Pt100 Offset adjustment with M308 command
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Hi,
I would have a question related to the offset value of my Pt100 sensors, attached on Duet 3 MB6HC 3.5.0-rc.3 firmware. I have +1C deg offset value in my sensor from which I would like to get rid off. Also, I have some small noise, which increases at high temperatures to approx. 1C variance between the target.
With the gcode instructions it is said that I should add Uu, Vv. I assume that I should place U-1 (for offset removal), but this didn't make any change to my temperature reading. Also, does this V (slope adjustment parameter) affect the noise?
I did not find an example on how to write the M308 command with those Uu and Vv, so it would be great to have instruction on that.
Br, Heidi
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Uu, Vv (RRF 3.5 and later, optional, default zero) Temperature reading offset and slope adjustment parameters. The temperature in °C read from the sensor is adjusted as follows: adjustedReading = (rawReading * (1.0 + v)) + u
Here is the formula, I've asked DC42 for an example on it's usage.
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Say you have a temperature sensor part way between a heater and the nozzle and you have verified that it reads 250C, when the nozzle temperature internally is only 240C. You may want to report this temperature in the UI or otherwise as 240C, not 250C. Let's also say that your room temperature is 20C, and you wantgthe difference between measured temp and reported temp to be 0C at room temperature.
We now have the values needed to calculate the U and V coefficients to this command.
It's of the form "y=mx+c" where
Using the values above:
Zero Difference Temp = 20C
Sensor Temp= 250C
Display Temp = 240CV = (Display Temp-SensorTemp)/(Sensor Temp-Room Temp) = (240-250)/(250-20)=-0.0435 U = -(RoomTemp*V) =-(20*-0.0435) = 0.8696
This can be seen in this graph:
If you want a positive offset then your display temperature will be higher than your sensor temperature (for example if your temperature sensor was insulated from the nozzle&heater).
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@HeidiH this facility was added after we released rc3 so you will need to use one of the later firmware builds.
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@dc42 @T3P3Tony @Phaedrux Thank you all for your help.
I had tried to modify the formula that was given for the M308, which Phaedrux instructed but I didn't understand it sufficiently to get it working. However, based on instructions on T3P3Tony I now got the point better and am now able to make the required modifications.
I would like to double check, that dc42 meant that my 3.5.0-rc.3 firmware version is not the latest release? I checked the version from https://github.com/Duet3D/RepRapFirmware/releases and it seems that this should be the latest?
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Please try the version linked here, or wait for rc4 which we will release today hopefully!
https://forum.duet3d.com/topic/35366/latest-3-5-post-rc3-firmware-builds/2
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@HeidiH we hope to release rc4 later today or tomorrow. Meanwhile there are unofficial builds at https://www.dropbox.com/scl/fo/6afk7iewlw1ycldb8dc4u/h?rlkey=21rm8jcz6aqy3gh523wkmxro7&dl=0.