Conductance, in μS (micro Siemens) per square metre, is an absolute value of the ability of the coating to pass current. Siemens is the reciprocal of resistance and is used in preference to ohms (resistance) as specified in NACE TM0102-02. Although C-SCAN will display μS/m², μS/m or Ω/m² if required.
C-SCAN conforms to the “AC Current attenuation survey” as specified in NACE RP0502.
The calculation relies on entering the following data into C-SCAN (or DCAPP if not available at survey time):
- Ground resistivity
- Coating type (FBE, coal-tar etc) or the electrical characteristics of coating
- Pipeline diameter
- Pipeline wall thickness
Without entering the above data only a (very) crude estimation of conductance can be given. NACE has laid down the limiting values of conductance, and are as follows:
Less than 100 μS/m2 Excellent coating
100uS to 500 μS/m2 Good to fair coating
500uS to 2000 μS/ m2 Fair to bad coating
Above 2000 μS/ m2 Bad to no coating
The graph below shows the relationship to Attenuation for Bitumen coating: