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Using Vent Caused Underfiring ?

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I just unloaded the first firing using the Envirovent that I installed on my Skutt 1027.  The cones were much more even, slightly hotter at the TC, but much improved over the cones with out the vent.  Problem is, I used the cone fire mode for 04 slow fire, that is supposed to compensate for irregularities and the Skutt tec said it would be more dependable that the Ramp mode I had written for my firings.  Kiln ran 12.30 hrs, 4 hours less that the program I usually run, and went to 1945.with no hold.  My Ramp firings are to 1920 with an hour hld.  When I unloaded this AM, none of the 04 cones are bent at all, the 05 are only slightly bent. BUT the firing is more even with the vent. GRRRR. what went wrong? I can't call Skutt until noon my time Monday AM. hoped to be running a glaze fire on this work before then.

Can I refire the bisque without risking the work?  Do I need to refire it?  I'm used to 04 bisque work and this isn't even 06 from the looks of the 05 cones.

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If I understand the question,yes I ran a bisque at cone fire mode at the Skutt tec's request before he suggested the vent purchase. He wanted to see what that results was as a check of the kiln function. Cones were 6 except for the cooler bottom. He felt the vent would do what I was hoping for. This under fire is nothing I was expecting.

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Ok, sorry if my question was a little confusing but you understood what I am asking :D Do you mean cone 6 if you are bisque to cone 04?

 

I have never used these vent systems but could you be drawing too much air out the kiln. I remember others saying you only want the smallest negative pressure in the kiln. This could be the problem.

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I have not used a vent but been eyeing them up for a while. I would be interested to hear what a seasoned vent-user has to say. 

 

What High Bridge says seem right about only the smallest negative pressure.  Otherwise, if air is being exhausted to forcefully it seems you might be drawing cold air in from somewhere (?) and that cannot be good for pots.  Can you adjust the draw of the envirovent?

 

However, that still does not seem to explain what you have seen (insert head scratching here....).  Since the top end of the firing is radiant heat (like from the sun) rather than the convection flows of air that move heat around at the beginning of the firing (like your oven),  it is hard to imagine that the vent is interfering with this, or?...

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The vent will not affect firing temperature. The kiln may have to work a tiny, tiny bit harder to compensate for the cool air being pulled through the kiln, but if it gets to the temp it needs, then it gets there regardless of how hard it had to work. 1945 is 1945, whether or not there's a vent.

 

You were using a totally different program and expecting the same results. Yes, the controller fired cooler than it should have, but that is likely due to an old thermocouple, or because it needs calibrating. If your thermocouple is good, then you need to either set an offset for the thermocouple, which will affect every cone, or do a cone offset for 04.

 

Personally, I would just keep using the program you have always used if it worked for you. That said, a 16 hour firing is super slow and unnecessary for anything but large or very thick work.

 

The unevenness issue is due to the fact that there's only one thermocouple in the kiln. You'll have to adjust how you load the kiln in order to even it out. Looser at the bottom and top, tighter in the middle.

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Thanks you for your replies.

The TC is not worn, has about 20 firings on it, and the previous test fire to 04 with out the vent and my own ramp hold program was pretty accurate except for the cool bottom.  By the way, 3 different Skutt tecs assured me the vent would even out the heat.  :unsure:   and the under fired load with the vent on was  noticeably more even. top to bottom, just under fired.   I have ordered a new TC anyway.

I am at a loss as to how to move forward with the vent.  I guess I could run a junk load of old cracked stuff and lots of shelves and posts to simulate a load, and do the Ramp Hold program for glazing that  I am used to running , put in plenty of test cones and see what they end up like.  I REALLY don't want to offer up a glaze load of good ware to the question.

 

OR;

I could re load the underfired bisque with plenty of 04 , 05, 06 cones and run the ramp-hold bisque program I'm used to running, forget the cone fire mode, and see what I get. 

 

Which would you do?

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The cone 04 fire mode if I understand it, is supposed to take care of whatever is needed to achieve the cone programed in for that program. The tramp hold bisque I have been using goesto1920 with an hour hold. The cone fire went to 1945 with no hold.

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The cone 04 fire mode if I understand it, is supposed to take care of whatever is needed to achieve the cone programed in for that program. The tramp hold bisque I have been using goesto1920 with an hour hold. The cone fire went to 1945 with no hold.

True if the thermocouples are correct and no offset.  The program to 1920 actually had more heat work than the program with no hold and a top temperature of 1945.  You should look at how much offset the thermocouple has and /or play around with some hold time but not 1 hour hold.

David

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Thanks, David and Neil, for you input.  I am really frustrated by this, guess you can tell by this entry in the middle of the night.  But after staring at the ceiling for the past hour, I think I am going to reload the under fired bisque, what can it hurt to refire it, using the ramp hold mode I have been using for years, put in the 04 , 05 cones ands see what that I get with the  vent running.  What has me so bumbed is the Skutt claim that the cone fire mode is put in, after many years of development on their part, to compensate for any variables and adjust to whatever to produce the correct cones.  I was told yesterday that if the cone fire mode did not produce the correct cones, then the control board had gone bad. Whaaaat?

After I run the programed ramp hold bisque firing with the vent, I will reevaluate.  May have a slightly used vent system for sale, cheap.  I've got to move forward, ware is stacking up in my studio, with nothing coming out the other end.

 

Neil, you are right, 16 hours is too long, but I was given that program by a super cautious teacher with many years of experience,  and I don't know where I can safely shorten it.

