dzach wrote:Conclusion:
Thermistor looks good at 0°C but at 100°C it only shows 96°C.
I'll have to find a way to correct the tables now.
Stephen Wright wrote:I always lived near the coast and never thought about it much, but consider your elevation for the boiling point of water, if you happen to be at 3750 feet, 96C is the boiling point: http://www.engineeringtoolbox.com/boiling-points-water-altitude-d_1344.html
frob wrote:Also, each thermistor has a tolerance - you can buy them as 10%, 5%, 2%, 1% etc. 96/100 would be within tolerance for a 5% thermistor.
frob wrote:That kind of thermistor isn't rated for temperatures over 125C, or 155 in some cases.
Its not a good choice to use on an extruder that runs in the 180-250C range.
Most Glass-encapsulated thermistors can handle 250C, or 300C in some models. Epcos makes those too.
frob wrote:... considered using thermocouples?
Hmm i may be wrong then - i assumed they were low temp epoxy dipped types because thats what they really look like in your pictures. usually the glass encapsulated ones are transparent, translucent, or very obviously made of glass. maybe its dipped in epoxy as a second protective coat, but i'd check to be sure - try cutting open the dead one, or scraping the top layer off to see whats inside.dzach wrote:frob wrote:That kind of thermistor isn't rated for temperatures over 125C, or 155 in some cases.
Its not a good choice to use on an extruder that runs in the 180-250C range.
Most Glass-encapsulated thermistors can handle 250C, or 300C in some models. Epcos makes those too.
The spec says it is good to 250°C, there is a Tmax for bath soldering 260°C, as I now see. I went (unintentionally) higher than 300°C and the little thing was permanently damaged.
Well that depends on how the ADC is set up in software. you can use ratiometric (Vcc as reference) or the internal precision reference. in this case we want radiometric. i assumed the firmware does that right, but haven't actually checked - I'll do that next time i need to go wading in there (a couple days hence..) and see what kind of digital filtering is used. It might be interesting to also do a test to see the noise and DC offset with and without the motors running.Vcc voltage would also play a role in the value the ADC reads.
Too late for that now, but I had already broken it open and it sure didn't look like glass in the inside. I might be wrong though.frob wrote: try cutting open the dead one, or scraping the top layer off to see whats inside.
frob wrote:It might be interesting to also do a test to see the noise and DC offset with and without the motors running.
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