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.
Lohms Vs. Orifice Size
Lohm Rate Vs. Orifice Diameter
Standard Conditions used by Lee
U.S. Standard Conditions at sea level are per ICAO STD ATMOSPHERE
|Pressure||14.70 psia (29.92 in.Hg.)|
|Temperature||59 deg. F (518.7 deg. R)|
Other References may use somewhat different conditions.
Gas vs. Liquid Calibration
Most EFS products are calibrated on gas for both gas and liquid service. Should it be necessary to use a gas calibrated component for liquid service, or a liquid calibrated component for gas service, the following factors should be considered:
- Allowance should be made for variations in liquids/gas correlation of up to ±15%. This is caused by the response of different fluids to the orifice geometry.
- Single-orifice restrictors will correlate directly from gas to liquid service, subject to the ±15% normal variation.
- Multi-orifice restrictors will correlate directly only when the pneumatic pressure ratio is very low. (P1/P2<1.2)
- When Multi-orifice restrictors are used at higher pressure ratios, the gas flow will be up to 30% higher than expected from a liquid calibration. This is caused by gas compressibility which results in a non-uniform distribution of pressure drops through the restrictor.
Warning: Do not substitute hydraulic restrictors in gas applications, or vice versa, without first considering the application and correlation accuracy.