It is possible to use an identified 'class' or 'group' of chattels as a security, even though individual items within this class are continuously changing. The value of the class of assets is the amount of money they are likely to be worth at any given time, notwithstanding fluctuations.
The appropriate arrangement is that the class of assets is 'charged' with the debt. This leaves the debtor as owner and in possession, free to use and replace the assets. Only when the debt in question becomes overdue does the charge become 'perfected' so that it can be enforced by selling those assets on behalf of the creditor.
Historically, such agreements were referred to as 'floating' charges, because the charge was thought of as floating above the identified class of assets until, the debt becoming overdue, the charge became perfected (or 'crystallised' or 'fixed' in the older terminology).