dzach's Hadron build

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Thermistor calibration

Postby dzach » Tue May 22, 2012 3:57 pm

This is a picture of the fried thermistor together with a new one:
2012-05-22-150029.jpg

I need to calibrate the thermistor so that the graphs I post are meaningful. To see how the thermistor performs I need two standard temperatures. Water offers an easy way to calibrate temperature. This is the setup to measure the temperature at 100°C and 0°C:
2012-05-22-170858.jpg
Measurement setup:
Boiling water

The plastic bag is there to protect the thermistor from the water.
2012-05-22-171344.jpg
Measurement setup:
Icy water

And the resulting graph:
thermistor2.png
From tap water to boiling water, icy water, stirred icy water and room temperature.


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.
dzach
 
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Re: Thermistor calibration

Postby frob » Tue May 22, 2012 6:00 pm

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.


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.

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.
Also with 100K thermistor, impedance is high enough that it can pick up a significant amount of induced noise from the motor cables,
which can throw off your measurements in a small but measurable way.

i suggest trying a glass-encapsulated, 1%, 10K thermistor with a shielded cable-
This one for example would be a pretty good choice: http://search.digikey.com/scripts/DkSea ... 95-2625-ND

Note that in the target temperature range, they will have a resistance somewhere around 1.5K-2.2K;
thus to get the best resolution in temp measurement, the pullup resistor should be in the same ballpark value, also a 1% or better,
though that would require an adjusted thermistor lookup table.
Also note that the 10-bit ADC in the AVR has its accuracy spec'd at 2%, and on the Arduinos there's no filtering on the AVcc supply.
So even with a 1% thermistor+resistor i wouldn't expect better than 5% overall accuracy.


... considered using thermocouples? :idea:
frob
 
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Re: dzach's Hadron build

Postby Stephen Wright » Tue May 22, 2012 7:29 pm

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
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Re: dzach's Hadron build

Postby loopingz » Wed May 23, 2012 8:56 am

I have to agree with Stephen here, altitude and day pressure can influente to this amount and the overall deviation is not that bad for a cheap captor...
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Re: dzach's Hadron build

Postby dzach » Wed May 23, 2012 12:57 pm

Guys, thanks for the input!
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

This is a very good point. My altitude is ~400m so the boiling temperature should be ~98.5°C. That accounts for 1.5°C of the difference.
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.

I believe I have the EPCOS B57561G0104F000 type which is stated at 1% tolerance.
That, together with the ADC and resistor tolerances may be the explanation for the remaining 2.5°C difference.
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.
frob wrote:... considered using thermocouples?

Yes, I've considered using thermocouples for testing, but the electronics are more involved and I had already ordered some extra thermistors. I will probably end up ordering some thermocouples now.

EDIT:
I measured the resistors that are connected to the thermistors with my average quality multimeter:
Extruder: 4.70kΩ
Heatsink: 4.68kΩ
It looks like their contribution to the deviation is max 0.5%, not very significant. Vcc voltage would also play a role in the value the ADC reads.
In any case, I know now what to expect from that end.
dzach
 
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Location: Athens, Greece

Re: dzach's Hadron build

Postby frob » Wed May 23, 2012 7:07 pm

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.
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.
Vcc voltage would also play a role in the value the ADC reads.
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.
frob
 
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Re: dzach's Hadron build

Postby dzach » Wed May 23, 2012 10:00 pm

It looked like epoxy to me too, I bought it as EPCOS B57561G0104F000 and had no problem working up to 250°C, I tried extruding ABS at this temperature.
frob wrote: try cutting open the dead one, or scraping the top layer off to see whats inside.
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:It might be interesting to also do a test to see the noise and DC offset with and without the motors running.

This would be interesting indeed. I still get errors with Marlin. I use shielded cables for the motors, but I don't have the necessary equipment to do a test for either the noise or the protocol.
dzach
 
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Location: Athens, Greece

A fan for extruding PLA

Postby dzach » Fri May 25, 2012 9:39 pm

I installed temporarily a 50mm fan for extruding PLA, driven from the D9 RAMPS port.
2012-05-26-003232.jpg
The fan is not close enough to the bed but nonetheless there is a remarkable improvement to the prints.

The results are absolutely positive. All small parts now print a lot better because they cool down and are ready for the next layer faster.
dzach
 
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Location: Athens, Greece

Re: dzach's Hadron build

Postby Enraged » Fri May 25, 2012 10:17 pm

nice, I'll have to hook one up on the weekend.

you should print yourself a fan duct and see if that helps
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Re: dzach's Hadron build

Postby naPS » Fri May 25, 2012 11:04 pm

I've been using a couple of 40mm fans mounted to a thing I printed off of thingaverse, and it's been amazing the difference in quality of prints that I've been getting. I'm still bummed that I've been getting a little bit of wobble out of the Z, but I've ordered some leadscrews to hopefully eliminate that. Here's the fan mount - works great for a MakerGear stepper plastruder : http://www.thingiverse.com/thing:12336

The only problem with the mount that I've found is that it doesn't blow on the back. When I try to really crank the speed up I get a little bit of warping on the side of the print closest to the back of the machine. I'm trying to rig some temporary things up, and will actually probably resort to printing this fan mount when I get a couple of my calibration issues done : http://www.thingiverse.com/thing:20459
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