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13/04/2006
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Tom Shelley reports on an ingenious device for detecting liquid levels in container for In vitro diagnostic machines
A US company has developed a sensor that detects the presence or absence of liquid at a specified fill level inside a plastic bottle or container, without it's having to be opened.
Developed for the in vitro diagnostic market, it is highly suited for systems filling and handling bottles of aggressive chemicals, those containing liquids deemed to be possible biohazards, and situations where it is essential to avoid the possibility of contamination.
The ExOsense is a PVDF based ultrasonic piezoelectric transducer with a 'Rexolite' lens that sticks to the outside of a bottle when an adhesive cover is removed. The amount of acoustic signal that is reflected from the inside surface of the bottle depends on the impedance mismatch between the bottle material and whatever is inside it. If the bottle contains liquid, the acoustic impedances are similar, 1.8 and about 1.5, and most of the ultrasound signal passes into the liquid without being reflected whereas if there is no liquid, the acoustic impedances of the bottle and air are sufficiently different, 1.8 and effectively 0, that most of the ultrasound energy is reflected back to the transducer.
The transducers are sufficiently low cost that they can be considered to be disposable in some applications. They are separate from the electronic switch units, being attached by a cable fitted with a miniature USB type connector, which plugs into the transducer head. The lens focuses the ultrasonic signal to a point on the inside of the bottle. The system requires no calibration: it is just plug together and go. It works with bottles made of polyethylene, polypropylene, polysulfone and polycarbonate but not those made of PTFE or any foamed core plastics. Heads are available for flat sided and round bottles and containers with a minimum bottle diameter of 50.8mm. Wall thickness can be from 0.5mm to 3.8mm. Response time is less than 100ms and the transducer works over the temperature range 0 degC to 70 degC. Sealing is to IP65. Power requirements are 4.75 to 5.25VDC or 6 to 32VDC. Power consumption is 50mA max and output configuration is open collector, 40mA max.
Fluids whose levels may be detected include: reagents, wastes, diluents, detergent washes, dialysate, coolants, saline or pure water. The device is used in equipment for clinical chemistry, haematology, immuno-chemistry, histology, medical laser systems, haemodialysis and cytology.
The device is commercially available world-wide from Gems Sensors
Gems Sensors
Pointers
* Sensors are non intrusive and stay outside the bottles, thereby maximising bottle volume and avoiding the need to test compatibility with the media inside
* The devices have a very small footprint, are robustly made, are simple to install and require no calibration
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Author
Tom Shelley
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