To remain in compliance with ISO 14644, you need a testing certificate. Frequency is dictated by application and ISO classification. In pharmaceutical manufacturing, more frequent testing (around every 6 months) is common, whereas general manufacturing usually tests only once a year. ISO classification also plays a role in determining this. In rooms classed as ISO 5 or cleaner, calibration should take place every 6 months at maximum. In rooms classed ISO 6 and up, calibration should occur at least once every year.