There are different answers depending on what you are trying to accomplish. We know that current SQF, BRC, and other certification requirements will accept one sample annually. If you want to determine if compressed air quality is in a state of control throughout the year, then certainly one sample is insufficient.
The following are sampling plans that you can use and combine to determine what is best for your company:
Percentage-Based – Pick a percentage of your outlets (25%, 33%, 50% etc.) to sample annually. We recommend sampling throughout the year to include seasonal changes, production variations, and maintenance schedules.
For example: you have 24 outlets with direct contact; you could decide on sampling 33% annually. Sample 2 outlets every quarter for a total of 8 samples for the year. Pick 8 different outlets the following year. After the 3rd year, you will have sampled 100% of the outlets and will have gathered valuable information that you can use to make an educated decision on a monitoring plan that works for your operation.
Maintenance Based – Take samples before and immediately after filter changes (include compressor and point-of-use filter changes). Also, sample after any major changes to the compressor or distribution piping.
The purpose of this strategy is to help you…
A. Ensure that air quality is in a state of control
B. Confirm that filters are being changed at appropriate intervals for your production levels
C. Ensure that major changes to your system did not negatively affect air quality
The Maintenance Based strategy can also be combined with the Percentage-Based sampling plan.
Minimal Based – Take three or four samples; one as close to compressor filtration as possible, one at the furthest point from compressor filtration, and then one or two somewhere in between. You can review results to see if there is any degradation of air quality the further you get from the main filtration. This will also take into consideration possible piping contamination.
Single Test – Take one sample from a critical control point. Select a different point-of-use each time you sample. Taking one sample is a good first step and will satisfy current minimum requirements for GFSI-required certification audits.