We have a similar problem at our trust, the cylinders are shared between our PICU and NICU.

If you speak with InoTherapeutics they will forward you a record sheet to complete for each patient, details included are cylinder numbers and commencement and finish times of treatment.

When we receive an invoice I locate this data and correlate a table of the cylinders used, total times and dispute any over-charges with the company, I hope this helps.