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.
I  =  Flow rate (gallons per minute). 
H  =  Differential pressure (psi). 
L  =  Lohms, a measure of resistance to liquid flow. It includes all density, viscosity, Reynolds number, coefficient of discharge & area units. 
When testing on water at 25 psi  and the above formulas simplify as follows: 
Some useful relationships:
1.  1000 Lohms will permit a flow of 50 lb/hr water at 25 psi ?P.  
2. 


3.  d = Orifice diameter (inches) 

4. 
where d = orifice diameter in millimeters 
Problem 1. What restriction will permit a flow of 1 gallon of water per hour at 50 psi ΔP?
Problem 2. An orifice with a hole diameter of .012" flows 18 lb/hr of water at 100 psi ΔP.
How many Lohms?
Problem 3. What ΔP 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 ΔP of 500 psi?
NOTE: For special flow requirements, The Lee Company can determine the required Lohm rating.
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:
Please note that this relationship is not the same as in electrical problems. The difference is due to the nonlinearity of
Please note that this relationship is not the same as in electrical problems. The difference is due to the nonlinearity of
When L_{1} = L_{2} = L_{3} , then 
For passageway size: DT = D/ N1/4
D_{T} = Diameter of a single equivalent orifice, with a Lohm rate = L_{T}
D = Diameter of the actual orifices, each with a Lohm rate = L_{1}
One of the reasons for using two restrictors in series is to allow fine tuning of a total resistance value. If L_{1} is known and is more than 90% of L_{T}, then L_{2} may vary by ±5% without altering the value of L_{T} by more than ±1%, even though the value of L_{2} may be as high as 40% of L_{T} . This effect becomes even more pronounced as L_{1} approaches L_{T}.
To determine the intermediate pressure between two resistances in series, the following formulas may be used.
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
L  =  Lohms 
H  =  Differential pressure 
I  =  Liquid flow rate: Volumetric 
S  =  Specific gravity* (click here) 
V  =  Viscosity compensation factor** (click here) 
w  =  Liquid flow rate: Gravimetric 
K  =  Units Constant – Liquid (click here) 
*S = 1.0 for water at 80°F. 
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
Volumetric Flow Units
VOLUMETRIC FLOW UNITS  
Flow Units 
Pressure Units  
psi  bar  kPa  
GPM  20  76.2  7.62 
L/min  75.7  288  28.8 
ml/min  75,700  288,000  28,800 
in^{3}/min  4,620  17,600  1,760 
Gravimetric Flow Units
GRAVIMETRIC FLOW UNITS  
Flow Units 
Pressure Units  
psi  bar  kPa  
PPH  10,000  38,100  3,800 
gm/min  75,700  288,000  28,800 
Problem 1. An orifice is required to flow 0.15 GPM of MILH83282 hydraulic fluid at 80°F and 100 psi ΔP. 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.
What is the maximum Lohm rate allowed for the Safety Screen?
Solution: