I was trying to use PVA on a duql head K8400, but were always struggling with extrusion problems of th PVA. Additionally a sparling was noticeable when the extruder was in “standby”.
I realised that this effect was related to a too hot nozzle, as it was reported already earlier (viewtopic.php?f=62&t=15032&p=76296#p76296)
For my understanding the poor thermal connection to the thermistor is the major problem of the deviation. Furthermore I was annoyed from the damage of the isolator at the bottom of the heat block which will be introduced by subsequent cleaning steps due to smeared material.
I replaced the NTC by a PT1000 in ceramic housing, which is assembled from the side of the heat block.
In this configuration I could measure a temperature drop in the nozzle tip of 10°C with respect to the nominal temperature (210°C -> 200°C nozzle temp). With fans working this drop becomes 2 - 3°C larger.
A drawback of this configuration is a noticeable noise in the temperature reading. But an uncertainty of ±1 °C is more acceptable than an unpredictable error of 50°C.
This may be an idea for an improved redesign.