Kiln controller troubleshooting

L

Lazy H

Joined
Aug 19, 2014
Messages
12
Hello, I just received a controller from ebay to replace the old infinitely variable knobs on my duncan ceramic kiln. It came with the controller, solid state relay w/ heat sink, and a type k thermocouple with some ceramic segments to protect it. I plugged everything in and it ran fine at first, perfectly matched my thermostat at room temperature, clicked it on and felt the elements start heating up, left it for a few minutes heating up to a set value (SV) of 30*C to see if it would keep the temp consistent, and when I came back the kiln was still warm and I couldn't tell if the elements were on or not, but the measured value digits (PV) on the screen didn't read a number, it just said LL. I tried to unplug and plug it back in again, and the same thing came up. Pressed the set button to try to adjust the SV and i could, but for some reason the PV switched to -15 while changing the SV, then changed back to LL when I was done changing the values. Tried undoing and redoing the wires a few times to see if something was loose in the thermocouple or anywhere else and no change. The kiln is 240 and controller is 60-240 volts, but i took one hot wire and one neutral wire to the controller to make it 120 volts, just in case of power surges, and there haven't been any problems I can see with wires being loose or the controller switching off it the connection isn't quite right. Any help would be greatly appreciated, I really don't want to keep trying to use pyrometric cones to get O1 tool steel to work right, especially since the cone shelf came broken when I got the kiln.

This is the kit I received, maybe trying to save money on the most affordable one I could find was a bad idea for reliability? The model number is DTA-48. The manual, both paper and digital, has no information on troubleshooting.
http://www.ebay.com/itm/Pottery-Cer...roller-Kiln-SSR-Thermocouple-HS-/121098643288
 
Slight update, left everything unplugged overnight, tried rewiring the thermocouple using standard copper speaker wire rather than the stainless braid shielded wire that came with the kit, it read the right temperature again, 20*C in my basement shop, but just to test the reading before firing I aimed a heat gun at the thermocouple probe and the reading went down, all the way to the negative teens before very very slowly coming back up to about 7*C again. Tried switching to autotune, thought maybe it just needed to run a bit and learn the settings of the probe or something like that, let it run with a set value of 30*C again and same result, came back in 10 minutes or so and came back with LL in the PV digits space, which switched to -15 again when I entered the set temperature mode or whatever.

I would really appreciate it if somebody could help me out, the person who sent it to me doesn't know anything about the problem, if it's the controller, the probe, or what to do about it and the O1 in the corner of my shop is starting to look pretty impatient for a proper heat treat.
 
Just tested the thermocouple with my multimeter, at room temp the resistance was just under one ohm. I got out my trusty heat gun and hit it for a few seconds and the resistance raised up to 15-19 ohms, does anyone know what values to expect for a type k thermocouple at a given temperature range? My google skills are failing me on that front at least.
 
Thermocouples output a millivolt signal. You should be measuring millivolts, not resistance. The table in the previous post gives the millivolt readings for different temperatures.

Your resistance reading is helpful insofar as it tells you the thermocouple is not broken (open circuit).

Can you find an online manual for the controller you have and post a link to it?

A wild guess as to the meaning of the LL message would be an input value below the Lower Limit of the selected range, but without the manual it's just a guess.

It's worth reading up on how thermocouples work, as it's less straightforward than it first appears. I recommend strong coffee and the Omega website.

Basically, a junction of 2 dissimilar metals will generate a voltage which depends on the temperature of the junction. It is true of any pair of metals, but only some metal combinations are useful. Type K thermocouples use Chromel and Alumel for the thermocouple legs and the voltage generated follows the table 629Hugo linked to.

When you wire the thermocouple, it needs to be wired correctly using the wire of the same composition as the thermocouple legs. That way, the new junctions are of similar metals and do not generate a voltage. If you use the correct wire but connect it the wrong way round, each of the new junctions will generate a voltage, but it will be of opposite polarity to the measuring junction, giving an error of twice the difference between the connection temperature and the instrument temperature. My guess is that this is what you did initially.

Changing to copper wire will have given erroneous readings as well.

To make a circuit, the thermocouple wires need to be connected at the other (usually cold) end as well. This is normally in the instrument and the controller will have a precision measuring device inside to measure the cold junction temperature and apply a correction.

I appreciate it's too late for you, but the best advice I can give on controllers, VFDs and other geeky stuff is never to buy anything until you have seen the manual online, read it, saved the manual location in favourites/bookmarks, and feel you understand it well enough to have a reasonable chance of making it work. That way, you can cry for help with a link to the manual and there is a reasonable chance someone familiar with the type of equipment you are using can assist.
 
Make sure the thermocouple is working and isn't installed backwards. If it is backwards, you might get an LL error.
 
You must log in or register to reply here.
Back
Top