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Differential pressure is the hardest-working measurement in the plant. It meters flow through orifice plates and pitot tubes, watches filters load up, measures tank level from the bottom up, and derives density. And it's the measurement most people under-specify — because a DP error doesn't just add, it multiplies. Here's why: flow from differential pressure follows a square-root relationship. A 1% error in DP becomes roughly a 0.5% error in flow — but more importantly, at low flows …