messed with the reading. It could be something to do with the crusty/worn surface layer of the element causing a bad reading or something. I see it about once a year.

Maybe something to do with what was fired?  I don't know if chemistry works like this, but maybe something like sodium or another low vapor pressure metal ion condensing on the elements over time can make the aluminum oxide layer slightly conductive?  I like thinking about these things and asking the questions even though no one has the answer hah.

[Bill Kielb]

1 hour ago, liambesaw said:

can make the aluminum oxide layer slightly conductive?  I like thinking about these things and asking the questions even though no one has the answer hah.

Likely would be an epic breaker tripper when ya light her up. I’m stumped.

[liambesaw]

14 minutes ago, Bill Kielb said:

Likely would be an epic breaker tripper when ya light her up. I’m stumped.

Well I assume it would burn out as soon as the elements get hot again, I just meant we are testing them cold with condensation from the previous firings on them.

[Bill Kielb]

7 hours ago, liambesaw said:

meant we are testing them cold with condensation from the previous firings on them.

Sounds plausible. Might even have a complaint that it blows the breaker then mysteriously only runs slowly. That would be tough to guess for sure.

[Smokey2]

14 hours ago, neilestrick said:

I have run into many kilns where the elements are worn out and actually measure lower resistance. I don't know why it happens, but it does. 

I'm going to add a wrinkle to this

I was testing the resistance on my L&L E28 I figured while I was at it I would check my new Olympic SQ169.

The factory spec resistance for elements should be 8 ohms.  After only 10 or so firings the reading was 7.2 ohms

[neilestrick]

15 minutes ago, Smokey2 said:

I'm going to add a wrinkle to this

I was testing the resistance on my L&L E28 I figured while I was at it I would check my new Olympic SQ169.

The factory spec resistance for elements should be 8 ohms.  After only 10 or so firings the reading was 7.2 ohms

Any chance you checked them when they were new, for comparison?

[neilestrick]

1 hour ago, Bill Kielb said:

Might even have a complaint that it blows the breaker then mysteriously only runs slowly.

Not that it couldn't happen, but I've never seen a breaker problem that wasn't obvious- loose/dangling wire making contact, wrong size breaker, worn breaker, etc.

[Smokey2]

9 minutes ago, neilestrick said:

Any chance you checked them when they were new, for comparison?

I meant to, I wish I did. Life got in the way.

[Bill Kielb]

3 hours ago, Smokey2 said:

I meant to, I wish I did. Life got in the way.

If ya ever run into that again you can simply measure the overall resistance and work backward from the known design voltage. Since wattage is key to design, approvals, electrical safety the rated wattage should really always be the thing. Since elements in general do not decrease in resistance then independent fusing of the appliance is likely less of a consideration or requirement for each model. So a 6000 watt kiln @240v should really never have elements that are less than let’s say 9.6 ohms gross reading otherwise it would begin to exceed its published electrical ratings. So the number that should give the greatest confidence would be the nameplate wattage and the minimum gross resistance would be:      the design voltage (squared) divided by the wattage. Maybe easier  to measure in a pinch.

[Smokey2]

@Bill Kielb

http://www.greatkilns.com/http-www-greatkilns-com-kiln-pottery-heat-treating-test-kilns-sq169he-html.html

[Bill Kielb]

2 hours ago, Smokey2 said:

@Bill Kielb

http://www.greatkilns.com/http-www-greatkilns-com-kiln-pottery-heat-treating-test-kilns-sq169he-html.html

16 ohms at 240 v and 3600 w. 12 ohms for the 208 v at 3600 watts. Total load, all elements in circuit and operating, meaning if there are two elements in parallel for the 240v they would generally be 32 ohms each and in parallel they measure 16 ohms together. If they differ then 1/R1+1/R2 = 1/Rtotal but still equal 16 ohms connected together.

[Dick White]

A few posts up Smokey said the published spec for the elements is 8 ohms, so two are wired in series to get the designed 16 ohms total  for 3600W @ 240V. He measured 7.2, which would suggest the kiln is pulling 4000W, or more than intended - if the measurement was correct. Another oddity on the spec sheet on that web page, not that web pages should always be believed, in the 208V column,  kiln draw is shown as 17 amps to get the 3600W (reverse calculating to 12 ohms total as you noted) on a 20a breaker. The 240V model is 15a on a 30a breaker. That's an odd implementation of the 125% rule...

[Bill Kielb]

2 hours ago, Dick White said:

kiln draw is shown as 17 amps to get the 3600W (reverse calculating t

I think the breaker size is switched on the web page. They have it right in the specs with a 30 for the 208v kiln which at 80%  of a 20 a breaker maxes out at 16 amps. Yes, in series they would need to be 8 ohms each for 16 ohms total or 6 ohms each for 12 ohms total. Good point though to make sure how they are connected. Measuring  the total  along with nameplate data gives a pretty high degree of confidence of what the total should be and therefore insight into the new value.