Abstract: The present invention is directed to a reusable respiratory measurement device having a discardable flow tube in which a counter is included in a detachable (discardable) memory unit that removably engages the respiratory measurement device body. The memory unit includes a counter that tracks usage of the respiratory measurement device and disables the respiratory measurement device when usage has reached certain predetermined thresholds. The memory unit is then replaced with a new memory unit which re-enables the respiratory measurement device for further measurements. The memory unit is typically sold as part of a batch or lot of disposable flow tubes in which case the counter value is commonly the same as the number of flow tubes in the batch or lot.