08/11/2011 | Author / Editor: Jennifer Breunig / Jörg Kempf
When specifying pressure measurement systems one of the most important aspects is the pressure gauge or transducer itself. If the process requires sterile connections measuring elements with a diaphragm seal should definitely be considered.
Many standard pressure sensors and gauges have crevices and dead spaces which make them unsuitable for hygienic applications. They may also be damaged by the high temperature of the steam used for sterilization in place. In such cases, a diaphragm seal not only provides a safe and sterile connection between the medium and the measuring instrument, but also allows simple cleaning between batches.
A diaphragm seal separates the pressure gauge, transmitter or switch from the medium being measured and ensures a process connection in which dead spaces are minimized or eliminated. Isolation is achieved by means of a flexible diaphragm, with the internal space between the diaphragm and the pressure measuring instrument completely filled with fluid. The elastic diaphragm transmits the process pressure to the incompressible fill fluid and hence to the measuring instrument. The link between the diaphragm seal and the measuring instrument may take the form of a direct connection or a capillary. For high temperatures a cooling section can also be fitted between the seal and the instrument.
The choice of fill fluid is important. The fluid needs to be compatible with the process medium, for safety in the rare event that the diaphragm ruptures. For sanitary applications it is important to choose an FDA-approved fill fluid, and further approvals — such as listing in the relevant national pharmacopoeia — may be necessary to ensure compliance with cGMP requirements.
To meet the demand for hygiene in pressure measurement, in-line instruments may be used to avoid dead space and promote cleanability. The patented in-line diaphragm seal design from Wika functions in exactly the same way as a conventional diaphragm seal, except that the diaphragm is cylindrical instead of flat. As with other diaphragm instruments, in-line diaphragm seals are available with either gauges or pressure transmitters.
In contrast to designs with beads or deviating geometry, the process medium flows unhindered through the measuring chamber of an in-line diaphragm instrument. The chamber is self-draining regardless of the orientation at which it is installed. Cleaning and sterilization in place (CIP and SIP) is easily achieved because there are no dead spaces or crevices in which products can settle or biofilms form.
In-line diaphragm seals are also available with built-in resistance thermometers, allowing them to measure temperature as well as pressure. In this case the temperature sensor is fixed to the inner wall of the diaphragm. This compact arrangement features the same excellent hygienic performance as the basic in-line diaphragm pressure sensor, and has the advantage of requiring fewer seals or welds than if two separate fittings were used for temperature and pressure.
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