Verifying Computer Clocks in Production

Another good question asked online, that we share here...

Q. In a mechanically driven process, when Time is a critical parameter, the timer would be qualified for accuracy and reliability. Probably challenged against another calibrated clock for various durations that would align with the process requirements/settings. No doubt Preventive Maintenance and Calibration schedules would be developed and approved during OQ.

For automated processes that have Time as a critical parameter and the Time is measured by the PLC internal clock, what strategies are best to provide documented evidence that the internal clock is accurate, reliable (especially over time).

A. It's a good question - I can remember a certain DCS (back in the 1980's) that had two versions of a real time clock chip. One had a ceramic chip and the other plastic. The plastic clock chips could lose more that 15 minutes in an hour (honestly - I had to write the code to work around the issue).

On the basis of risk, and following ASTM E2500 / GAMP principles of leveraging the work of your supplier, if you believe that things have improved in the last 20 years you can assume that the system is acceptably accurate and include a check of elapsed time as part of your OQ or PQ.

Confirming this across a number of intervals and process runs over time should allow you to confirm that the clock is sufficiently accurate across a range of intervals with negligible drift over time (you will need to define what 'acceptable' is in your protocol and use a time reference to measure real time).

Taking a risk based approach, I would only expect to specifically test the clock if I had reasons to suspect that their might be an issue.