Are thermocouples more accurate than thermistors?
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@zapta said in Are thermocouples more accurate than thermistors?:
@deckingman, having +/-5C error is reasonable as long as the error is stable over time. The question, is, how do you determine that your tolerance is 5C and not 10C or 20C?
I never said that having a +/- 5 degree error is reasonable or otherwise. I only said that one can assume a +/- 5 degree tolerance is acceptable. Measurement tolerance and measurement errors are completely different things.
And just because one might use an accurate sensor, does not mean that measurements taken with that sensor will be accurate. With 3D printers, the sensor is often some distance away from the part that we are interested in. Usually hot ends have a sensor somewhere in the heater block but not in the nozzle itself. From the work I have done measuring temperatures inside the nozzle, I can pretty well guarantee that sensor in the heater block will not show an accurate temperature of the nozzle itself - regardless of how accurate that sensor is.
Take a look at this write up that I did https://somei3deas.wordpress.com/2020/05/21/the-effect-of-deflected-part-cooling-air-on-brass-and-steel-nozzle-temperatures/ and you'll see that the temperature inside a brass nozzle could be around 1.7 degrees cooler than the nozzle block without a fan running, and as much as 8.5 degrees cooler with some deflected air from a part cooling fan. And for a hardened steel nozzle, the difference is much more dramatic.
So I say again, what is the point of using a sensor with a claimed accuracy of 1 deg C or better, when there could be measurement errors of 8 degrees or more which depend on the position of that sensor in relation to the nozzle tip?
With regard to temperature drift over time, once I've established a particular temperature to use for a particular filament, I've never had recourse to re-evaluate it. And I can't say that I've ever seen any evidence that temperature sensors used in 3D printers will drift sufficiently over their lifetime to require re-evaluation. But I have no doubt that if you search the internet long enough and hard enough, you'll find "evidence" to the contrary.
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@deckingman said in Are thermocouples more accurate than thermistors?:
So I say again, what is the point of using a sensor with a claimed accuracy of 1 deg C or better, when there could be measurement errors of 8 degrees or more which depend on the position of that sensor in relation to the nozzle tip?
Because you use a sensor/electronics/firmware that gives you 10C error than the reading can be much more than 8C away from the nozzle's temp.
BTW, I am using platted copper 0.4mm nozzle with flow < 10mm^2/sec and material fan (which is weak in the first place) at 50%. What's your guts feeling about the nozzle temp drop in my system (nozzle vs heating block temp), when printing at ~230C?
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When you are shipping thousands of machines, the support burden of in-accurate sensors is non-trivial. Plus finding a good thermistor for temps above 250C is still a challenge.
But for the solo hobbyist, no it's not critical.
@zapta PT100's are more readily available, although the sensors for FDM are already a custom part so that's kinda academic.
Isolate, substitute, verify.
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@zapta said in Are thermocouples more accurate than thermistors?:
@deckingman said in Are thermocouples more accurate than thermistors?:
So I say again, what is the point of using a sensor with a claimed accuracy of 1 deg C or better, when there could be measurement errors of 8 degrees or more which depend on the position of that sensor in relation to the nozzle tip?
Because you use a sensor/electronics/firmware that gives you 10C error than the reading can be much more than 8C away from the nozzle's temp.
It's highly unlikely that a thermistor would give a 10 degree error but so what if it did? If you accept that there are measurement errors due to factors such as the position of the sensor relative to the nozzle tip, or the affect of deflected part cooling air, or EMF induced noise in the wiring, or numerous other factors, and for that reason you going to print at whatever temperature gives you the best print quality, then effectively you are saying that you are going to ignore the sensor value in absolute terms. So if you are going to ignore it, why does it have to be highly accurate as opposed to reasonably accurate?
BTW, I am using platted copper 0.4mm nozzle with flow < 10mm^2/sec and material fan (which is weak in the first place) at 50%. What's your guts feeling about the nozzle temp drop in my system (nozzle vs heating block temp), when printing at ~230C?
I have absolutely no idea. But if I did come up with a number, would you take what I say as being the absolute truth? Would you always use that temperature and never deviate from that value? Or would you print a temperature tower to check? I would hope that you would do the latter and if necessary adjust the setting to give you the best print quality. Which is the point that I'm trying to make. It doesn't matter where or how you measure the temperature, or if you use complicated mathematical formulae to calculate the correct setting, you are still going to adjust that value to give you the best print quality. Which effectively means that you are going to ignore the absolute temperature reading and use some other value. So why does it need to be a highly accurate sensor rather than a reasonably accurate one if you are not going to take much notice of the value? Whether your chosen temperature setting is compensating for the position of the sensor relative to the nozzle, or the effect of deflected part cooling air, or the accuracy of the sensor or a combination of all of those things is largely immaterial.
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@theruttmeister said in Are thermocouples more accurate than thermistors?:
When you are shipping thousands of machines, the support burden of in-accurate sensors is non-trivial.
True, but so is cost. If you have a reasonably accurate, good quality sensor which does the job well enough, you wouldn't choose to use a more expensive sensor which offers no material benefit.
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@deckingman said in Are thermocouples more accurate than thermistors?:
I have absolutely no idea. But if I did come up with a number, would you take what I say as being the absolute truth?
Of course, until you will change your mind.
I seldom run heat towers. I have one printer and have one slicer settings per material (ABS, PLA, etc) and am getting good results in general. I switched recently to a new printer and it seems to print at lower temperature than before so I would like to know if the heat block temperature reading is reasonably accurate.
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@zapta said in Are thermocouples more accurate than thermistors?:
@deckingman said in Are thermocouples more accurate than thermistors?:
I have absolutely no idea. But if I did come up with a number, would you take what I say as being the absolute truth?
Of course, until you will change your mind.
I seldom run heat towers. I have one printer and have one slicer settings per material (ABS, PLA, etc) and am getting good results in general. I switched recently to a new printer and it seems to print at lower temperature than before so I would like to know if the heat block temperature reading is reasonably accurate.
I feel like I'm hitting my head against a wall. You say that you are using a copper coated nozzle. So I can only assume that it's hardened steel with a copper coating on the outside yes? In which case, there would likely be good thermal transfer from the block to the outside skin of the nozzle. But poor thermal transfer from that outside skin to the inner core and the filament. If those assumptions are correct, then you would need to set the nozzle block to a higher temperature in order for the filament to be at the correct temperature. It doesn't matter if you use a relatively cheap thermistor or the most expensive and accurate sensor known to mankind. You'll still need to ignore that reading and set the nozzle block to a higher temperature.
There are two ways that you can establish the temperature inside the nozzle. You can either do as I have done and fit a sensor inside it, or you can print a temperature tower to determine the best temperature to use. In either case, you will end up using a different nozzle block temperature regardless of how accurately you have measured it. -
@deckingman, it's copper core that is played with some hard material. Supposed to have better thermal transfer.
EDIT: It's 'plated copper' rather than 'copper plated'. I wonder if this may be the reason that my printing temperature setting is lower than I would expect.
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@deckingman having shipped thousands of "reasonably accurate, good quality' thermistors... I wouldn't do that again.
It's hard enough to just get the same damn thermistors over several years. A PT100 is an actual standard, a 100k thermistor is whatever the manufacturer felt like making that day. -
The E3D PT1000 cartridge arrived, I installed it, and the printer prints well, as it did before. I can sleep better now. Definitely worth the money.
