Hotend Temperature Readings Changed. Cause?

Hello All.

I've had a wonderfully functional Reprap i2. I replaced my Wade's extruder, which naturally required me to remove the hotend and move it into a different extruder base. Ever since, I need to set the hotend temperature higher to get the extruder to work. In other words, I used to extrude Octave green at 230, but now I have to set it to 235 to get the same results. Does anyone have any idea what may have happened?

One thing I suspect i that the teflon sheath around the thermistor lead may have pulled back a little, and the lead is touching the brass hotend nozzle piece. Could that cause the behavior I'm seeing? Should I look into something else?

Thanks in Advance.

Yes it could.

The thermistor only reads a temp equilibrium. it is an average of the hot nozzle and the outside surrounding temp. outside temp influence should be minimized around thermister.

Another note brass is not the most thermally conductive material, but easy to machine. Even moving the thermistor 2mm up would cause the temp equilibrium to change by a few degrees.

The hot end is hottest near the heating element of the heated block. Temp difference increases the farther away you go from the heat source, because outside of the heater air is cooling the hot end down.

For more precision in measurement and in design, use a heater block that is as close as possible to the nozzle, insolated as to reduce temp variation, and have thermistor as close as possible to the nozzle. Insolate everything that is supposed to be hot, but be careful to allow enough exposed area so that the cold region stays cool enough to avoid glass transition temps.

Thanks, james.

I forgot to mention that I'm working with a J-Head IV, which includes a small drilled-out section where the thermistor is inserted. I therefore assume I'm getting the same temperature profile at the thermistor as I did before, and always.

I'll be disassembling and reassembling the hotend tonight, and I'll let you know if I manage to fix my issue, and how I did it.

My speculation appears to have been right. The wire going into the thermistor was touching the brass piece of the j-head, changing the resistance profile, and therefore my temperature readings. Making sure that the teflon sheath was all the way down appears to have fixed it. I'm back in action.

What a pain this one was. On the bright side, all the time I put into making my hobbed bolt grip more tightly (thinking that filament slippage was occurring because of it) may now pay off handsomely.