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The design and construction of a prototype automated low level gas mixing system is described. The basic design of the instrument is based on a commercial, manually operated gas blender which was produced by CR labs. The apparatus uses a simple principle based on volume proportioning, where a certain volume of pure gas is mixed with a known volume of air. This instrument is used for mixing a specific gas such as carbon monoxide with clean air to produce an accurate mixture in the parts per million range. A gas simple of selected concentration between 1 to 1000 ppm can be automatically prepared and subsequently used for gas sensor characterisation. Development of the instrument can be divided into four main areas. Firstly, the design of a suitable mechanical system. The mechanical system itself comprises three main parts, namely a motor driven mixing cylinder, a motor driven syringe as a gas injector unit and a stainless tube network. All these components are then assembled to form a complete mechanical system. Secondly, the design and development of an automatic controller based on a Z-80 microprocessor is undertaken and then interfaced with the mechanical system. Thirdly, experiments are carried out to determine the optimum steps of the process for preparing the gas mixture accurately within an acceptable processing time. Finally, control software in Z-80 assembly language is developed in order to dictate the instruments operation through the predetermined steps of the blending process. The instrument is then calibrated for both low and high concentration ranges to ensure that highly accurate output gas mixtures can be produced over a wide concentration range. The calibration process involves several experiments in order to determine the <I>'calibration parameter</I>'. Data obtained from these experiments were plotted to show the relationship between the number of motor steps and the concentration of the output gas mixture produced. The gradient of the graph (in motor steps/ppm) and an offset at the y axis (in number of motor steps) are considered as instrument calibration parameters. Both parameters are entered into the respective input of the control software, so that the instrument is ready to be used for actual operation.

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