function copy_volume,old_volume,new_field_type=new_field_type

    new_volume = old_volume
    new_volume.h.field_type = new_field_type

    for i=0,new_volume.h.nsweeps-1 do begin
        new_volume.sweep(i).h.field_type = new_field_type
    endfor

    return,new_volume
    end