Interpreting Diagnostic Values

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The diagnostic values output by the LI-7500A and LI-7200 CO2/H2O gas analyzers indicate whether or not your instruments are working normally, and what might have gone wrong in case of a malfunction. Here are the two steps for interpreting the diagnostic values:

  • Convert the diagnostic values from decimal to binary format
  • The LI-7500A and LI-7200 diagnostic values are labeled as “Diagnostic Value” in their outputs. You can use the method of division by two to manually convert them from decimal to binary format as shown below for converting decimal 254 to binary 11111110, where the remainder of the first division is the zero bit (Table 1‑1).
  • Table 1‑1. Division by two to convert from decimal to binary.
    Division Quotient Remainder
    254/2 127 0
    127/2 63 1
    63/2 31 1
    31/2 15 1
    15/2 7 1
    7/2 3 1
    3/2 1 1
    1/2 0 1
  • If you have the Microsoft Office suite, you can easily do the conversion using the Windows Calculator or the DEC2BIN () function in Excel. You can also use an online converter or other tools to do the conversion.
  • Interpret the binary diagnostic values
  • The binary diagnostic values of the LI-7500A have 8 bits, while those for the LI-7200 have 16 bits. Table 2 shows the assignments for each bits and what their values represent. The diagnostic values of the LI-7500A should be in the range of 0 to 255 and those for the LI-7200 from 0 to 8191. The maximum values occur when all the used bits are equal to 1 for the LI-7500A and LI-7200, respectively. You can interpret the diagnostic values by looking up each bit in Table 2.
  • Here are two examples showing how to interpret the LI-7500A and LI-7200 diagnostic values:

    1. A diagnostic value of 254 for an LI-7500A is equal to 11111110 in binary format as shown. From Table 2, the AGC is equal to 6.67 × (1×23 + 1×22 + 1×21 + 0×20) = 93. A high AGC value such as this indicates the LI-7500A is malfunctioning. The optical path is partially blocked. Clean the windows and check if there is anything wrong with the optical path.
    2. A diagnostic value of 8061 for an LI-7200 is equal to 0001111101111101 in binary format. The AGC is equal to 6.67 × (1×23 + 1×22 + 0×21 + 1×20) = 87. The zero at the seventh bit indicates the chopper temperature controller is out of range, hot or cold. Check that the sensor head cables to the LI-7550 are tight and the chopper temperature is set properly (5 vs 30 °C).