Absolute pressure switches

Most pressure sensors detect the difference between the measured value and a reference. In the case of absolute pressure sensors, the reference chamber cannot be evacuated to absolute zero, because it can only be approached within a few thousands of a millimeter of mercury (torr). In the case of positive pressure detectors, when the barometric pressure is the reference, atmospheric pressure variations can cause errors up to 25 mmHg. Intelligent transmitters can also operate with multiple references and switch them as required. 

The balanced-pressure injection entails the headspace vial being pressurized and allowed to reach an equilibrium then a valve is switched to direct part of the sample into the transfer line and the GC for a specific time interval (Fig. 2B). The absolute volume of the sample injected into the GC is unknown because this technique uses a theoretical amount of time to inject the sample. A number of contact parts are minimized in this design which should in theory lessen the chance of analyte adsorption or condensation within the system. An example of an instrument utilizing the balanced-pressure technique is the Perkin-Elmer TurboMatrix model HS-40. 

The passive function of the BC system causes a spontaneous decrease in pressure in the hermetically sealed areas with the continuous accumulation of a significant amount of released thermal energy. Full localisation of the accident is accomplished by active spraying of the SG compartments, which gradually reduces the pressure in the containment to a value of 80 kPa (absolute), when the spray system is automatically switched off. A moderate vacuum in the sealed area prevents the release of radioactive substances this vacuum is maintained by the controlled actuation of active spray systems [228]. 

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