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Here is the outline of a problem that I'm trying to solve. I have a tank that contains a certain level of liquid. The flow into this tank is not controlled for the purposes of this loop. The flow out of the tank is pumped by a variable speed pump. So the task is to use PID control loop to keep the level in the tank constant by varying the speed of the pump. Simple - the level rises, the pump goes faster, the level drops the pumps slows. OK, but there's a complication. The device that the pump is pumping to can only accept flow up to a maximum rate (10 meters cubed/hour). So we have to limit the speed of the pumps so that the flow out of the tank doesn't exceed 10 M3/hr. I'm aware that this can cause the tank to overflow, but the customer doesn't care about this, they need to limit the flow to less than 10M3/hr. How can I go about limiting this? Should I vary the parameters of the PID loop as the outflow gets near 10M3/hr? Or should I disable PID control when the flow is near 10M3/Hr and re-enable it later? Or should I try something else entirely? All suggestions are gratefully received. Best regards, Peter Thornton EMR Radio & Telemetry [email protected]