Determining the calibration interval for UV measuring devices
A calibration certificate does not automatically expire on a uniformly prescribed date. The appropriate recalibration interval is derived from stability, use, environmental exposure, the permissible measurement deviation and operational obligations of proof. An interval of twelve months is a workable starting point for many UV measuring instruments, but it must be confirmed or adjusted on the basis of the documented measurement history.
Why the reading shifts at all
Three causes act together and none is visible from the outside: the detector ages under UV exposure, the transmission of diffuser and filter falls through contamination and solarisation, and the electronics drift with temperature and operating hours. All three shift the reading slowly and steadily – an instrument showing a plausible value may be well outside its specification.
How the appropriate interval is determined
ILAC-G24:2022 and OIML D 10:2022 describe the approach: an initial interval is set and then revised on the basis of your own calibration results. If the measured deviation stays small over several cycles, the interval can be extended; if drift appears, it is shortened. The following influences indicate the direction:
| Influence | Indicates a shorter interval | May indicate a longer interval |
|---|---|---|
| Use | Frequent, high doses, changing sources | Infrequent, controlled use |
| Environment | Heat, humidity, contamination, process chemicals | Clean laboratory environment |
| Measurement history | Visible drift or a repair | Several stable calibration sequences |
| Tolerance | Tight process or safety limit | Wide permissible measuring span |
| Evidence obligation | Customer or normative deadline | Manageable internally on a risk basis |
Twelve months is therefore a recommendation and a starting point, not a general legal or normative requirement. Binding deadlines may, however, follow from customer contracts, management systems or application-specific regulations – in drinking water disinfection, for example.
Storage and intermediate checks
A stored instrument ages more slowly, but it does not stop ageing altogether: humidity, temperature and incident light continue to act. For long idle periods an intermediate check against a second, likewise calibrated instrument or against a stable source is advisable. It does not replace a calibration, but it shows whether anything has changed between two dates.
The calibration procedure itself is described under UV calibration.
Frequently asked questions on the calibration interval
How often do industrial UV sensors have to be calibrated?
There is no generally prescribed date. Twelve months is a workable starting point for many UV measuring devices, to be confirmed or adjusted against the instrument's own measurement history – this is the procedure described in ILAC-G24:2022 and OIML D 10:2022. If the deviation stays small over several cycles, the interval can be extended; if drift appears, it is shortened. Frequent use, high doses, heat and contamination argue for a shorter interval.
How do you tell that a calibration interval was too long?
From the deviation reported by the recalibration – not from how the instrument behaves in service. Detector ageing under UV exposure, falling transmission of diffuser and filter through contamination and solarisation, and drift in the electronics shift the reading slowly and steadily. An instrument that displays plausible values can lie well outside its specification; the stated ageing of under 3 % per year applies only to operation within specification. If the deviation exceeds the permissible measurement error, the interval is shortened.
Author: Dr. Mark Paravia
Dr.-Ing. Mark Paravia is the managing director of Opsytec Dr. Gröbel GmbH in Ettlingen and heads the accredited calibration laboratory. Following his research on pulsed xenon excimer discharges at the Institute of Lighting Technology at KIT, his current focus is on optical radiation measurement technology. He is vice-chair of the DIN Standards Committee FNL 7 “Optical Radiation,” and a member of the DVGW Project Group on UV Disinfection.
