Extruder temp reading is extremely inaccurate

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09-27-2016, 08:37 PM
iineviitable

Extruder temp reading is extremely inaccurate

I updated my I3 to repetier from the anet firmware that came with it and everything was working fine until I ended up shorting out my thermistor.
I replaced my thermistor and the reading was about 10 degrees lower than ambient so I just rolled with it for a while and experimented with temps until I found 150° was about right.

Now after another firmware change it reads ambient at -15... I did the same thing I did before and took an infrared temp gun and just rolled with it, well now I'm about to start experimenting with different kinds of filament and would love for my temperature reading to be accurate so I'm not going in blind.

Does anyone know what in the hell could be going on? Or have anything for me to look at, or something I should upload like firmware?
09-28-2016, 07:52 AM
printbus

How did you "short out" the thermistor? A short only between the two thermistor leads wouldn't have caused any damage. Remove the short and everything should have been fine. If, however, a thermistor wire was shorted to some other exposed wire, the input on the electronics board that the thermistor connects to may have been damaged. For example, people occasionally report frying their thermistor input when a heater failure inadvertently applies 12V to a thermistor wire that wasn't insulated properly.

On your new thermistor, were you aware that not all thermistor characteristics are the same? It could be that your new thermistor has different resistance-vs-temperature characteristics than the one you replaced. I can't speak to Repetier firmware, but I believe all printer firmware variations have a means to specify the type of thermistor you are using. You'd have to figure out how Repetier handles this.

It's generally a bad idea to try measuring hot end temperatures with an IR thermometer, and to an extent, any other thermometer/thermocouple scheme. IR thermometers don't do well reading the small heated area and the shiny aluminum block typical to most hot ends. Another problem is that the thermistor is (usually) measuring a temperature within the heated block. Measuring the external temperature of the block will naturally read lower due to surface cooling. Between these two issues, people typically report IR readings that are 30-40 degrees C lower than what the printer firmware is seeing.

What kind of extruder do you have on your i3? If it is something like a Greg's Wade with reduction gears that you can rotate by hand, it's often suggested that you determine the optimal temperature setting for your filament by getting a feeling for how easy it is to manually extrude by rotating the extruder gearing with the hot end set to different temperatures.
09-28-2016, 03:11 PM
iineviitable

Quote:

Originally Posted by printbus

How did you "short out" the thermistor? A short only between the two thermistor leads wouldn't have caused any damage. Remove the short and everything should have been fine. If, however, a thermistor wire was shorted to some other exposed wire, the input on the electronics board that the thermistor connects to may have been damaged. For example, people occasionally report frying their thermistor input when a heater failure inadvertently applies 12V to a thermistor wire that wasn't insulated properly.

On your new thermistor, were you aware that not all thermistor characteristics are the same? It could be that your new thermistor has different resistance-vs-temperature characteristics than the one you replaced. I can't speak to Repetier firmware, but I believe all printer firmware variations have a means to specify the type of thermistor you are using. You'd have to figure out how Repetier handles this.

It's generally a bad idea to try measuring hot end temperatures with an IR thermometer, and to an extent, any other thermometer/thermocouple scheme. IR thermometers don't do well reading the small heated area and the shiny aluminum block typical to most hot ends. Another problem is that the thermistor is (usually) measuring a temperature within the heated block. Measuring the external temperature of the block will naturally read lower due to surface cooling. Between these two issues, people typically report IR readings that are 30-40 degrees C lower than what the printer firmware is seeing.

What kind of extruder do you have on your i3? If it is something like a Greg's Wade with reduction gears that you can rotate by hand, it's often suggested that you determine the optimal temperature setting for your filament by getting a feeling for how easy it is to manually extrude by rotating the extruder gearing with the hot end set to different temperatures.

I thought about all that and it seems as though the input is damaged, i swapped the connector for the bed's thermistor and it reads at -12 when i put it in the hot end's thermistor. I brought the outside temp of the hotend to about 120 then slowly increased by 10 until it extruded by hand which was at like 140. So i ordered a ramps 1.4 as a temporary solution until i completely rebuild the printer in the future.