UL is the uncertainty in the path length;UΔT is the uncertainty in the transit time difference (due to averaging this is sometimes assumed to be negligible);UT is the uncertainty in the transit time (often negligible);Ucosθ is the uncertainty in the path angle allowing for skew or oblique flow (see Table 1; for example for an angle of 60° the uncertainty is ±3 % for ±1° of skew flow).