Flow

In the recent past, isolator manufacturers have tended not to fit instrumentation to demonstrate the airflow rate through their isolators. This has been a notable omission, since the airflow through an isolator is the single most influential parameter defining the conditions within the isolator. Airflow rate is likely to have a much greater effect on air quality than the isolator pressure, but unreasonable emphasis has often been placed on pressure measurement at the expense of flow measurement. Perhaps one reason for this is that airflow rate is not easy to measure at low pressure and flow rates. The most precise method is to apply a suitably sized orifice plate and then measure the pressure drop across the plate using a pressure transducer. The airflow rate or, more precisely, the mass flow rate, can be related to the pressure drop by mathematical formulae; specialist companies can provide the equipment and the calibration data. One problem with this arrangement is that it introduces a large pressure drop in the ventilation system, requiring fan power to overcome this. Mechanical devices, such as vane anemometers, might be used, but such instruments are not designed for continuous use and also introduce resistance to flow. Probably the best method is to use a version of the hot-wire anemometer, usually using a small thermistor as the heated element. This can give an electronic signal that may be calibrated by using a standard orifice plate during setup. This flow transducer can be used to sense velocity off the face of a HEPA filter for unidirectional flow or to measure the overall flow in the ventilation system. Pitot tubes linked to pressure transducers may also be used to measure flow rate, but they tend to require relatively high duct velocities to give a satisfactory signal-to-noise ratio. As with pressure, flow rate information may be linked to alarm systems, building management, or process control. Recirculating isolators will very likely feature instrumentation of the main flow as well as the makeup flow. The isolator airflow rate meter is normally designated a critical instrument.