Extruder temperature measurement accuracy?

I'm wondering what the tolerance of the 100K thermistor is, and hence how accurate the temperature readings are. Although an accurate temperature reading isn't essential, because you can find out by experiment what works well on a particular machine, it does make it difficult for people to compare results and swap settings. I'm now using 210C first first layer and 205C for the rest, but someone else said 185C worked best for him, whereas 190C didn't work at all for me.

I'm considering making a new board to run the IR sensor with modulation, and I'm wondering whether to add a thermocouple interface on it as well. A thermocouple and MAX31855 chip would provide temperature readings accurate to +/- 2C.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I've just found the answer to my own question. A comment in the firmware source says the extrusion thermistor is RS part # 198-961, which has a 10% tolerance and a B value of 3960. By my calculations, at 200C the 10% tolerance could result in a reading as low as 194.7 or as high as 206.0. So the tolerance is about +/- 5.5C at PLA printing temperatures.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I'm a bit worried about the extrusion temperature. My machine is telling me I have a fault on the sensor when I turn everything on. Having checked it out I'm not sure there is an error. I established the termistor was working in principal by removing it from the block and holding it between finger and thumb, no error. My room is in not very warm and doing the heater tests shows stable temperatures although I obviously don't know whether its 200 was really 200. For now I am assuming my ambient temperature is too low for the termistor reading to show a sensible value. However I do worry as when I heated the hot end up the first first reported temperature were below 0 and it's not that cold in here!

Hi picnic

If it's reading -273C, it's disconnected, or not getting enough power. The temperature isn't accurate at the very extreme of the range - that means at the low end. Because of the high resistance and the low voltage (3.3V), it can be quite varied at room temperature - I get between 5C and 15C. By the time the hot end heats up, accuracy is much improved. However, if you're NOT powering your board from the ATX PSU (put a jumper on JP9 (5V_EN) on the Duet, as per the 'Now connect the ATX power supply unit' in 'commissioning' instructions), and powering it from 5V USB power, and that power is below 4.7V, it can cause problems with things like temperature reading. There is a warning about this, second paragraph in the commissioning instructions.

dc42: I thought we had changed the thermistor for a 1% one. I'll check what it is tomorrow.

Ian
RepRapPro tech support

Quote
picnic
My machine is telling me I have a fault on the sensor when I turn everything on. Having checked it out I'm not sure there is an error.

FYI that means the Duet thinks the temperature is below -30C or above +300C. I know the extruder thermistor isn't accurate at low temperatures, but I think perhaps it's time to ask for a new one.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

My extruder temperature reads -9 at room temperature (+/- 20 deg). By experiment, printing with PLA is best at 185 and ABS 210, so I suspect it is under-reading more than most.

I double-checked this today. It seems Adrian was unaware of the thermistor updates, and was using old values in the firmware:

From Firmware, Platform.h, line 117
#define THERMISTOR_BETAS {3480.0, 3960.0} // Bed thermistor: RS 484-0149; EPCOS B57550G103J; Extruder thermistor: RS 198-961

We use this bed thermistor:
[uk.farnell.com]
Beta = 3988

And this hot end thermistor:
[www.digikey.co.uk]
Beta = 4138

This is probably enough to make a few degrees difference on both hot end and bed. The firmware has been updated so that the thermistor settings are correct, though the descriptions aren't yet! (edit: they are now!)

On-the-fly changing of the thermistors (M304 in our version of Marlin on the Melzi) isn't available on the Duet, at the moment. Mostly because in the main branch of Marlin, M304 has a different function, so we're trying to work out what g-code we can give this function.

Ian
RepRapPro tech support

Edited 1 time(s). Last edit at 01/06/2014 04:57PM by droftarts.

does it have to be a g-code? Wouldnt it be better to have a config/prefs/whatever file to store machine-specific (rather than print-specific) variable information in, and read this on start up? It would allow more flexibility than defining constants in the headers (though these could still be in place and then overidden in a similar manner to the network updating at present, but without having to use up a g-code for every option).

Ray

Ray

There's plenty of available gcodes, see: [reprap.org] . And it can be added to the config.g, with is machine specifid, just like all the other setup gcodes.

Ian
RepRapPro tech support

like M304 for example? :p

Quote
droftarts
I double-checked this today. It seems Adrian was unaware of the thermistor updates, and was using old values in the firmware:

From Firmware, Platform.h, line 117
#define THERMISTOR_BETAS {3480.0, 3960.0} // Bed thermistor: RS 484-0149; EPCOS B57550G103J; Extruder thermistor: RS 198-961

We use this bed thermistor:
[uk.farnell.com]
Beta = 3988

And this hot end thermistor:
[www.digikey.co.uk]
Beta = 4138

This is probably enough to make a few degrees difference on both hot end and bed. The firmware has been updated so that the thermistor settings are correct, though the descriptions aren't yet! (edit: they are now!)

On-the-fly changing of the thermistors (M304 in our version of Marlin on the Melzi) isn't available on the Duet, at the moment. Mostly because in the main branch of Marlin, M304 has a different function, so we're trying to work out what g-code we can give this function.

Ian
RepRapPro tech support

Does that mean that I will need to change all my hotend temperature settings after updating my firmware?

This is interesting, I puchased some thermistors for my home built head while waiting for the missing parts. They were 100K NTC with a beta of 3950 and give a similar reading to the bed before heating, so I know they are fine and they look like caps and not in the diode type package. I must have damaged the first one or it failed because after some testing it also read -9'C cold while the bed was at 28'C so I replaced it with another exact same one which is working fine still, they were very cheap.

So if your cold bed temp is different to the cold head temp, you have a problem or maybe the wrong thermistor as above. Just compensate for it by comparing the temp difference at cold to the bed and take this off the hot final print temp until you either update the software or get the correct part.

Dieter

Ormerod #257

Quote
dmould
Does that mean that I will need to change all my hotend temperature settings after updating my firmware?

Yes. By my calculation, the temperature which the old firmware reported as 200C will be reported by the new firmware as 188.3C. In other words, if the new firmware is accurate, then the old firmware was over-reporting by about 12C [EDIT: at typical PLA extrusion temperatures].

Edited 1 time(s). Last edit at 01/07/2014 10:17AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

@dc42: That sounds about right, as people seem to be struggling to print (extruding stops) at under 200C. Usually PLA prints in the 190 to 205C range, though I have had some that needs to be at 175C to produce good prints!

We tend to use an IR thermometer to check bed temperature, but have to set up a multimeter with a thermocouple probe to read hot end temps, and even then getting a good contact point is tricky. So we generally go by the temperature known PLA extrudes nicely at. We had noticed our Ormerod temps were 5 to 10C higher, but hadn't put 2 and 2 together until now!

Ian
RepRapPro tech support

Quote
dc42

Quote
dmould
Does that mean that I will need to change all my hotend temperature settings after updating my firmware?

Yes. By my calculation, the temperature which the old firmware reported as 200C will be reported by the new firmware as 188.3C. In other words, if the new firmware is accurate, then the old firmware was over-reporting by about 12C [EDIT: at typical PLA extrusion temperatures].

Odd. I found I had to make all my temperatures about 15 degrees colder than recommended - e.g. I print PLA at 185 intead of 200. The difference you state therefore appears to be the wrong way around. Now it looks as if with the new firmware I will be setting a temperature of 170 or so for PLA to get the same temperature as I am using now!