Confusion about Semitec 104GT-2 B value in documentation



  • I'm in the process of configuring a printer that uses the E3D thermistor cartridges, which are said to be 104GT-2 (though, that thermistor is apparently discontinued, that's not relevant here.)

    In this old thread, somebody raised some confusion about different sources of documentation. In this thread, it is clarified that B4725 should be used in combination with C7.06e-8, instead of the previously recommended B4388 when no C value is used.

    However, here, you can find the following quote:

    For greater accuracy, use a C parameter as well (note, the correct B value to use will be lower with a C parameter than without one).

    4725 is clearly higher than 4388, but it is stated that with a C parameter the B value will be LOWER, not higher.

    The datasheet for the Semitec 104GT-2 lists a B value of 4267 -- confusingly similar to 4725. and actually lower than 4388.

    I'm left more confused about which values to use than I was when I started double-checking the values provided by different sources.

    Any input?



  • dont confuse the beta value with the Steinhart Hart equation

    https://en.wikipedia.org/wiki/Steinhart–Hart_equation

    The problem with beta values is that they are only valid for the specified temperature range. ie. 25-50.
    Once you go beyond that the error can become significant. There the Steinhart Hart equation will provide better results.

    You can not compare Beta with Steinhart Hart Constants.



  • @veti

    Thanks for the reply but I do not think I'm confusing those... there is a direct contradiction in the documentation as I quoted -- that is the confusion.



  • that statement is not correct



  • @bot said in Confusion about Semitec 104GT-2 B value in documentation:

    @veti

    Thanks for the reply but I do not think I'm confusing those... there is a direct contradiction in the documentation as I quoted -- that is the confusion.

    I agree. There is a contradictory statement. A quick gander at E3D's instructions https://wiki.e3d-online.com/E3D-v6_Assembly#Configure_Firmware_.28Easy.21.29 shows that they also recommend using the (higher) B4725 value in combination with the C7.06e-8. So my best guess is that is what we should use, despite the apparent contradictory words which state that the B value to use will be lower when a C values if also used.



  • i calculated the b and c value for a b3950 thermistor table and the b value went up as well.



  • @veti said in Confusion about Semitec 104GT-2 B value in documentation:

    i calculated the b and c value for a b3950 thermistor table and the b value went up as well.

    Can you confirm that when you include a C value, the B value increases (which is contradictory to the statement in the Wiki)? I think we can all understand that when the B value is quoted for 25 deg C, then a higher value will be needed for around 200 deg C. The confusion arises from the inclusion of a C value as well as a B value, where the Wiki indicates in words that the B value should be lower, yet the example given shows a higher value is used.



  • using this table

    https://www.makeralot.com/download/Reprap-Hotend-Thermistor-NTC-3950-100K.pdf

    using 25 and 50 gives = Beta 3948 K
    using 25 180 and 220 gives = R25: 100000 Ω β: 5071 K C : 1.793806e-7



  • @veti You haven't directly answered the question but from the numbers you've quoted, I'll take it that the words in the Wiki are incorrect and that when using a C value, the B value increases, rather than decreases as stated.


  • administrators

    @bot said in Confusion about Semitec 104GT-2 B value in documentation:

    However, here, you can find the following quote:

    For greater accuracy, use a C parameter as well (note, the correct B value to use will be lower with a C parameter than without one).

    4725 is clearly higher than 4388, but it is stated that with a C parameter the B value will be LOWER, not higher.

    The note was wrong so I have corrected it. Thanks for pointing this out.



  • Excellent thank you everyone for the help.


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