CPM Calculation Mode
CPM mode picks how the displayed CPM is calculated. Set it on the Config page, Input tab, in the advanced settings. Changes apply at once, no reboot.
Picking a mode
- Not sure? Leave it on Bucket (the default).
- Watching a hot source and want the fastest response? Pick Adaptive fast.
- Want the steadiest reading from a steady source? Pick Live ring.
- Want a fixed one-minute window for comparing devices? Pick Fixed 60s.
- Want a ring mode without the slow start after boot? Pick Fast start.
The five modes
| Value | Menu name | Behaviour |
|---|---|---|
| 0 | Fast start | Uses recent pulses just after boot, then blends into Bucket once a full window of data exists. |
| 1 | Live ring | Rate over the last 256 pulses. Very smooth at background, quick at high rates, slow to follow change at very low rates. |
| 2 | Fixed 60s | Rate over the last 60 seconds, capped at 64 pulses, so the window narrows above about 64 CPM. Can read 0 if no pulse landed in the last minute. |
| 3 | Bucket (default) | Average of the per-second counts over the last 60 seconds. Stable, predictable, slowest to respond. |
| 4 | Adaptive fast | Rate over the last 19 pulses or 5 seconds, whichever is shorter. Fast, but jumps around at background rates. |
On serial builds for counters that only report CPM, Bucket uses the CPM Window setting (default 30 s) instead of 60 s. These counters already smooth their own CPM, so the display lags by roughly their window plus yours. See Serial Counters.
What changes when I switch modes
The mode changes the live CPM, CPS and dose shown to your own consumers:
/statuspage top numbers- OLED display
- Live serial output / GeigerLog connection
- MQTT (your own broker / Home Assistant)
- Webhook (your own URL)
- ThingSpeak (your own channel)
- Local UDP broadcast between your ESPGeiger devices
Public aggregators always get a steady 60-second value, so the network sees consistent data from every device:
- Radmon (radmon.org)
- URADMonitor (uradmonitor.com)
- Safecast (safecast.org)
- ESPGeiger Stations (stations.espgeiger.com)
- GMC / GeigerLog protocol
The mode never affects the 5- and 15-minute averages, total and lifetime counts, the 24-hour history or the inter-pulse histogram.
How reliable is a reading?
The Poisson nature of radiation means every reading has statistical noise that shrinks as more pulses accumulate. The 95% confidence interval as a percent is roughly 196 / sqrt(N), where N is the number of pulses behind the reading. Quick reference:
| Pulses behind reading | Confidence | Meaning |
|---|---|---|
| 4 | ±98% | Mostly noise |
| 16 | ±49% | Noisy |
| 64 | ±24% | OK |
| 100 | ±20% | OK |
| 256 | ±12% | Solid |
| 1000+ | ±6% | Very solid |
At background rates, collecting ~256 pulses takes around 8 minutes; at higher rates it happens much faster. Live ring and Fixed 60s tend to give the most solid readings at low rates because they widen the window to collect more pulses.
Details of the calculations, rate limits and memory use are on the CPM mode internals developer page.
Cross references
- Counter Configuration - debounce and dead time settings that affect all modes.
- HV Configuration - high voltage and tube ratio settings that affect what counts as a pulse in the first place.