How-to: Thermistor calibration

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Captain Starfish
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Re: How-to: Thermistor calibration

Postby Captain Starfish » Sun May 22, 2016 7:30 pm

1. Through the PID signal chain, the glass bed acts as a filter and delay. You have a point inasmuch as it would make sense for the thermistor to be mounted on the top of the glass so the PID processing includes that stage in the process chain, but that's impractical. So we get to temperature and hold the bottom of the bed at temp, relying on the steady nature of the system (ie we don't have temps running around everywhere) to allow time for the bed to equalise. To calibrate the thermistor you want the thermocouple to be reading the same temp as the thermocouple. Putting one on top and looking for the same value would mean that you'd be overshooting or undershooting the temperature by whatever the difference was across the glass. Which would then lead to over/under heating of the bed at steady state.

2. It's likely the power supply caps or HF transformer singing under load. These printers are built to a price and the power supplies used are barely adequate to the task. That song happens on mine too, always has since the day I bought it two years or so ago. A bigger PSU of better quality will fix it but, if everything else is working ok then there's nothing to worry about. If I were to do something to deal with the noise I'd be looking at an SSR and much higher voltage supply for the bed. But I'm happy enough with just the noise.

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Re: How-to: Thermistor calibration

Postby DeltaCon » Mon May 23, 2016 2:09 am

Thanks captain.
That makes a lot of sense actually, about measuring at the same position as the thermistor.
The singing however is not at full load. It starts when pwm kicks in and decreases the power, or rather starts giving it frequencies. I already gathered everything I need for the 24v psu upgrade so that will be next. But first I want everyrhing running as far as fsr and callibration are concerned.
Thanks for the quick reply.
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Re: How-to: Thermistor calibration

Postby SPICETRADERS » Fri Jun 10, 2016 8:34 pm

I would like to look at using an LM35 instead of a thermistor (or actually a simular device that can take the heat). Of course one more wire to add to the head, as well as more changes to the Arduino. And the issue with temperature would be the first issue to tackle.
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Xenocrates
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Re: How-to: Thermistor calibration

Postby Xenocrates » Fri Jun 10, 2016 9:16 pm

SPICETRADERS wrote:I would like to look at using an LM35 instead of a thermistor. Of course one more wire to add to the head, as well as more changes to the Arduino.


An LM35 would be rather interesting* to work with. The normal package is too large to fit inside the Onyx's hole, being nearly a quarter inch in diameter (.214), if you wanted to pot the entire thing, or at 3/16 (.1875) just the top portion (Which would likely reduce the accuracy of it to the bed surface, might be able to be press fit if you enlarged the hole slightly. It's also got a nearly linear output, which from what I've gathered, is usually a good thing, however I'm also told that it's a less than optimal thing for printers, as the temperature sensing systems are optimized towards thermistors almost exclusively, with some allowance made for things like thermocouples and PT100s. Is there a particular reason you want to work with an LM35 instead of a thermistor? some plastics seem to want a bed temp that pushes very close to the edge of it's safe range (I've got some filaments marked for 140C beds, and the LM35 tops out at 150). I would think a small package RTD might be perfect if you want high accuracy, and is also more temp stable and easier to integrate.


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Re: How-to: Thermistor calibration

Postby SPICETRADERS » Fri Jun 10, 2016 9:40 pm

Spec sheet shows a TO220 package which would allow a machine screw mount. Yes still issue of the 150 max temp. Maybe the KYT84 silicone temperature sensor as it is suppose to be virtually liner.
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Re: How-to: Thermistor calibration

Postby Eaglezsoar » Sat Jun 11, 2016 2:17 pm

Captain, I wanted to thank you for this interesting and informative thread. I have added it to my calibration tips section of my 3D links file.
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Re: How-to: Thermistor calibration

Postby Khayrisill » Thu Mar 16, 2017 6:05 pm

Hi,

I have a Prometheus and the manual say to change the thermistor type to 8.
When I do this I'm far from the good temp. So i want to do this but if a change GENERIC_THERMISTORTABLE_1 I change the value for the bed too (the bed is set to Type 97).

Can I used the GENERIC_THERMISTORTABLE_2 for my extruder ?

thx

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Re: How-to: Thermistor calibration

Postby Xenocrates » Thu Mar 16, 2017 11:27 pm

Yes you can. Just make sure you have the right numbers and you'll be fine.
Machines:
Rostock Max V2, 760W corsair modular PSU, PT100 enabled E3D V6 and volcano, self adjusting carriages, Raymond style enclosure
Automation Technology 60W laser cutter/engraver

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Re: How-to: Thermistor calibration

Postby Khayrisill » Fri Mar 17, 2017 9:48 am

It works. I used the Type 98. Because before I change the table 1 and my bed was off.

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Re: How-to: Thermistor calibration

Postby dc42 » Fri Mar 17, 2017 4:25 pm

My advice to anyone wanting an accurate temperature reading is to use a PT100 sensor, not a thermistor. No calibration is needed, although most current electronics boards do need an adapter of some sort.

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Re: How-to: Thermistor calibration

Postby Qdeathstar » Fri Mar 17, 2017 8:12 pm

Unfortuately the cyclops doesn't support PT100 :( I got a sensor and one of your boards, but can't do anything wth it.


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