Sensor-based feedback loops
In the late 1970s and early 1980s, the increased availability and reduced cost of electronic air velocity sensors made sensor-based feedback loops a viable solution for biosafety cabinet manufacturers. The first of these cabinets used an airflow sensor, specifically a thermal (hotwire) anemometer, to continuously measure the downflow velocity in a single spot in the work area. The velocity was reported to the biosafety cabinet’s speed controller via a feedback loop. As downflow velocity dropped due to filter loading, the speed controller increased the blower speed to return the velocity to its nominal setpoint.
The biggest advantages of this technology are real-time airflow monitoring and the display of airflow in the biosafety cabinet. However, there are shortcomings to this design. The thermal anemometer consists of a small wire through which an electrical current is passed.
The air passing over the wire cools it proportionately to the air’s velocity and the resulting temperature differential is converted to a voltage. The voltage is sent to the controller, which must interpret the voltage as an air velocity. Each sensor element responds differently to changing velocities. Therefore, either the controller must be calibrated with its unique sensor, or a calibrated sensor with an integral compensation circuit delivering a standardized output must be used. Replacement can be expensive and requires a trained certifier and recertification of the unit after repairs are completed.
LabconcoThe most significant drawback to this technology is in the sensor’s lack of accuracy. Typical thermal anemometer sensors used in biosafety cabinets have an accuracy of +/-10 percent, which allows for a considerable amount of fluctuation. Finally, the sensor itself requires annual recalibration to compensate for changing airflow patterns in the cabinet as well as sensor “drift” as it ages.
When the design was first introduced, a thermal anemometer to maintain biosafety cabinet performance was a vast improvement over the manually adjusted speed controls that were originally used. However, its inherent drawbacks have led manufacturers to seek more robust and reliable methods to automatically compensate for changing airflows as the HEPA filters load.
Sensorless airflow control
In 2007, Labconco solved the intrinsic problems associated with using sensors to monitor and automatically adjust motor speed to compensate for filter loading. One goal in the development of the Purifier® Logic® Biosafety Cabinet was to incorporate better, more efficient motor technology. To that end, a direct current (DC) electronically commutated motor (ECM) was installed in place of the conventional alternating current (AC) permanent split capacitor (PSC) motor.
The ECM offers numerous advantages over earlier PSC technology. Its inherent efficiency offers an energy savings of 50 percent or more, while its rugged design provides an operational lifespan approximately three times longer than that of the PSC motor. The cooler operation of the ECM minimizes the increase in air temperature in the working environment of the biosafety cabinet, promoting user comfort. Microprocessor sensing and control of motor speed and torque allow for the programming of the motor to deliver constant air volume to the biosafety cabinet even as HEPA filter loading changes.
Constant Airflow Profile (CAP) technology
The process of “teaching” the ECM to deliver constant airflow volume, the Constant Airflow Profile (CAP), was developed by Labconco. In order to program the ECM to maintain a nominal airflow, engineers recorded the speed and torque requirements of each size cabinet at a variety of different airflows and HEPA filter differential pressures. The speed, torque and airflow data was processed using software provided by Regal Beloit to generate a unique performance profile for the ECM (Figure 1).
LabconcoWith this process, CAP technology has solved the previously encountered problems with airflow monitoring. As discussed above, thermal anemometers require routine calibration. With CAP technology, there are no sensors to recalibrate or replace. Therefore, maintenance and equipment replacement costs for these airflow monitoring devices have been eliminated. In addition, this robust design is not susceptible to temperature and humidity fluctuations that can plague thermal anemometer- based systems. Perhaps the most beneficial advantage to this design is its inherent accuracy. Testing performed at Labconco Corporation has demonstrated that airflow is maintained with only a 1 to 2 percent change as the HEPA filter loads. Figure 2 shows a representative data sample from this study.
Significant strides have been made in the last 40 years to maintain constant airflows in biosafety cabinets. Simple chopping circuits and differential pressure gauges have given way to sensor-based control systems. These, in turn, are now being supplanted by sensorless microprocessor-motor systems, which are capable of maintaining accurate airflow volume even as the cabinet’s HEPA filters load. One sensorless system uses CAP technology, which offers the advantages of tenfold accuracy and reliability, and the elimination of periodic recalibration of airflow sensors to ensure proper airflow.