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Lohm Rate vs. Orifice Diameter Lohm Laws Liquid Flow The Lohm has been selected so that a 1 Lohm restriction will permit a flow of 100 gallons per minute of water with a pressure drop of 25 psi at a temperature of 80°F.
Some useful relationships:
Liquid Flow – Examples Problem 1. What restriction will permit a flow of 1 gallon of water per hour at 50 psi ∆P? Problem 2. An orifice with a hole diameter of .012" flows 18 lb/hr of water at 100 psi ∆P. Problem 3. What ∆P will be required to flow 20 GPH of water through a 2000 Lohm orifice? Problem 4. What water flow will result from a restriction of 500 Lohms and a ∆P of 500 psi? NOTE: For special flow requirements, The Lee Company can determine the required Lohm rating. Liquid Flow – Two Forumlas for Combinations of Restrictors PARALLEL FLOW, the total Lohm rating is: Please note that this relationship is identical to the electrical equation. SERIES FLOW, the total Lohm rating is:
For passageway size: DT = D/ N1/4 DT = Diameter of a single equivalent orifice, with a Lohm rate = LT D = Diameter of the actual orifices, each with a Lohm rate = L1 Liquid Flow – Series Flow One of the reasons for using two restrictors in series is to allow fine tuning of a total resistance value. If L1 is known and is more than 90% of LT, then L2 may vary by ±5% without altering the value of LT by more than ±1%, even though the value of L2 may be as high as 40% of LT . This effect becomes even more pronounced as L1 approaches LT. To determine the intermediate pressure between two resistances in series, the following formulas may be used. Liquid Flow – Flow Formula The following formulas are presented to extend the use of the Lohm laws to many different liquids, operating over a wide range of pressure conditions. NOMENCLATURE
Liquid Flow The following formulas are presented to extend the use of the Lohm laws to many different liquids, operating over a wide range of pressure conditions. These formulas introduce compensation factors for liquid density and viscosity. They are applicable to any liquid of known properties, with minimum restrictions on pressure levels or temperature. The units constant (K) eliminates the need to convert pressure and flow parameters to special units. Volumetric Flow Units Gravimetric Flow Units Liquid Flow – Units Constant K
Gravimetric Flow Units
Liquid Flow Calculations – Examples Problem 1. An orifice is required to flow 0.15 GPM of MILH83282 hydraulic fluid at 80°F and 100 psi ∆P. What restriction is required? Solution:
Problem 2. What pressure drop will result from a flow of 5 PPH of SAE #10 lubricating oil at 20°F, flowing through a 1000 Lohm orifice? Solution:
Problem 3. A Safety Screen is required to flow 775 lb/hr of diesel fuel @ 80°F with a maximum pressure drop of 5 psid. Solution:
Viscosity Compensation Factor For Single Orifice Viscosities of Typical Fluids vs. Temperature

