Setting calibration intervals from your own history

Most instruments start life with an interval copied from somewhere: the maker's suggestion, a corporate standard, or the interval of the instrument it replaced. Twelve months is common. It is a reasonable place to start and a poor place to stay.

What an interval is for

The interval is a bet that the instrument will still be inside tolerance when it is next checked. Too long and the bet is lost: as-found failures, and measurements you can no longer trust. Too short and the workshop spends its time confirming instruments that never drift.

Start from what matters

• How critical is the measurement? A safety trip or a product release deserves a shorter interval than an indication on a local panel.
• How harsh is the service? Heat, vibration, pulsation and dirty fluids all speed up drift.
• How tight is the tolerance compared with the instrument's own accuracy? Little margin means less time before a failure.

Then let the as-found results decide

Every calibration produces evidence. The as-found readings say how far the instrument moved since last time. Collected over a few cycles, they tell you whether the interval is right.

• Several passes in a row with plenty of margin: the interval can be extended, a step at a time.
• Passes that sit close to the limit: keep the interval and watch the trend.
• An as-found failure: shorten the interval, and look at what the instrument was used for since its last calibration.

Change intervals in steps, record why each change was made, and apply the same reasoning to similar instruments in similar service.

Keep the plan visible

An interval only works if the due date is in front of the people who do the work. Calibrator sets the next due date when a calibration is approved, and the overview shows each month's workload and every instrument that is overdue or due soon, area by area.