What should the external temperature of E3D hot end be?

Hi all -
I have a RUMBA controller (Marlin firmware) on a Mendel90 printer, with an upgraded E3D V6 hotend. I was having some jamming-like problems printing with ABS, but my hot end thermistor (via Repetier) is reporting adequate temperatures for ABS (215-220 deg C). When I point an IR thermometer at the hot end, I get a temperature that is much lower - not 220, but more like 130 deg C. This seems low, but I figured most heat is inside the hot end where the thermistor is. Anyone know a good rule of thumb to use in determining the true internal temp from the external (IR) temperature? O also considered the firmware setting for the thermistor, which may be a factor.
Thanks
Rob

Hello Rob,

Did you set the marlin firmware for the correct thermistor with the correct beta value?

It's difficult to get accurate temperature measurements of small objects with an IR thermometer. If you're uncertain of the accuracy of your thermistor, why not take it out of the hot end and use it to measure a couple of knowns, like melting ice and boiling water. If it can report those accurately then you can assume it's giving you a good value for the interior of the hot end. Or you can get a K-type digital thermometer for about $10 that will give you accurate point-contact measurements.

Kafledelius -
Yes, my first working theory is that the firmware is wrong. This thermistor came with the hotend as part of a kit I purchased on EBay, and is the following:

Item:NTC Thermistor
Model:B2-100-3950-1
R @ 25: K 100K-1%
B25/50:K 3950-1%
Element:Thermistor,type MJB2

I found this discussion on the forums for someone who had the same problem:
[forums.reprap.org]

This has some good suggestions which I will try.
Rob

I suspect that surface temperature of the hot-end won't give a very accurate reading of the temperature inside.

I've done a couple of experiments, and found that my (cheap Chinese) multimeter reads about 40 degrees lower than what Marlin reports when the sensor is held against the outside of the hotend. The temperature reading varies 10-20 degrees depending on exactly where it is touching the hotend, how hard it is pressed, etc. I also tried removing the nozzle and inserting the multimeter temperature sensor in the hole. In this case, it was reading about 20 degrees lower than what Marlin was reporting.

I haven't done any calibration checks of my multimeter. I believe I have set the Marlin firmware to the right setting (5) for my thermistor, I'm not sure how to tell what is the right value... the thermistor came with my (cheap Chinese) E3D V6 hotend.

Have you tried holding various types of filament against the outside of the hotend? That will at least give you ballpark figures of whether it is hot enough.

You can feed a thermocouple probe into the hot end (where the filament normally goes) to get a more accurate temperature measurement.

---

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].

Quote
rtrahms
When I point an IR thermometer at the hot end, I get a temperature that is much lower - not 220, but more like 130 deg C.

IR thermometers accuracy varies greatly based on the emissivity coefficient of what is being measured. Every substance ranges from 0 to 1 with 1 being a perfect radiator, or a "blackbody". Most heat blocks are made of aluminum and have a emissivity coefficient a small fraction of that of a blackbody. Unless your IR thermometer was specifically designed to measure something with very low emissivity, it will be useless in getting an even remotely accurate reading.

I second that. My E3D's heater block shows up as about 120C with an IR thermometer, even through it extrudes perfectly.