Check the reaction time of the variable speed drive air compressor. The variable speed drive air compressor reaction time is the time taken to regulate between its 'minimum' and 'maximum' speed (and output). A variable speed air compressor with a slow reaction time may be insufficient to satisfy the control strategy of the Metacentre system controller. As a consequence, the Metacentre system controller may react to the lack of pressure by loading an additional air compressor. This characteristic can be avoided by either reviewing the variable speed drive air compressors VSD reaction time, or by adjusting the Metacentre system controllers PL & PH values. Check relationship between individual equipment capacity and air consumption levels! Typically variable speed air compressors have a minimum speed of between 25 and 50% of full speed (depending on type) whereas fixed speed compressors operate either fully loaded, unloaded or stopped. If air demand varies above the maximum output of the variable speed air compressor, this will induce the loading of a fixed speed air compressor. The fixed speed compressors addition will induce the variable speed air compressor to regulate down, which could be as low as its minimum speed. If the fixed speed compressors output and the output of the variable speed air compressor running at minimum speed is greater than air demand, the Metacentre system controller will be forced to unload the fixed speed compressor. This characteristic called 'dead band' may be repeated. 'Dead bands' which can appear more than once, can be identified by mapping the steps in generating capacity available between 0 and 100% of total generating capacity. Where significant 'dead bands' occur, a careful review of the generating equipment available to the Metacentre system controller may be necessary. For more information about dead bands, refer to the 'Challenge Convention' case study on the website or consult your Metacentre equipment provider.