Chilly Posted December 28, 2013 Report Share Posted December 28, 2013 Following on from the discussion on re-firing and cracks........... I've tried to convert the below ramp from deg F to deg C, and calculate how long the firing will take. (Theory of course, I know the kiln may not be able to keep up/down with the ramp rate, but it will give me a guide as to firing time. Cone 6 Glaze Firing 100 degrees F per hour to 220 degrees F, no hold 350 degrees F per hour to 2000 degrees F, no hold 150 degrees F per hour to 2185 degrees F, hold 15 minutes On the way down: 500 degrees F per hour to 1900 degrees F, no hold 125 to 175 degrees F per hour to 1450 degrees F, no hold Cool naturally from 1450 degrees F [Deleted text that didn't format - see post after this for ] The maths on both sides looks OK. I converted the deg F to deg C using the Excel Convert formula, and double-checked the results using online conversion sites, so they are correct. But the results to work out time taken don't make sense. I used (EndTemp-StartTemp)/Ramp, looking at degF and degC separately, the results look sensible, but I would expect them to be the same as each other. So, why does firing in Deg C takes longer than firing in Deg F. I don't believe in Santa Claus, and I don't believe these figures either. Can anyone throw any light on this? Link to comment Share on other sites More sharing options...
Mart Posted December 28, 2013 Report Share Posted December 28, 2013 Your post is unreadable and I can only guess what went wrong. please use preview next time. Next to "Post" is "More Reply Options" > "Preview Post" Link to comment Share on other sites More sharing options...
Chilly Posted December 28, 2013 Author Report Share Posted December 28, 2013 Ummm, typed lots of spaces, and previewed it, looks, better, hope it stays..... Deg F Deg C Start temp Ramp To hours Start temp Ramp To hours F F F C C C 32 100 220 1.88 0 37.8 104.4 2.76 220 350 2000 5.09 104 177 1,093 5.60 2,000 150 2185 1.23 1,093 66 1,196 1.57 The maths on both sides looks OK. I converted the deg F to deg C using the Excel Convert formula, and double-checked the results using online conversion sites, so they are correct. But the results to work out time taken don't make sense. I used (EndTemp-StartTemp)/Ramp, looking at degF and degC separately, the results look sensible, but I would expect them to be the same as each other. So, why does firing in Deg C takes longer than firing in Deg F. I don't believe in Santa Claus, and I don't believe these figures either. Can anyone throw any light on this? Link to comment Share on other sites More sharing options...
Mart Posted December 28, 2013 Report Share Posted December 28, 2013 You used wrong math.Converting F to C or C to F is one calculation.Converting "rate of change" is something else. The formula for converting an F rate to a C rate is C = F / 1.8The formula for converting a C rate to an F rate is F = C x 1.8 Link to comment Share on other sites More sharing options...
Mart Posted December 28, 2013 Report Share Posted December 28, 2013 What this means is that 100 F per h = 55.56C per h Now, do not forget to subtract the ambient (usually 20-22C) from correct sides of the calculation You do not start form 0 but from ambient temperature. 20C is 68F Link to comment Share on other sites More sharing options...
Mart Posted December 28, 2013 Report Share Posted December 28, 2013 Now, do not forget to subtract the ambient (usually 20-22C) from correct sides of the calculation You do not start form 0 but from ambient temperature. Essex, this time of year?? Ambient temp is likely to be closer to 0°C than 20°C !! Sure, if you keep your kiln outside or in a room, whit no insulation. Most kiln controllers do not like that 0 C temp at all and think something is broken. Actually, on second thought, forget the subtraction. It's not needed. Link to comment Share on other sites More sharing options...
Chilly Posted December 28, 2013 Author Report Share Posted December 28, 2013 You used wrong math. Converting F to C or C to F is one calculation. Converting "rate of change" is something else. The formula for converting an F rate to a C rate is C = F / 1.8The formula for converting a C rate to an F rate is F = C x 1.8 32/1.8= 17.7778. So you're saying that 32F does not = 0C? Everyhere I look says 32F=0C What this means is that 100 F per h = 55.56C per h Now, do not forget to subtract the ambient (usually 20-22C) from correct sides of the calculation You do not start form 0 but from ambient temperature. 20C is 68F ""What this means is that 100 F per h = 55.56C per h"" WHY? Now, do not forget to subtract the ambient (usually 20-22C) from correct sides of the calculation You do not start form 0 but from ambient temperature. Essex, this time of year?? Ambient temp is likely to be closer to 0°C than 20°C !! It was 7 deg C when I switched the kiln on last night, with a forecast low of 3C, perhaps I should have started my example at one of those two, but 0C is the lowest "operating" temp for my controller, and anything colder than 0C I'm not interested in - tooooo cold! Link to comment Share on other sites More sharing options...
Chilly Posted December 28, 2013 Author Report Share Posted December 28, 2013 What this means is that 100 F per h = 55.56C per h While eating my lunch I decided it must need a % calculation to convert the ramp rate. I found a chart in one of my books that gives heating rates for Orton cones, and surprise surprise, the difference between the F ramps and the C ramps is 55.56%. So I need to multiply the F ramp rate by .5556. Yippeee. The time taken is now the same for both C and F. So, Ramp Rates of 100, 350, 100 F become 56, 195, 83 C OK, so what was I doing before I needed to drain the swamp? Oh yes, I remember............................... Link to comment Share on other sites More sharing options...
Mart Posted December 28, 2013 Report Share Posted December 28, 2013 You used wrong math. Converting F to C or C to F is one calculation. Converting "rate of change" is something else. The formula for converting an F rate to a C rate is C = F / 1.8 The formula for converting a C rate to an F rate is F = C x 1.8 32/1.8= 17.7778. So you're saying that 32F does not = 0C? Everyhere I look says 32F=0C No, I did not say that "32F != 0C". I wrote: "Converting "rate of change" is something else" and you need to use different math. What this means is that 100 F per h = 55.56C per h Now, do not forget to subtract the ambient (usually 20-22C) from correct sides of the calculation You do not start form 0 but from ambient temperature. 20C is 68F ""What this means is that 100 F per h = 55.56C per h"" WHY? Because you do not need to subtract 32 like you have in C to F equation and 9/5 = 1,8. Here is very good picture, that illustrates this: temperature-conversion You are wondering, where that "9/5" comes from? It's from (°C × 9/5) + 32 = °F Notice the 1.8x difference between 100 and 180 Link to comment Share on other sites More sharing options...
Chilly Posted December 28, 2013 Author Report Share Posted December 28, 2013 Mart: Your first explanation did not make sense to me, your second explanation would have left me more confused. I'm pleased I was able to work out the answer for myself, as I understand it. I was not wondering where "9/5" comes from. I am well aware of the calculations required to convert from F to C and vice versa, but converting the ramp rate clearly needed a different theory. Link to comment Share on other sites More sharing options...
bny Posted December 28, 2013 Report Share Posted December 28, 2013 The methods that Norm Stuart gives below are correct. Rates are converted with multiplication and division only. Typical trick warm up problem in week 1 of college physics. You've got it. Degrees per hour ramp rate is C = F / 1.8 or F = C * 1.8 But for actual temperature you have to also add or subtract 32, C = (F - 32) / 1.8 or F = (C * 1.8) + 32 Multiplying/Dividing Adding/Subtracting has never been easy for me without a computer spreadsheet program. Sometimes I'll forget and use a temperature converter for ramp rate, resulting in the wrong answer. I am well aware of the calculations required to convert from F to C and vice versa, but converting the ramp rate clearly needed a different theory. Link to comment Share on other sites More sharing options...
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