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| Solenoids Used in Medical Analysis of Fluids |
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A medical manufacturer needed to control the flow of body fluids from a patient to a medical analyzer with contaminating the body fluids with external substances and without getting air into the circulating system. This problem is critical for blood analyzers, dialysis machines and heart/lung machines. The prevention of external substances and air from getting into the body fluid flow is easily accomplished by using tubing manufactured from an inert material such as polytetrafluoroethylene that connects directly from the body to the analytical section of the machine without an opening for valves or other type hardware.
The problem then becomes one of how do you turn the flow of the body fluids ON and OFF without having the fluids pass through the inside of a valve. This can be done simply by pinching (collapsing) the tube at some point between the body and the analytical section of the machine. The device used for pinching off the tube is called a pinch valve. The valve can be either manually operated or electrically operated. For remote controlled applications, electrically operated pinch valves are required.
Linear solenoids are the primary type of electrical actuator used to operate pinch valves. They offer the following advantages:
· They act directly upon the tube without any other linkages thus allowing for simplicity of design.
· They have a high force to size ratio allowing for smaller package size.
· They are simple to control electrically. You simply turn them ON and OFF.
· They are reliable and cost effective.
Pinch valves are either normally open or normally closed. A normally open valve uses a push type solenoid where the solenoid's spring allows the tube to be open. The tube is closed by energizing the solenoid.
The normally closed valve uses a pull type solenoid where the solenoid's spring has enough force to close the tube. It is opened by energizing the solenoid.
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