Repeatable precision dispensing of precise, small-volume fluid doses is critical for maintaining the accuracy of ingredient concentration, product efficacy, and batch-to-batch consistency for clinical diagnostics, pharmaceutical, food and beverage, and countless other controlled precision dispensing applications. While a full-bodied, pressurized system with valve assembly undoubtedly offers the greatest dispensing accuracy, the cost of capital equipment often requires researchers, product developers, and manufacturers to adopt a more economical alternative to handle their precision dispensing needs.
|Figure 1. A cutaway diagram of the diaphragm pump illustrates four subassembly components.|
Perhaps the most cost-effective solution is a solenoid-actuated diaphragm pump. Simple in construction as well as in operation, the diaphragm pump basically consists of four subassemblies: an electric coil, a movable arm, the pump seat, and inlet and outlet check valves (Figure 1).
The operation of the diaphragm pump is analogous to a two-stroke engine. When the solenoid is activated, the diaphragm moves up and creates negative pressure over the pump seat. The inlet check valve opens up and fluid is drawn into the chamber. At least 100 milliseconds should be allowed to completely fill the chamber. Subsequently, the solenoid is deactivated by shutting off the electric current. The spring then pushes the diaphragm back down and the resulting pressure forces the outlet check valve open and dispenses the liquid dose.
The pump manufacturer can precisely pre-set the pump stroke, causing it to dispense an exact amount of fluid every time the pump is actuated. Accurate dispense settings range from 8 µl up to 250 µl, with a tolerance of +/- 3 percent from the setpoint. Under normal operating conditions, a diaphragm pump can be expected to perform up to 20 million open and close cycles.
The accuracy of the miniature diaphragm pump, as well as its dependability, ease of operation, and affordable cost, has contributed to a dramatic rise in popularity in recent years. Diaphragm pumps are ideally suited for the transfer of fluids between unpressurized containers. It should be noted however, diaphragm pumps have the ability to generate only about five PSI (1/3 bar) of pressure, which means the diaphragm pump is not a good fit for higher-pressure applications. When placed in a pressurized system or when working against high resistance, it quickly loses accuracy.
Miniature diaphragm pumps are used in the instrumentation, industrial, medical, and electronics industries. As the requirement for precise dosing of liquids continues to grow, solenoid diaphragm pumps are gaining popularity as the solution of choice.
About the Author
Ping Lin heads the engineering department at Bio-Chem Valve Inc. where he researches fluid dynamics, mechanics, and materials. Mr. Lin holds a master’s degree in Mechanics and has been awarded three U.S. patents for valve design. He has more than 10 years experience in valve design, engineering, and material management. Mr. Lin can be reached at 973 263-3001.
For More Information: www.bio-chemvalve.com
A version of this article appeared in the March 2004 issue of MicroTEC magazine.
Calculating Liquid Flow Resistance
Minature diaphragm pumps are not a good fit for applications where they must work against high fluid resistance. The Lee Company (www.theleeco.com) 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 one 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, it is possible to 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 flowrates. Lohm laws predict the actual performance of fluidic devices beyond the definition conditions of water at 25 PSID and 80 F. In liquid flow, several variables must be related, including:
I = Flowrate
H = Differential pressure
V = Viscosity correction factor. V factors compensate for the interaction of viscosity and device geometry and are unique to each class of device.
S = Specific gravity. Use 1.0 for water @ 80 F
K = A constant to take care of units of measure. Use 20 for PSI and GPM.
The Lohm Law for liquid flow is:
For more information on Lohms, visit The Lee Company Web site at www.theleeco.com.