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A drug delivery device includes a pressurized reservoir in communication with a flow path to an outlet. The flow path includes two normally-closed valves and a flow restriction. A pressure measurement arrangement measures a differential fluid pressure between two points along the flow path which spa

A drug delivery device includes a pressurized reservoir in communication with a flow path to an outlet. The flow path includes two normally-closed valves and a flow restriction. A pressure measurement arrangement measures a differential fluid pressure between two points along the flow path which span at least part of the flow restriction, one of the points being between the valves. A controller selectively opens the valves to deliver a defined quantity of the liquid medicament to the outlet.

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What is claimed is: 1. In a drug delivery device having a pressurized source of a liquid medicament delivering the liquid drug to an outlet via a flow path, the flow path including a flow restriction and first and second valves, a method for identifying the presence of a gas bubble in the flow path

What is claimed is: 1. In a drug delivery device having a pressurized source of a liquid medicament delivering the liquid drug to an outlet via a flow path, the flow path including a flow restriction and first and second valves, a method for identifying the presence of a gas bubble in the flow path, the method comprising: (a) operating the first and second valves in such a manner as to ensure a pressure between the first and second valves significantly below a pressure of the pressurized source; (b) during a pressure accumulation period, maintaining the first valve open and the second valve closed; and (c) at the end of the pressure accumulation period, identifying the presence or absence of a gas bubble in the flow path based at least in part on a measurement of pressure between the first and second valves. 2. The method of claim 1, wherein measurement of pressure between the first and second valves is performed by differential pressure measurement between the pressurized source and fluid in the flow path between the valves. 3. The method of claim 1, wherein the first valve is closed at the end of the pressure accumulation period. 4. In a drug delivery device having a pressurized source of a liquid drug delivering the liquid drug to an outlet via a flow path, the flow path including a flow restriction, a method for identifying the presence of a gas bubble in the flow path, the method comprising: (a) monitoring at least one parameter affected by a pressure drop across the flow restriction; and (b) when the at least one parameter satisfies a bubble-detection condition indicative of a reduced pressure drop across the flow restriction, indicating the presence of a gas bubble in the flow path. 5. The method of claim 4, wherein said at least one parameter is a pressure measurement affected at least by a fluid pressure downstream relative to at least part of the flow restriction. 6. The method of claim 5, wherein the pressure measurement is a differential pressure measurement indicative of a pressure differential between the pressurized source and a part of the flow path downstream relative to at least part of the flow restriction. 7. The method of claim 5, wherein the bubble-detection condition is a value of the pressure measurement indicative of a fluid pressure above a threshold value downstream relative to at least part of the flow restriction. 8. The method of claim 7, wherein the threshold value is defined relative to the pressure of the pressurized source. 9. The method of claim 5, wherein bubble-detection condition is evaluated during flow of the liquid drug at a rate limited primarily by the flow restriction. 10. The method of claim 5, wherein bubble-detection condition is evaluated using a pressure measurement indicative of a rate of pressure increase after closure of a valve downstream of the flow restriction. 11. The method of claim 10, wherein said pressure measurement is taken after closure of a valve upstream of a pressure measurement location. 12. The method of claim 4, wherein the at least one parameter is indicative of a rate of pressure increase in fluid pressure after closure of a valve downstream of the flow restriction.