Hot End Thermistor reads ~7 to 4 degrees cooler than bed (and also room).  

RE: Hot End Thermistor reads ~7 to 4 degrees cooler than bed (and also room).

Posted by: @john-h64

Thank you for viewing my post.

Recently, I unplugged my Prusa MK3 printer and plugged it into a different outlet, since I needed to use that outlet for something else. It was after this that I noticed my heat bed would report about 24 degrees celsius, but the hot end would report about 7 degrees cooler. I was still able to print, but I was... suspicious. It printed alright, maybe a bit stringier than I remembered.

I turned the printer on a few days later, and it was still reporting a bit low, roughly 18 degrees. The hot end's time to warm up seemed about what I expected, and I did another print, and it was relatively fine.

So, today, I'm looking at it, and it's about 4 degrees cooler than the heat bed, 22 degrees versus 26. I put a thermometer in the room, and that's measuring about 25-26 as well. (It is a dollar store thermometer).

So, I have a few options, it would seem:

1. "This is fine." (Image of dog and mug)
2. Adjust the temperature on prints to go down ~4 degrees
3. Take my limited mechanical skills, and attempt to replace the thermistor using the directions here: https://help.prusa3d.com/en/guide/how-to-replace-thermistor-in-the-extruder_22208 and these thermistors I bought a while ago, for my previous printer: https://smile.amazon.com/gp/product/B01HCKI7NW/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

As I believe it is relevant, I have built a printer before (JG Aurora A3), but my Prusa was shipped to me built. If that informs my mechanical skills at all. The instructions don't look that imposing, thankfully, but I am not terribly mechanically minded.

Also, I haven't updated the firmware in a while, since I have to pick up the printer and move it to the room where the computer is, in case that is relevant (but I doubt it).

I had had a few massive blob situations that I cleaned off, which might have caused the thermistor to be wacky, but these were months ago, and I had done several prints since then without noticing any issues. I can't really get a good look at the thermistor because the air flow foot thing is in the way. (Mine snapped while cleaning one of the aforementioned blobs, and my friend gave me an upgraded replacement instead of the standard one from when I got my printer).

If anyone could offer some advice or tips, I would appreciate it greatly.

Thank you very much.

For an optimal function you need the original Prusa thermistor type Semitec 104GT or comparable. If you measure resistance, the value should be 100kohms at 25 degrees C. room temperature.

Temperaturtabelle für Thermistor Semitec 104GT

RE: Hot End Thermistor reads ~7 to 4 degrees cooler than bed (and also room).

Thank you for the reply! Does this mean that you recommend I replace the thermistor, then?

Do you think it would be okay to keep using the printer while I wait for a replacement, if so?

Thank you very much!

RE: Hot End Thermistor reads ~7 to 4 degrees cooler than bed (and also room).

I think the important question is "Is the temperature correct in the operating range?"

Thermistors are not linear so it may be that yours does not match the correction at the low end but be fine at operating temperatures.  I would try measuring the temperature after the hot end was warmed up before getting drastic and replacing the thermistor.

Jerry

RE: Hot End Thermistor reads ~7 to 4 degrees cooler than bed (and also room).

What can I use to corroborate the temperature read by the thermistor? Do I poke the hotend with an instant read digital thermometer?

I have noticed stringier prints lately, but this could be due to trying to print several pieces at once. Incorrectly raised temperatures could lead to stringier prints?

Hmm, it sounds like I should print a solo part and see if it looks better than the stringy multi-part prints.

Thank you all very much for replying and helping so far!

RE: Hot End Thermistor reads ~7 to 4 degrees cooler than bed (and also room).

You could try a temp tower.

A string test, or general test stl files.

The sensors are not as precise as we think, just adjust it a little, a temp tower you can see what is better, and every filament is different.

RE: Hot End Thermistor reads ~7 to 4 degrees cooler than bed (and also room).

Goodness! I'd seen some general test stl files, but I'd never seen a temp tower before!

I'll grab one of the ones from the prusaprinters.org site and give it a go.

If I can get good results, then I will hold off on attempting to replace the thermistor. I did go ahead and order a replacement from Prusa, though, just in case. I'm hoping I won't need it, but it can't hurt to be prepared, just in case.

Thank you for your suggestion!

RE: Hot End Thermistor reads ~7 to 4 degrees cooler than bed (and also room).

Replacement parts you need always, check if you need more parts, otherwise you will have to wait till you order them.

Also the fan duct you can better print a few, with a crash you need these.

RE: Hot End Thermistor reads ~7 to 4 degrees cooler than bed (and also room).

@john-h64

I use an infrared thermometer and aim it very carefully at the heater block.  I think the easy way is raise the Z axis to the top and aim from the bottom.

You could do the same thing with a mechanical thermometer I think. Just hold it on the heater block near the thermistor.

RE: Hot End Thermistor reads ~7 to 4 degrees cooler than bed (and also room).

I think my digital kitchen thermometer works up to about 200 degrees Celsius, so I might be able to see if 190 degrees is actually 190 degrees. An infrared thermometer sounds like a much, much safer idea, though....

I printed out a temperature tower today, and it looks like it has way fewer strings at the bottom, by far. The back of the tower, with the little overhang thing looks best on the 190 level as well. I guess I should try one of my over-stringified models at 190 degrees, maybe? See how that comes out? I had been printing at 210 successfully, without too much craziness, until the thermistor started reporting different than the bed when cold.

Everything looks like it gets really stringy as it gets hotter.

Thank you all very much for your help thus far!