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.
|The Purifier® Logic® Biosafety Cabinet is one example of a biological safety cabinet that uses sensorless airflow control.|
The 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).
|Figure 1. This graph illustrates how the ECM motor maintains constant airflow. The CAP (red) line indicates the motor torque and speed required to maintain a constant volume of 800 cubic feet per minute (CFM). This line is programmed into the motor as a series of constants generated during the characterization process. The green dashed line represents the starting filter pressure in the biosafety cabinet. As the HEPA filters load, the new pressure will be represented as the blue dashed line. The biosafety cabinet is operating stably at point “A,” until the filters load. The blower then speeds up to point “B,” a result of increased pressure and reduced airflow. This increase in speed (referred to by some as “self-compensation”) happens with any type of motor (see the “Self-Compensating Blowers” sidebar for further discussion). Unlike the PSC motor, which would remain at point “B,” the ECM checks its speed and torque. Because point B is not on the CAP line, the ECM increases its speed and torque to points “C,” “D,” and finally “E” until its speed and torque fall back onto the red line.|
With 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.
|Figure 2. This graph illustrates actual test results from NSF International on a Purifier Logic Type A2 Biosafety Cabinet powered by an ECM with CAP technology and a biological safety cabinet powered by a PSC motor. In the Motor/Blower Performance Test as defined in NSF/ANSI Standard Number 49, a new biological safety cabinet’s total volume of air displaced by the blower is measured. The cabinet’s front grille is then restricted to simulate an additional 50% load on the HEPA filters. The total volume of air is measured again and compared to the initial value. In the graph shown, the Purifier Logic Biosafety Cabinet with the CAP technology saw its volume decrease from 784 to 778 CFM, a loss of 0.7% (represented by the red line). The biosafety cabinet with the PSC motor saw a loss of approximately 60 CFM, or 8% (represented by the blue dashed line). These results demonstrate that the Purifier Logic Biosafety Cabinet maintains airflow at more than ten times the accuracy of the PSC-powered biosafety cabinet.|