BLtouch and electrtical/thermal interference

[jens55]

How can one determine if one is seeing interference to the height measurements the BLTouch is reporting?
I would like to scan my bed at full operating temperature but am wondering if the results I am seeing are affected by electrical or thermal interference.
Would the readings become very erratic or would they tend to wander in one direction or another?
My 500x500 bed (Creality CR10-S5) shows a substantial change (0.2mm) in some areas when it is hot vs when measured cold and I wonder if it is best to use the cold compensation values or the hot compensation values.
Measured cold my readings swing between + 0.45 to -0.21 but when hot the readings range from +0.67 to -0.07 so everything has shifted by about 0.2 mm. The map is smooth without any erratic single point ups and downs which would indicate that I don't see electrical interference from heaters switching on/off but I could see temperature related effects which would presumably be much smoother.

[dc42]

@jens55 you can use the B1 parameter of the M558 command to turn off heaters temporarily during probing.

[jens55]

@dc42, I have generated maps with and without heating (B0 and B1) but I would like to interpret what I am seeing. How much of the difference between a hot map and a map without heating is actual change in dimension and how much of what I am seeing is caused by interference from either electrical interference or temperature interference to the BLTouch.

[droftarts]

@jens55 you can measure the bed manually at the nozzle by changing the probe type to 0. Then at each probe point you will be asked to jog the hot end down manually, and use a piece of paper to check when it makes contact. Laborious, but accurate. Then you can compare that mesh to any mesh generated by the BLTouch.

Ian

[jens55]

@droftarts, thanks, that will be a good way to double check and since I would only take the measurements at a handful of points it shouldn't be too difficult. (the full mesh is 441 points .. LOL)

[dc42]

@jens55 said in BLtouch and electrtical/thermal interference:

@dc42, I have generated maps with and without heating (B0 and B1) but I would like to interpret what I am seeing. How much of the difference between a hot map and a map without heating is actual change in dimension and how much of what I am seeing is caused by interference from either electrical interference or temperature interference to the BLTouch.

B1 only turns the heating off briefly during probing, not long enough for the bed to cool down significantly. So you should be able to compare height maps taken at the same bed temperature with and without using B1 directly.

[jens55]

@dc42, that was my understanding as well but in real life, the net result was that the bed temperature gradually dropped during the roughly 2 hours that a full probe (441 points) of the surface took. Since the temperature kept dropping (slowly) I eventually decided to take the one set of measurements with a cold bed without attempting to heat the bed. I did not attempt to try a smaller sample size and then compare just the data points of the smaller set with each other. I will give a smaller sample set before I try the manual method that @droftarts suggested.
Thanks!
BTW, this is a very large bed slinger and I probe very slowly (and do up to 10 repeats) which is why it takes 2 hrs to do the full scan.