The head of the agency or an employee designated by him or her shall have full responsibility as custodian for the petty cash account and its proper use under this chapter and applicable regulations of the director of financial management. The custodian of the petty cash account shall be covered by a surety bond in the full amount of the account at all times and all advances to it, conditioned upon the proper accounting for and legal expenditure of all such funds, in addition to other conditions required by law.