These note documents consistent discrepancies of about 2 percent between values of skin friction coefficients in incompressible turbulent flows deduced from Preston tube measurements, using the generallyaccepted calibration of Patel (1965), and values deduced by fitting velocity profiles to the logarithmic law, using the generallyaccepted constants of Coles (1968). It is shown that the discrepancy cannot be explained by any of the admitted uncertainties in either procedure, and since Patel's direct calibration has received numerous confirmations while Coleus’s constants were chosen indirectly it is suggested that Coleus’s law of the wall should be modified to force agreement with Patel's calibration. This could be done by increasing Coleus’s additive constant from 5.0 to 5.2.
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