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- Jun 11, 2006
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As some of you know I have been Questioning this topic for awhile. The current 15n20 thread brought this topic back to the surface and I thought I would see if we can hash this over a little. I have done quite a bit of research into this since my oven build. What I found is I was correct in doubting the depth I (we) immerse our TC in our ovens. I have never measured the TC immersion depth of ovens like EvenHeat or Paragon. But the few pictures I have seen that show the TC seem to show it not having much of an immersion depth. But I am far from an expert so we will leave there ovens out of this, unless someone has one and wants measure the length of the TC that is exposed in the ovens chamber.
So what I found with my research is that an insufficient immersion depth is the leading contributor to temp errors. I came across a paper talking about this and thy had some handy charts. One of which I will place below so you can see how length affects accuracy. The problem we are dealing with is the thermal conductivity of the thermal couple. As we heat the TC tip (part that reads temp) that heat is also traveling down the TC and through the fire bricks and out to our junction block. What clever people have calculated is the amount of TC that needs to be exposed in relationship to its diameter. Now this is in accordance with the TC being a solid sheathed TC. The TC we use are normally exposed and have very large diameter leads. I don't know how this affects the numbers on the chart but we could make some guesses. Normally a sheathed TC uses fine wire inside and a tiny TC junction. This is inclosed within a metal sheath and filled with a insulating compound. The kind we use are just 2 large wires that are tig welded together on the end.
This chart lists accuracy as a % of reading compared to the diameter of the TC times its immersion depth. You will quickly see that as the diameter goes up so does the error. Now your guess is as good as mine as to what the "diameter"of our TC's are. But logically thinking this over its mass that transfers heat. So a 1/4" diameter tube would transfer less heat then a 1/4" solid rod. But if we just take a number like say 9mm. According to the chart a 9mm TC would need an immersion depth of just over 3" which would give us an error of 1%. That sounds low but at say 1500 that's 15deg. That's not taking into account any error of the wires, connections and controller. If we want say a .1% error which is 1.5deg we need close to 5" of immersion depth.
So I don't know what this tells us exactly because I don't know what the equivalent diameter of our thermal couples are. But it does show us that there is actually a very large error that can occur if we don't have enough of our TC exposed to the inside of our ovens. I think this is one big reason why our heat treat Temps are not universal all ovens. My 1450 might be your 1475. So this if anything else hammers home the need for solid testing of your oven to find what gives you the best outcome. Don't just take someones word that X temp is the best temp. Do any of you have any more data on this or where our TC's fall on this graph? Thanks guys
So what I found with my research is that an insufficient immersion depth is the leading contributor to temp errors. I came across a paper talking about this and thy had some handy charts. One of which I will place below so you can see how length affects accuracy. The problem we are dealing with is the thermal conductivity of the thermal couple. As we heat the TC tip (part that reads temp) that heat is also traveling down the TC and through the fire bricks and out to our junction block. What clever people have calculated is the amount of TC that needs to be exposed in relationship to its diameter. Now this is in accordance with the TC being a solid sheathed TC. The TC we use are normally exposed and have very large diameter leads. I don't know how this affects the numbers on the chart but we could make some guesses. Normally a sheathed TC uses fine wire inside and a tiny TC junction. This is inclosed within a metal sheath and filled with a insulating compound. The kind we use are just 2 large wires that are tig welded together on the end.
This chart lists accuracy as a % of reading compared to the diameter of the TC times its immersion depth. You will quickly see that as the diameter goes up so does the error. Now your guess is as good as mine as to what the "diameter"of our TC's are. But logically thinking this over its mass that transfers heat. So a 1/4" diameter tube would transfer less heat then a 1/4" solid rod. But if we just take a number like say 9mm. According to the chart a 9mm TC would need an immersion depth of just over 3" which would give us an error of 1%. That sounds low but at say 1500 that's 15deg. That's not taking into account any error of the wires, connections and controller. If we want say a .1% error which is 1.5deg we need close to 5" of immersion depth.
So I don't know what this tells us exactly because I don't know what the equivalent diameter of our thermal couples are. But it does show us that there is actually a very large error that can occur if we don't have enough of our TC exposed to the inside of our ovens. I think this is one big reason why our heat treat Temps are not universal all ovens. My 1450 might be your 1475. So this if anything else hammers home the need for solid testing of your oven to find what gives you the best outcome. Don't just take someones word that X temp is the best temp. Do any of you have any more data on this or where our TC's fall on this graph? Thanks guys
