def rho_to_units(rho, ref_density=1250):
    """Convert dimensionless growth rate to real units for a growth process.
    
    Returns
    -------
    density  :  number or numpy array (dtype float)
        Time in units (hours, days, etc.)
    
    Parameters
    ----------
    
    rho  :  number or numpy array
        Cell density in dimensionless units.
    
    ref_density  :  number or numpy array
        The cell density at a dimensionless density of `rho = 1`. 
        Defaults to 1250 cells / mm^2.
    """
    return rho * ref_density