Postgis DB to osm pbf file

Hi I would like to output changes I have made to the data in my PostGIS database to an OSM PBF file so that I can convert it to a graph file (gph) using Osm2PO.

I am trying to use Osmosis for this but can't seem to get it working. Here are some commands I have tried:

osmosis --read-apidb host="localhost" database="gis" user="username" --write-xml file="filename"

but this fails with various exceptions with an error saying:

ERROR: relation "schema_migrations" does not exist

I also tried using --read-pgsql instead of apidb but it gives an error:

The following named pipes () and 1 default pipes have not been terminated with appropriate output sinks.

Anyone know the best way to do this?

Thanks