Medical Device Link.

MANUFACTURING

Using the right toolkit

Manufacturers of medical devices are concerned about particle and microbial contamination control in their clean rooms, and take steps to reduce the risks of contamination adversely affecting production. These steps include commissioning the right classification of clean room, ensuring suitable clothing is worn, adopting appropriate cleaning procedures and controlling the quality of air introduced to the area.

Courtesy of Airflow Development Limited. (Click image to enlarge)

To have a clean room that is working efficiently, it is necessary to have the right filter with the ideal number of air changes, and more importantly, to ensure that the airflow into the room is sufficient. To achieve optimum flow and cleanliness in the clean room, computerised simulations can assist in designing, planning and visualising the potential airflows. Simulations can help determine where equipment and furniture should be situated to alleviate any “dead” areas. These simulations can also visualise the effects of bringing new items into the clean room.

These simulations can also be used in existing clean rooms to show how the airflow is being obstructed by equipment, people and design or if turbulent airflow is being created. However, it may not be so easy in an established clean room to rectify any problem areas. One solution is to create a mini environment such as a softwall clean room over the troubled area that provides localised laminar airflow and uniform air change rates. This can be a more cost-effective solution to the problem than relocating equipment or machinery.

Although air changes per hour give an indication of air supply, they relate to the volume of the room, which is an incorrect unit of measurement. Measuring air velocity is a more accurate way of determining the cleanliness of the room: the higher the velocity, the cleaner the room. However, a recently published article¹ raised concerns over the inaccuracies of measuring air volumes supplied to a clean room. Thus the question arises, what is the correct way to measure the airflow to a clean room? There are three instruments to employ: a measuring hood, a Pitot-static tube and an anemometer. Table I provides definitions of the equipment and the terms used.

Measuring hood

A measuring hood (balometer) is the most common way of measuring airflow into a clean room. The hood is placed over the supply of air into the room (usually a filter unit in the ceiling), and the volume of air is measured as it exits from the hood.

However, there are concerns that the measurements are inaccurate by as much as 20% compared with measurements from taken with a Pitot-static tube.³ The inaccuracies were found with a swirl type diffuser, where the air is not distributed evenly across the measuring grid. However, if the hood has been calibrated in a rig using the right type of diffuser, this problem can be eliminated.

Pitot-static tube

This is a small diameter open-ended tube that faces the air stream and senses total pressure. The static tube has holes at right angles to the flow and senses static pressure. The velocity pressure is the difference between the two readings. The tube is effective at taking readings in a duct, because it only requires small access holes. However, this method is difficult and time consuming and its use is avoided during regular testing.

Anemometers

These instruments measure air speed and need to be calibrated against recognised standards. A rotating vane anemometer records the linear movement of air in metres or feet over a period of time. The rotating vane forms part of an electronic transducer, which offers little resistance to flow and provides good average readings. The design allows for the measured velocity to be displayed instantaneously, thereby reducing the need for external timing. This instrument is valuable for large ducts/areas, but it is not easy to insert into smaller ducts. Readings can be inaccurate if there are any “swirls” in the flow.

A thermal anemometer records the drop in temperature from air passing over a heated element, which is mounted at the end of a probe. This probe can be inserted directly into the airflow, and the small sensing head allows easy access to systems with small access holes. As with the rotating vane, readings are direct and instantaneous. When taking readings, compensations for variations in the ambient temperature, pressure and composition of gas/air need to be made. Some instruments also have the ability to measure temperature. These offer an advantage over rotating vane anemometers and Pitot-static tubes because they are more effective at lower velocities. However, to obtain an average reading, a transverse method is also needed because they only take a point velocity reading.

Accuracy is critical

Whichever method is used to measure airflow, the accuracy of the instrument is important. Instrument accuracy can be quoted as a percentage of full-scale deflection or as a percentage of reading for all or part of the scale. The accuracy of the equipment can also be affected by site conditions. Because these will differ from the conditions in the laboratory, the stated accuracy may be 61%, but in reality the end result may have an uncertainty of 610%.

In addition, it is important that the user is competent in taking accurate readings and interpreting the results. Inadequacies in any of these areas can lead to inaccurate results.

Frequency of measuring

There are no industry guidelines in place at the moment, but there is debate over periodical or continuous monitoring as an effective measurement. Discussions are currently underway to assess any changes that need to be made to ISO 14644, Cleanrooms and Associated Controlled Environments. For manufacturers to meet the standards laid down by the International Organisation for Standardisation, they should perform periodical testing to ensure they pass any audits specified by customers. However, best practice would lean towards a continuous measurement of filter pressure, particles and daily airflow to identify and remedy any problems at an early stage. For continuous measurement, the use of Pitot tubes, a pressure gauge and a particle counter would be suitable.

J. Govier is Managing Director of Connect2Cleanrooms Ltd, Unit 1 Willow Mill, Fell View, Caton, Lancaster LA2 9RA, UK, tel. +44 1524 771923,
e-mail: joe@connect2cleanrooms.com
www.connect2cleanrooms.com