In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: This flow was measured by using a differential pressure transducer , passed through a polygraph , and from this to a computer with custom software that derived ventilation ( VE ) , tidal volume ( VT ) , inspiratory time ( TI ) , expiratory time ( TE ) , breathing frequency ( f ) , and mean inspiratory flow ( VT / TI ) on a breath - by - breath basis .
### Output Text:
This flow was measured by using a differential pressure transducer , passed through a polygraph , and from this to a computer with custom software that derived ventilation ( VE ) , tidal volume ( VT ) , inspiratory time ( TI ) , expiratory time ( TE ) , breathing frequency ( f ) , and mean inspiratory flow ( VT / TI ) on a breath - by - breath basis . 