Leveraging contamination monitoring data to set action and alert limits
First, let’s look at what the international standard says. ISO 14644-2 Annex B states that “Statistical process control principles can be used to set alert and action levels based on analysis of historical data.”
When performing a cleanroom certification, there is a specific formula to use for the contamination monitoring data to determine where to take the sample, in addition to sampling time and volume. There are also pass/fail criteria for the certification. For cleanroom monitoring, the values for alarm or alert should be based on historical information as well as what will affect the product quality.
As an example, if you are doing a room certification for ISO 5 with a particle size of 0.5 microns, the limits are 3,520 particles per cubic meter. However, this number should not be used as the alarm limit, as a very clean room would have a significantly lower average, such as 100 particles, and never trigger it. In this case, it is advisable to set the alarm limits to 150 or 200 particles, enabling a more active monitoring of the room’s environment. Ensure the alert limit is reflective of the entire room, which is typically much lower than the certification value.
Managing high volumes of data to set effective action and alert limits
You’re going to have a high stream of data if you want a fast response to contamination events. Decide how long you want your samples to run to determine how fast. One minute samples are an immediate indicator that aren’t completely overwhelming to deal with when taken by FMS systems. They show what is occurring in the cleanroom while allowing for a effective response if something is out of control. Likewise, ten second sampling can provide a higher quantity of data with no loss to data quality. No matter what you choose, the intervals between samples is an important decision for setting effective action and alert limits – cleanroom managers don’t want to miss events.
What’s the technique for interpreting contamination monitoring data?
You have a few options when getting data for action and alert limits. In the following examples, we’ll use one minute samplings.
- Option 1: Trigger the threshold value based on out of limit readings.
- Option 2: View one minute data and provide an alarm if you have N number of samples above a threshold
value out of X samples.
- Option 3: Multiply the particle readings by 35.3 to obtain normalized cubic meter values.
- Option 4: Average the last 35 minutes and provide it as cubic meter data, then begin the next sample.
- Option 5: Average the last 35 minutes and update every minute dropping the oldest one minute sample and
adding back the newest one.
Remember this for your cleanroom action and alert limits
There is no specific rule regarding where to set an alarm limit. However, in order to ensure a quality product and identify where action and alert limits should be set, limits should be based on a careful review of the contamination monitoring data and the process. Particle Measuring Systems (PMS) can help you by providing guidance on the regulations you need to be compliant with, develop contamination control strategies, operator training, performance optimization, and more.
For a full description of the sample interpretation described in this blog, download the full application note here.