Sensor calibration
Correct a sensor that reads consistently high or low, and filter out spikes, without touching your hardware.
Every station drifts. A barometer reads 2 hPa low, a thermometer catches an hour of afternoon sun, a tipping bucket under-catches in heavy rain. Pro Weather already discards impossible readings automatically, but a reading that is merely wrong looks perfectly plausible and gets stored like any other.
Calibration is where you correct that. It lives under your site → Calibration in the dashboard, and it is set per station.
How a correction works
Each reading is adjusted as:
corrected = raw x multiplier + offset- Offset shifts every reading by a fixed amount. Use it for sensors that read consistently high or low: temperature, humidity, pressure, wind direction.
- Multiplier scales every reading. Use it for sensors that mis-scale
rather than mis-read: a rain gauge catching 4% too little needs a
multiplier of
1.04, not an offset.
Leave a field blank to change nothing. Values are entered in your site's unit system, so an imperial site types offsets in °F and inHg.
What you can correct
| Sensor | Knob |
|---|---|
| Outdoor temperature | Offset |
| Outdoor humidity | Offset |
| Pressure (sea level) | Offset |
| Pressure (station) | Offset |
| Indoor temperature and humidity | Offset |
| Wind direction | Offset, in degrees |
| Wind speed and gust | Multiplier |
| Rainfall | Multiplier |
| Solar radiation | Multiplier |
| UV index | Multiplier |
Derived values are recalculated from the corrected readings, so nothing contradicts itself: correct the air temperature and your dew point, heat index, wind chill, humidex, apparent temperature and cloud base all follow.
Finding the right numbers
- Temperature: compare against a reference thermometer in a properly shielded, ventilated spot, over a full day rather than one moment.
- Pressure: compare against a nearby official station or an airport METAR at the same minute. Pressure is the most commonly mis-set sensor and the easiest to verify.
- Humidity: ageing capacitive sensors usually read low. A salt test (a saturated salt solution in a sealed jar reads 75% at room temperature) gives you a reference point.
- Rainfall: pour a measured volume through the funnel slowly and compare the total. Set the multiplier to the ratio of what you poured over what was recorded.
Start small. A large correction usually means something else is wrong, such as a sensor in direct sun or a blocked rain funnel, and no amount of arithmetic fixes a badly sited sensor.
Spike limits
A spike limit is the largest change you accept between two consecutive readings. Radio interference, a dying battery or a loose connector produce single readings that jump far beyond anything the weather can do. A reading that fails its limit is dropped rather than corrected, exactly like an impossible value: it is treated as if the station never reported it.
- Temperature, humidity, pressure, wind and gust limits cap the change between readings.
- The rain limits are absolute caps on the rate and on the amount recorded in a single reading, because a phantom bucket tip has no previous value to be compared against.
Two readings more than an hour apart are never compared, so a gateway coming back online after a quiet night is not mistaken for a spike.
Keep limits generous. A real cold front can move the temperature several degrees in minutes, and a limit that is too tight quietly deletes real weather. If the page shows that readings are being dropped on calm days, raise the limit.
What calibration does and does not change
- Corrections apply the moment data arrives, before anything is shown or stored. Your public site, your charts, your records and your CSV exports therefore always agree with each other.
- Older history is not rewritten. A correction changes new readings, plus roughly the last hour, which your station's archive still hands back on every refresh and which is re-saved each time. Everything before that keeps the values it was stored with, so your charts show a small step at the moment you saved the correction. This is deliberate: silently rewriting months of stored observations is not something we will do behind your back.
- Imported history is not corrected. Data brought in from weewx or CumulusMX was already calibrated by that software.
- Calibration is per station, so a site with more than one station calibrates each separately.