[jyxent]

They measure a series of points to get a bed mesh, the same as other leveling sensors do. You shouldn't need to live-adjust z, because you've measured the bed surface with the nozzle tip.

With a BLTouch, you need to adjust the z-offset, because the nozzle tip and the BLTouch trigger height are not the same. Once that is calibrated, it should continue to work fine as long as the sensor is working properly.

I think it mainly comes down to the accuracy of the sensor. If your sensor gives you a +-0.03mm reading when probing the same spot, then you will have high and low spots in your bed mesh that you can't correct with z-offset.

This could explain your issues with the BLTouch you mentioned in another thread here. I have various brands of knockoff BLTouch sensors, and the worst one had a +-0.02mm range when measuring points, which made it impossible to get a good bed mesh. The one I replaced it with had about a +-0.005mm accuracy, which fixed my issues with the low and high spots.

[stavros]

I've measured the repeatability on mine, and it was pretty good, though I don't remember the number right now.

The pressure sensor on the hottend is superior to any other method, because it has much better resolution (though maybe not better than the Beacon), and it can adjust to print conditions in real time, which no other start-of-print sensor can do.

[jyxent]

I'm not aware of any hotend sensors doing as you describe. The Prusa MK4 only uses it to generate a bed mesh the same as any other bed-leveling sensors. It does not do any live calibration while printing.

[stavros]

Really? That's very disappointing, if that's the case.

EDIT: I'm not sure, this article says it can detect spaghetti just from the pressure difference, for example:

https://all3dp.com/1/nextruder-prusa-xl-deep-dive/

It also hints at detection of the height while printing.

[jyxent]

Without watching the video referenced in the article, I'm guessing this was based on what they theoretically could do, rather than something that they had actually implemented.

They currently only use the load cells for bed leveling: https://www.youtube.com/watch?v=F7ETSVu04ao&t=940s

I could be wrong, but I would expect that the load cell readings would be far too noisy in order to determine the height of the nozzle while printing. Even if they were accurate, different materials would also behave differently due to different viscosities.

[TaylorAlexander]

To be fair they really shoved the Mk4 out the door (read up on the release schedule for Input Shaping if curious) so there is probably room for development in the firmware. Perhaps they will have live pressure response in a year or two if such a functionality is deemed beneficial.