Reviewing water activity standards

Since water activity testing is critical to ensure the quality and safety of manufactured products, it is common to find water activity instruments in most quality assurance laboratories. As with any laboratory test method, good laboratory practices require routine verification assessments for water activity to make sure the instrumentation is working properly. This can be accomplished by running tests on standards of known value and comparing the test results to the actual values.

While this can be difficult for some common quality assurance tests that do not have independent standards, such as moisture content, water activity standards exist in the form of electrolytic (salt) solutions. These water activity salt standards are one of the most important tools available to laboratory personnel to provide them with confidence in their water activity testing results and facilitate troubleshooting.

A change in results when testing salt standards indicates some change in the instrumentation that could be impacting the integrity of product test results. For instruments that rely on a chilled mirror sensor, changes in standard readings are typically the result of contamination of the mirror, which can happen at any time, making frequent verification with salt standards necessary. For instruments using electrolytic sensors, due to their resistance to problems from contamination, salt standards can be used less frequently and serve the purpose of determining if an instrument calibration is needed.

Verification with standards

Water activity standards are used to verify water activity instruments by placing a standard in a testing cup and then the cup in the instrument. For some tools, verification is performed like any other water activity test. The user can then compare the result to the actual value at the appropriate temperature and if the test result is within the instrument tolerance of the actual value, the instrument passes verification. If not, the instrument is inspected for cleanliness, or the calibration is adjusted until successful verification is achieved.

With more advanced instruments such as the LabMaster NEO from Novasina, the process is automated. The instrument recognises which standard is being used, knows the true value, determines if the verification passed, and if not, applies a calibration adjustment.