FloSonex vortex flow meters are designed for accurate flow rate measurement of ultra-pure and ultra-aggressive liquids. The flow sensor consists of a solid flow tube and a removable PTFE or PVDF sensor. The flow tube is compatible with most processing liquids including DI water, hot (up to 160°C), 95% sulfuric acid, phosphoric acid, SC1, SC2, "Piranha", and many others. The flow tube is completely passive containing no moving parts. The only active components are the two ultrasonic transducers sealed within the sensors. The sensors are easily removed from the flow tube with a blade screwdriver. All repairs may be made without shutting the process down. Each FloSonex unit is permanently calibrated at the factory. Removal of the sensors does not affect calibration. In the unlikely event of an electronics failure, calibration data may be downloaded to the new module.

HOW IT WORKS
The flow tube contains a small flat-faced pin called a bluff body that spans the internal bore. As fluid moves across the bluff body, vortices or eddies are formed and shed alternately from one side then the other. The vortices form at a rate that is directly proportional to flow. The formation of the vortices is directly behind the bluff, and the spacing is the same at all flow rates. The frequency is sensed outside of the flow tube by directing a beam of ultra-high frequency sound from a transmitter in one leg of the yoke, through the flow tube wall, through the liquid, and out again to a receiver in the other leg of the yoke. The received signal is demodulated and the vortex shedding frequency is computed by a microprocessor and transformed into a 4-20 mA signal.

Most liquids compatible with PFA may be measured. Limitations include liquids with the following characteristics:

• High viscosity (e.g.cold concentrated acids)
• Contain an excessive amount of bubbles
• Near boiling point (hot low-pressure water)
• High acoustic dampening (e.g., glycerin)

Viscosity affects the minimum flow rate that may be measured. Flow rates below the minimum will be indicated as zero flow rate. The higher the viscosity, the higher the minimum flow rate. Viscosity of liquids depends on temperature. Therefore, many viscous liquids, such as concentrated acids, can be measured when they are hot, but not when they are cold. To determine the minimum flow rate, first find the viscosity of the fluid at the operating temperature. The minimum flow rate, in lpm, may then be determined for a given flow meter size from the Minimum Flow Rates Chart. In general, it is not recommended that fluids with viscosities above 6 centistokes be measured. These fluids will not damage the meter, but the flow meter may not be able to indicate the flow rate. For example, when a hot 95% sulfuric acid system is first started, the liquid may initially be cold and have a very high viscosity, on the order of 30 cSt. The flow meter will indicate zero flow rate until the acid temperature rises to 120°C.

To sense the vortex shedding frequency, an ultrasonic beam is passed through the fluid. If a bubble passes through this beam, it will cause modulation to the beam that is similar to the modulation caused by vortex shedding. Signal processing routines in the microprocessor based electronics module discriminate between the signal signatures caused by bubbles and by vortices rejecting bubble signals. This will allow trouble-free operation in most cases. However, if excessive numbers of bubbles pass through the beam, vortices can no longer be detected. For this to happen, the bubbles must have a diameter of at least 0.02 mm and must be spaced closer than one pipe diameter. Bubbles less than this size and more closely spaced can also cause interference. However, bubbles less than 0.005 mm diameter will not cause interference, even in large numbers (such as cloudy water) unless the concentration is so heavy that the ultrasonic beam is blocked below detectable levels. A common source of gas bubbles in semiconductor processing equipment is ozone injectors. If possible, ozone injectors should be placed in the system either downstream of the flow meter, or far enough upstream to allow the gas to dissolve. Oxygen outgassing from hydrogen peroxide is another source of bubbles. Typically, liquids that contain hydrogen peroxide, such as SC1, SC2, and Piranha, do not generate enough outgassing to cause interference.

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