Casting a string containing a negative value (e.g. '-24') to float fails with

com.osisoft.rdsa.RdsaException: [PIOLEDBENT] Cannot convert -24 from String to Single.

This seems to be a bug in Osisoft PI SQL DAS (server-side not the JDBC driver), as connecting to the PI server using Osisoft's SQL Commander and running

select cast(N'-24' as single)

results in the same error.

I think the best course of action would be to document this behaviour of the PI server as known issue and NOT work around it.