True dust mass of the model (blue), the optically thin dust mass at an observing wavelength of 1.3 mm (yellow dotted) and 9 mm (green dashed), as a function of time. The observations assume a disc temperature of 20 K during the formation of the disc (<0.18 Myr, grey vertical line) and 30 K at later times. The pink dashed-dotted line shows the 9 mm, optically thin dust mass, where the opacity used to calculate this in Eq. (1) has been set to the opacity that the Birnstiel et al. (2018) model produces at a wavelength of 9 mm and at a maximum particle size of 9 mm, i.e. the same opacity that is used to calculate the disc continuum emission. This is to highlight how the resulting optically thin dust mass can be affected by a difference in between actual disc opacity, and the scaling law (Eq. (2)) that is often used to determine the opacity in observational surveys of protoplanetary discs.