Pressure Control: Without Deadtime, I Might Be Out of a Job 41 Me: The deadtime from inertia is negligible, and the high gain of the furnace pressure response means the fan speed only needs to change a percent or less to correct for an excursion. The time required for a very small speed change is therefore very small-- confirming Pressure Control Fact of Life #4 and adding #10. You: Why aren't variable speed drives considered more often to improve the control of difficult loops? Me: Valve manufacturers like to downplay the effect of valve stick- slip, drive manufacturers don't understand Pressure Control Fact of Life #3, and users tend to select among the alternatives handed to them on silver platters rather than to look for uncommon hardware. REFERENCE 1. McMillan, G K; Tuning and Control Loop Performance, Momentum Press, 2010