eFunda: Theory of McLeod Gages
 McLeod Gages: Theory
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 Boyle's Law Boyle's Law, proposed by Robert Boyle in 1662, states that for a closed isothermal (constant temperature) system, the product of the pressure (P) and volume (V) is a constant, Equivalently, the product at two different states must be the same,
 McLeod Gages Suppose that initial pressure and volume in a McLeod Gage are given by, P1 = PiV1 = V + A·h0 where V is the reservoir volume and A is the cross-sectional area of the sealed tube, as shown schematically below. Suppose that the final compressed pressure and volume are given by, P2 = PgageV2 = A·h According to Boyle's law, we have, For a typical manometer, . The unknown pressure Pi can be reduced to a function of the height difference h, Furthermore, the volume of the reservoir is usually much larger than the tube, V » A·(h0-h) This allows us to drop the area term, resulting in a simple quadratic function for the pressure,
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