Overload
The IEC 60846-1:2009 §8.8.2 overload evaluation — what an instrument must do in a field far above its range, why reading low is the dangerous failure, and what recovery must look like afterwards.
Last reviewed
Detectors do not necessarily saturate in an intense field. A Geiger–Müller tube can enter a regime in which the count rate falls as the dose rate rises: dead time after each event prevents the tube from registering subsequent events. The indication does not blank and does not peg at full scale — it reads low, within the normal display range, and is indistinguishable from a genuine measurement.
The consequence is that the indication decreases while the actual dose rate increases. This test exists to establish that the instrument does not present that failure mode.
Requirements
Two requirements, both of which must be met.
During exposure, the instrument must maintain an off-scale-high reading or an unambiguous overload indication. It must not present a number that could be mistaken for a real measurement.
Five minutes after the exposure ends, the instrument must return to normal operation within its specification. If it cannot — because of damage, a latched state, or incomplete recovery — it must display an unambiguous warning. An instrument that resumes an apparently normal display while reading incorrectly is the second failure mode this requirement addresses.
Exposure conditions
You expose for five minutes at a dose rate defined relative to the instrument’s own range, and the multiple depends on which range bracket the instrument falls into — a large multiple for low-range instruments, tapering for the high-range ones where a large multiple would be impractical or unattainable. The tool carries the standard’s bracket for each condition; enter the exposure you actually used in the condition field so the report records it.
Test each decade or each range separately if the instrument switches ranges — an instrument can behave correctly on its most sensitive range and badly on another, and one condition row per range records that correctly.
Checklist evaluation
There is no relative response here and no acceptance band. The result is two observations:
- did the off-scale or overload indication hold throughout? Yes / No
- was normal operation within specification restored five minutes later? Yes / No
Both must be Yes for a pass. The tool gives no verdict until both are answered, which is deliberate — “we did not check the recovery” and “the recovery was fine” must not look the same in a report.
Record the observation in the remarks field, not only the Yes or No. “Display showed OVL throughout; returned to 0,08 µSv/h background reading after 3 min” is auditable. “Yes” is not.
Continuous observation
The observation must be continuous. An instrument that goes off-scale, returns into the display range after ninety seconds, and goes off-scale again before the next observation has failed the requirement; an evaluation based on the state at the start and the end alone would record it as a pass.
Where the geometry permits, observe through a shielded window or on a remote display. Where it does not, record the display on camera.
Interpretation of a failure
The standard sets the pass condition; it does not tell you what to do with an instrument that fails. A failure here is among the more serious findings available, because the instrument is not merely inaccurate: it presents a plausible low indication in the conditions where an underestimate has the greatest consequence.
An instrument that fails overload but passes everything else is not usable for emergency response or for approaching an unknown source. It may still be perfectly serviceable for routine monitoring in a facility where the maximum credible dose rate is known and bounded. That distinction belongs in the report as a restriction, not as a footnote.