A true zero volume fitting is one in which no part of the fitting actually becomes a part of the flow path. The only Valco fittings which fit this description are our through-bore unions, which allow tubing to butt end-to-end. Even these are only zero volume if the tube ends are perfectly square.
All other fittings are designed with zero dead volume: that is, there is no volume introduced by the fitting which is not cleanly swept.