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When you feel up to experimenting, try your old firing schedule without the one-hour hold at end and compare the cones to the alternate Skutt gave you.  As David mentioned, the heatwork from the hold is bringing your cones down at end -- not the 15 hours of slow firing to get to the hold point.  If you run the schedule without the hold, you'll find out a more apples to apples comparison.  It might be you can get the same result by adding a hold to the Skutt schedule -- and cut down your firing time from 16 to 13 hours. 

 

At the clay studio where I fired, we always did a one or two hour preheat . . . mostly because students/residents would put damp wares on the bisque cart.  At home, where I only fire my own wares, I skip the preheat as I know my greenware is dry.  In a class setting, you error on the side of caution.  But you may not need that much caution where you have more control over the stuff going in the kiln. 

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In the effort to shorten the firing time,

what temp is when the burnout occurs?

One of the reasons why I have not been really interested in the cone fire mode is that with the ramp mode I've developed from many firings with cones I know what is happening in the kiln and have been able to adjust various things to get the cones like I want except for the cooler bottom. What I was looking for with adding the vent was more dependable use of the bottom shelf. I don't think I have glaze that need to be fired a cone cooler that I can put there. I'm wondering if I have simply fallen for some good advertising.

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What has me so bumbed is the Skutt claim that the cone fire mode is put in, after many years of development on their part, to compensate for any variables and adjust to whatever to produce the correct cones.  I was told yesterday that if the cone fire mode did not produce the correct cones, then the control board had gone bad.

 

Skutt does not build their own controllers. Bartlett Instruments does. It's the same technology in the V6-CF, the Dynatrol, and others. I doubt the control board has gone bad.

 

Single zone kilns are notorious for firing unevenly. The computer only knows the temp in the middle. Even though Skutt kilns have graded elements, they tend to fire unevenly unless you are careful about how you pack the kiln. The vent will help to even out the firing a bit, but it will by no means fix it entirely. For Skutt to tell you it would was misleading.

 

I have seen many Skutt kilns (and other brands) that do not fire accurately. I have a friend who sets her controller to cone 3 with a short hold to get an actual cone 5. That's an extreme case, but it's not uncommon to be 1/2-1 cone off. Again, I think Skutt is not being honest is claiming that their cone fire mode is 100% accurate. If you glaze fire at cone 5/6, and want to continue to use the cone fire mode, you need to do another firing to determine if it's firing too cool at all temperatures, or just at cone 04. Run a cone fire (you can just put in some shelves) to cone 6 with large cones spaced throughout the kiln and see what you get. If it runs cold, then set a thermocouple offset, which will correct all temps. If it's fine, then just do a cone offset for cone 04.

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You can likely speed up your custom firing at the beginning and middle. Just leave the last 200 degrees the same. You can probably get it down to 9 hours or so, and just add a preheat when needed. I often fire bisque in under 6 hours with no issues.

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Yes, It is.

4 ramps ,cone will come out  between 05 hot and 04 cool

1-100*per hr.to 200* ,hold 2 hours

2-150*per hr. to600*, hold 30 ,minutes

3-200* per hr. to1650*,hold 1 hour

4-100* per hr. to 1920, hold 1 hour

I use this with my work and also when I have heavy student pieces,  If the work seem a bit damp, I add to the 1st ramp.  I would like to go faster, but not risk loosing anything.  Thanks for your help. 

 

The refire is cooling today, I will have info on cones tomorrow night after work.

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Keep the hold on the first ramp only if you need a preheat. Unless things are still damp or very thick it's unnecessary. The holds on ramp 2 and 3 are also unnecessary. There's no gain to doing them. This schedule is still plenty conservative for most work, but will save you a few hours:

 

1- 100/hr to 200, hold as needed

2- 250/hr to 950

3- 125/hr to 1100

4- 250/hr to 1650

5- 100/hr to 1920, hold 1hr

 

I'm quite surprised that the 1hr hold at 1920 doesn't put you well beyond cone 04. That last ramp is right on schedule to put you at 04 without the hold. I think your thermocouple is not calibrated, which would explain why the Cone Fire mode ran cold.

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Keep the hold on the first ramp only if you need a preheat. Unless things are still damp or very thick it's unnecessary. The holds on ramp 2 and 3 are also unnecessary. There's no gain to doing them. This schedule is still plenty conservative for most work, but will save you a few hours:

 

1- 100/hr to 200, hold as needed

2- 250/hr to 950

3- 125/hr to 1100

4- 250/hr to 1650

5- 100/hr to 1920, hold 1hr

 

I'm quite surprised that the 1hr hold at 1920 doesn't put you well beyond cone 04. That last ramp is right on schedule to put you at 04 without the hold. I think your thermocouple is not calibrated, which would explain why the Cone Fire mode ran cold.

Neil is right and if your thermocouple was reading right cone 03 would have bent to about 30 deg with 1 hour soak @ 1920 deg F.  If everything was correct than you only needed 1887 deg F for i hour soak to get cone 04  to 90 deg bend. So if you want to pursue this, short of getting someone like Niel to look at every thing you will have to talk to Skutt about what you can do.

David

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How does a thermocouple get calibrated? Or become uncalibrated? I have ordered a new one. Will it be calibrated out of the box or is this something to do with the control board?

There is no calibration of the thermocouple itself, but there is an offset adjustment which is usually one of the option keys on the front panel of the controller scroll to TCOS, the instruction manual tells how to increase or decrease the offset.  It is best to just write down what the offset is set to for both the thermocouple TCOS and cone CNOS and think about it before making changes or call the Skutt technician.  You can get into trouble. Cone offset increases or decreases the heatwork for each cone individually TCOS adjusts calibration of the temperature readings, and affects all cone numbers and user programs.

David

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