# Measuring H values

• Although the wiki includes instructions on computing the H corrections for autoprobing, I found a different approach made more sense to me. Is what I describe a reasonable approach? Should I replace what's on the wiki, add this as an alternative, or simply leave it as it is?

First of all, for each point in bed.g, the H value is the difference between sensor trigger height and actual height. So to probe each point, you should start with a G30 and then measure the height at which a paper is pinned. The prior calibration settings should be irrelevant since all we're doing is vertical moves, and the G30 sets a meaningful zero. So the procedure I suggest is:

• For the center point: G1 X0 Y0 Z10

• G30 (with no paper); this sets the Z=0 based on the G31 probe Z offset

• Put the paper under and use G30 Z_number_ to find the height at which the paper is pinned by a repeatable amount

• Record this value

• Now for each point in bed.g, use G1 X_value_ Y_value_ Z10 to go there, then repeat the previous three steps

• Now you have a list of Z heights, one for each point in bed.g (including X0 Y0). The appropriate H value is value at X0 Y0 - value; put these in your bed.g

Once you have these values in your bed.g, as long as your printer's offsets are repeatable and your bed is physically flat, your autocalibration should give nice low consistent residuals. If you want to adjust the vertical height of the nozzle when printing the first layer, do that with the G31 Z offset value in config.g.

Unfortunately I can't claim fantastic results with my printer; I'm not sure why.

• If I understand you correctly, what you are doing should produce the same results as the method on the wiki. The reason I prefer the method on the wiki is that I can probe each point several times to check that the trigger height is consistent, but I only have to lower the nozzle to grip the paper once per point. Also it gives me a direct readout of the trigger height at each point.

• If you want to re-probe with this method, just add G1 Z10 and G30 S-1 as many times as needed. I found the instructions a little confusing because they seemed to need consistent autoprobe results (which I still don't get, although individual probes do seem to be repeatable) and it also seems to make some assumptions about the paper thickness and its relation to the desired print height.

I agree that there's no point computing H values if the probe heights are not repeatable, but when I run a manual probing sequence (i.e. ending with G30 Pn S-1) I do get more or less repeatable results. Unfortunately even with the computed H values I still don't get good pre-fit deviations out of the autoprobe (and the post-fit deviations are not always great either).

• Unfortunately even with the computed H values I still don't get good pre-fit deviations out of the autoprobe (and the post-fit deviations are not always great either).

That's to be expected to some extent. If you need to use H values to correct for effector tilt, then you have a geometrical error that the model used by the printer (including the auto calibration code) does not account for.

If you can't track down the geometrical error that cause effector tilt. then in the very near future you will be able to use grid-based bed compensation to compensate for the residual height errors.

Looks like your connection to Duet3D was lost, please wait while we try to reconnect.