Flux density of 3C 196, as measured after initial calibration. As the data was calibrated against a (Scaife & Heald 2012) flux scale model of 3C 196, the deviation from the model should be minimal. Deviations from the flux scale indicate instability in the calibration, which means that the corresponding data should be discarded. This figure shows that data below 15 MHz is too unreliable to be used.