Real-Time Particle Monitoring

With real-time particle monitoring, a single particle counter or sensor is used at a specified location. Each event is detected and counted, and there are no gaps in the particle counting data. And particles are monitored in particles per cubic foot or per cubic meter. This system is best suited at very critical or sensitive operations, where events can occur suddenly or without warning.

There are several kinds of particle counters available. One type is a stand-alone portable particle counter that comes equipped with a display and built-in carbon vane vacuum pump. The remote counter, on the other hand, has no display and should be connected to a computer, a facility monitoring system or data acquisition system. Vacuum for sampling with the remote counters are furnished via a seperate centralized carbon vane vacumm pump that serves several, or all, particle monitor sensors.

Whether a single particle counter or sensor or several sensors are used, real-time monitoring offers a number of important benefits. For example, it provides for the continuous detection of all particle events and emergency reaction to those events. It is also ideal for crucial monitoring, as well as watching equipment for failure and preventive maintenance. Real-time particle monitoring allows for immediate:

• notification or alarming of yield-destroying particle levels
• feedback to staff when procedures are not being followed
• feedback after shut down/evacuation procedures to determine if the area is in specification

Sequential Particle Monitoring

Sequential particle monitoring is also known as Pneumatically Multiplexed Particle Counting and, more simply, manifold particle counting. This type of monitoring involves the addition of a sequential manifold sampler that connects the particle counter to multiple sample tubes. Each tube is sampled in sequence one at a time.

During the sampling process, air is constantly being pulled through the sample tubes through a blower. When the manifold switches to the next tube being sampled, the particle counter stops counting and pauses to allow any air from the previous sample to be purged. This eliminates any “air hammering” that may free particles in the sample tubing due to the starting and stopping of the air flow. Particles are monitored in particles per cubic foot or per cubic meter, as they are with real-time monitoring.

The frequency of each sample is based on the number of points. Each location is generally sampled for one minute and then purged for 10 seconds, as the sampling arm moves to the next location. The ordered nature of manifold particle monitoring offers a number of advantages. For example, fewer counters can be used to cover a specific area. This, in turn, translates in decreased costs, greater sensitivity per cost and easier service. Sequential particle monitoring is excellent for trending the overall performance of a cleanroom.

Real-Time vs. Sequential

Comparing real-time and sequential particle monitoring reveals distinct differences between the two systems. Real-time monitoring allows for the detection of every single event in the cleanroom—regardless of how short the duration. It continuously detects everything with no loss in the data. The system uses multiple sensors, and the sensoring points can be located anywhere, as tubing distance is not a factor.

However, more sensors require more calibration and overall service. Operation and maintenance costs are also higher with real-time monitoring because of the individual counters involved.

Sequential particle monitoring, in contrast, has a lower cost for the same coverage area. Fewer sensors are used, which means less calibration and service. However, detection can be made only to events that happen over a longer span. Short events are missed and reported only when sampled. Sequential particle monitoring can only detect trends, not single events.

Cleanroom standards require that a number of factors be kept under tight control, including airborne particles. Selecting the most appropriate monitoring system can make the process of particle monitoring much easier to manage. Continuos cleanroom particle monitoring and cleanroom automation reduces process and product defects and contributes to refining quality control.

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