Abstract: During operation of a computer system, index records and data records of a data structure are organized in a hierarchical manner, with index records being at a higher level of the hierarchy than the data records referenced by the index records. While the data structure is manipulated, the data records are stored in a random access memory. Modifications made to the data structure are chronologically written to a disk storage in a bottom-first/top-last order of the hierarchy for persistent storage as a database. A back-up copy of the database is made by storing the data and index records of the database in a top-first/bottom-last order on a sequentially readable media. A temporal and spatial view of a portion of the database can be obtained by accessing the index records and the data records of the database or the back-up media in the top-first/bottom last order.

Abstract: A fluid flow meter comprises a pair of transducers spaced apart in the direction of fluid flow. A transmitter causes acoustic signals to be transmitted in both directions through the fluid by the transducers. A processor determines information relating to the fluid flow by monitoring the time of flight of acoustic signals received by the transducers. Part of the space between the transducers defines a flow path consisting of a flow structure having a plurality of parallel fluid flow passages which extend axially in the direction of flow, wherein the cross-sectional diameters of the passages are chosen such that substantially plane acoustic waves only are transmitted through the passages by the fluid, and wherein under working conditions the fluid flows through each passage such that the ratio of the volume flow rate through the passage to the total volume flow rate remains substantially constant with respect to the total volume flow rate.

Abstract: An analogue to digital converter comprising a first analogue to digital conversion stage (14) for performing a relatively coarse conversion on a sample of an analogue input signal; a sample and hold stage (13) for holding the sample of the analogue signal fed to the first conversion stage while it is converted by the first conversion stage; a digital to analogue conversion stage (15) to which the digital output from the first conversion stage is fed; a difference circuit (16) for generating an analogue difference signal representing the difference between the signal held by the sample and hold stage (13) and the output from the digital to analogue conversion stage (15); a second analogue to digital conversion stage (17) for generating a digital output corresponding to the difference signal; and a combining unit (18) for combining the digital outputs of the first and second conversion stages to generate a digital output representing the sampled analogue input signal.