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G Code for showing current thermistor impedance values available?

Posted by gdachs 
G Code for showing current thermistor impedance values available?
August 14, 2017 02:13PM
Is there a g code command that allows me to see the current impedance of a thermistor?
I want to find the Steinhart Hart coefficients empirically and I am afraid that my multimeter
will change the impedance if I measure it parallel to the board, at least at low temperatures.

Gerald
VDX
Re: G Code for showing current thermistor impedance values available?
August 14, 2017 02:46PM
... you can read the voltage value with the "M42" command, if you remove this thermistor pin from the "reserved pins" list ...


Viktor
Re: G Code for showing current thermistor impedance values available?
August 15, 2017 12:02AM
Thanks for the fast answer. Can you explain why I have to remove it from the "reserved pins" list. What other consequences does this have?

I can't find something in the config that would remove the pin from the list, can you help me with this too?

Gerald

Edited 1 time(s). Last edit at 08/15/2017 12:24AM by gdachs.
VDX
Re: G Code for showing current thermistor impedance values available?
August 15, 2017 01:14AM
... look into the file "pins.h" - there are some pin-groups "excluded" from G-code (or M-code):

Quote

// List of pins which to ignore when asked to change by gcode, 0 and 1 are RX and TX, do not mess with those!
#define _E0_PINS E0_STEP_PIN, E0_DIR_PIN, E0_ENABLE_PIN, HEATER_0_PIN, analogInputToDigitalPin(TEMP_0_PIN),
...

To reenable a pin for the "M42" command you have to remove it from this lists ...


Viktor
Re: G Code for showing current thermistor impedance values available?
August 15, 2017 05:10AM
Um, this isn't Marlin -smoothieware has no "reserved pins list". There's no config.h either.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: G Code for showing current thermistor impedance values available?
August 15, 2017 10:15AM
@the_digital_dentist: so no possibility to do this?

Then I might try to hook the heater directly to 12V and the multimeter directly to the thermistor and make a video winking smiley.
Have only to switch off before it gets too hot.

Gerald
Re: G Code for showing current thermistor impedance values available?
August 15, 2017 08:13PM
Do you have an unknown thermistor you're working with, or do you know the manufacturer/part no.? Thermistors are cheap. If you have an unknown part, replace it with a known part, then use a few values from the manufacturer supplied temperature vs R table and have Smoothieware calculate the Steinhart-Hart coefficients for you.

If you absolutely must try to create the T vs R table yourself, you can easily calculate the effect of the multimeter's input resistance on the measured thermistor resistance at low temperatures. Typical DMM Rin is in the 10 M Ohms - 100 M Ohms range. Look at your DMM's specs to see input resistance. If your meter doesn't have any specs, your chances of being able to create an accurate T vs R table with an unknown thermistor and an no-spec DMM are pretty slim. How are you planning to measure temperature?

Edited 1 time(s). Last edit at 08/15/2017 08:19PM by the_digital_dentist.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: G Code for showing current thermistor impedance values available?
August 16, 2017 04:53AM
Yes I have an unknown thermistor. I have looked already for a replacement. The thermistors that are known by smoothieware are not easy to get. And most other thermistors that are cheap have only beta values for temperatures between 0-80°C.

A multimeter with a Rin of only 10M Ohms will give me a failure of 1% at 100K of the thermistor at 25°C, this failure gets much less at the more important higher temperatures, because the resistance of the thermistor will decrease very fast. I will use a K-Type thermometer with a failure of ±0.75% in the interesting temperature range.

Gerald
Re: G Code for showing current thermistor impedance values available?
August 16, 2017 08:37AM
You can use Ohm's law to calculate the effect of the meter's input resistance, no matter what its value is, but it doesn't really matter. You don't have to get one of the thermistors that are preconfigured in smoothieware. Get any 100k thermistor (100k Ohms at 25C) for which the manufacturer provides an R vs T table (just about all of them have such tables in the data sheets). You can use that table to generate Steinhart-Hart coefficients. That is likely to be far more accurate than trying to create your own table from an unknown thermistor.

The absolute accuracy of temperature readings isn't essential or even generally possible in a 3D printer. There is always a temperature drop from the thermistor reading to the bed surface that depends on the materials, thickness, and construction of the bed. The thermistor is there to get the temperature into the approximate range and to serve as feedback for the PID algortihm. You are going to set the temperature of the bed/extruder to a value that works for the filament you're printing, regardless of numerical reading. If the ABS won't stick when you set the bed temperature to 100C, you'll set it to 110C, or whatever temperature you experimentally determine works for ABS in your printer. That's why your slicer stores printer and filament parameter files.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: G Code for showing current thermistor impedance values available?
August 17, 2017 12:13AM
Quote
the_digital_dentist
Get any 100k thermistor (100k Ohms at 25C) for which the manufacturer provides an R vs T table (just about all of them have such tables in the data sheets). You can use that table to generate Steinhart-Hart coefficients. That is likely to be far more accurate than trying to create your own table from an unknown thermistor.
I know this all, why do the people always think I am stupid. Is it my bad English?

I repeat myself, I have already looked for a replacement. I found the cheap unbranded ones from China that are not documented at all. And I found the proper documented thermistors on digikeys or mousers websites. Despite the fact that for 2 of them I have already to pay the price for the thermometer, they take 18€ to 20€ for shipping. I don't name this cheap.

Maybe I am really stupid, so provide me a link that allows me to order them cheap, please.

Gerald

Edited 1 time(s). Last edit at 08/17/2017 04:27AM by gdachs.
Re: G Code for showing current thermistor impedance values available?
August 17, 2017 06:36AM
How about [www.maplin.co.uk]

I live in the US. It took 3 seconds to find that link. You gotta make some effort...

here's another, complete specs including R vs T table:
[www.rapidonline.com]

Edited 1 time(s). Last edit at 08/17/2017 06:49AM by the_digital_dentist.

Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: G Code for showing current thermistor impedance values available?
September 04, 2017 01:19PM
Maplin doesn't ship to my country, the second thermistor is only for up to 110°C, too less for my hotend.

Anyway, I had answered a clear question and had the hope to get an answer to it.

I have found an easy solution myself. Just configure the thermistor with any beta value, then heat the hotend up to 230 °C.
Now calculate the impedance of the resistor with this formula:
r2 = r1 / exp(beta*(1/t1 - 1/t2));
with:
r1 = 100K Ohm,
t1 = 25 + 273 °K and
t2 = 230 + 273 °K

Now measure the real temperature t2 add 273 for Kelvin and calculate a new beta:

beta = log(r1/r2)/(1/t1-1/t2);

Instead of using this new beta I could repeat this with another beta for temperatures between 0 and 80°C and a temperature of 80°C and at end I would
have 3 pairs of temperature and impedance values for the calculation of the Steinhart and Hart coefficients.

Gerald

Edited 1 time(s). Last edit at 09/04/2017 01:23PM by gdachs.
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