Abstract: Several improvements are disclosed for a drug delivery pack with dry reagent in a housing, each of which can be employed independently or in conjunction with others of the improvements. A novel spring is provided with two spring columns extending in parallel between a spring top and a spring bottom. The spring is particularly designed for fitting within the housing and compacting a dry reagent bed. A plunger mechanism to ratchet housing portions between two positions. In one position, a compression component adjacent the reagent bed is relatively uncompressed, such that it does not “set” over time. When the user is about to use the drug delivery device, the plunger is slid to compress the compression component so that it exerts pressure on the reagent bed. A similar sliding mechanism can also divert a stream of diluent through the housing from a priming path, where diluent bypasses the reagent bed, to a functional path passing through and eroding the reagent bed.