A Simplified System of Defining Fluid Resistance Over the years, The Lee Company has developed the Lohm system for defining and measuring resistance to fluid flow. Just as the "ohm" defines electrical resistance, the "Lohm" or "liquid ohm" can be used as a measure of fluid resistance. The Lohm is defined such that 1 Lohm will flow 100 gallons per minute of water with a pressure drop of 25 psi at a temperature of 80°F. Since resistance is inversely proportional to flow, by definition: By using Lohms, one can specify performance without concern for coefficients for discharge, passageway geometrics, physical dimensions or tolerances. The resistance of any flow can be expressed in Lohms and confirmed by actual flow tests. Lohm Laws generalize the Lohm definition and allow the system designer to specify Lohm requirements for particular application based on the desired pressures and flow rates. See below for a graph relating Lohms to hole diameter and flow coefficient, Cv.
