http://www.bamr.co.za/portalevel.shtml is still the best bet.
The old BS 5306-4 for CO2 requires:
a) Every 3 months Test and service all electrical detection and alarm systems.
b) Every 6 months Perform the following checks and inspections.
1) Externally examine pipework to determine its condition. Replace or pressure test and repair as
necessary pipework showing corrosion or mechanical damage.
2) Check all control valves for correct manual function and automatic valves additionally for correct
automatic function.
3) Externally examine containers for signs of damage or unauthorized modification, and for damage to
system hoses.
4) Check contents of pilot containers. Liquefied gas type should be within 10 % and non-liquefied
within 5 % of correct charge. Replace or refill any showing a greater loss.
5) Carbon dioxide containers, check weigh or use a liquid level indicator to verify correct content of
containers. Replace or refill any showing a loss of more than 10 %.
c) Every 12 months Carry out a check of enclosure integrity using the method given in Annex A. If the
measured aggregate area of leakage has increased from that measured during installation which would
adversely affect system performance, carry out work to reduce the leakage.
d) As required by BS 5430, but otherwise when convenient, remove the containers and pressure test
Lets you weigh them or Portalevel them
when necessary.