Bed leveling (inductive) probe inaccurate when heating bed

Hello,

I have built a prusa i3 mk2 clone from scratch, following Thomas Sanladerer series. and am experiencing trouble with the bed leveling (inductive) probe. It seems to be highly sensitive to heat, since running a repeatability test on the probe with the nozzle and the bed being hot results in a standard deviation much higher than the test with those parts being at room temperature (0.09mm vs 0.01mm). The width of the hysteresis is also much larger when heated (i.e. the probe "untriggers" way higher above the triggering height when the heating elements are hot). Thomas Sanladerer tackled this issue in one of his videos, stating that this is a common issue. However, the repeatability in our case is much worse than in the video. This results in a very different home position each time I run a G28 command when the heating elements are hot, and the printer is therefore unable to reach a correct height when printing, and just hovers a few millimeters above the bed.
Looking this up on google, I couldn't find anyone with a similar problem, and Tom didn't seem to be bothered at all, even though he bought the cheapest sensor available on the market. Therefore I am wondering if there may be other factors causing this fail, or if I just own a defective sensor and should buy another.
The inductive sensor is this one, and the heatbed is an AptoFun mk3, which is placed bare aluminium side up to have a flatter print surface.

Any thoughts on this?
Thanks in advance.

Edited 1 time(s). Last edit at 07/24/2019 08:51AM by bathal.

This is a common issue

Its is very dependent on the probe you use...

Prusa solved this by putting a thermister in the probe and compensating for the temperature see "Pinda v2"
Not sure marlin has the firmware for this compensated probe yet though.

Otherwise you just preheat the bed with the probe at a specific height, then probe once everything has settled.

Thanks for your answer,

unfortunately, I can't do that, since the repeatability is very bad when it's hot, It will never give me the same height reading twice... Homing before heating up and not homing after avoids relying on the probe when it is inaccurate, however I have to wait for everything to cool down between attempts to print anything

Edited 1 time(s). Last edit at 07/24/2019 08:45AM by bathal.