Why an ECU cares about humidity
Water vapor takes up space in the intake charge that oxygen would otherwise fill. On a hot, sticky day an engine makes a little less power and can be less prone to knock than on a dry day at the same temperature. Most street cars never account for it, but for consistent racing, dyno comparisons or water injection tuning, knowing the humidity helps explain changes you'd otherwise blame on the tune.
On a Nexus ECU, the humidity channel can be used as a table axis or in tuning like any other input. If you don't have a sensor but know the day's humidity, you can set up a humidity input calibrated to a dial and enter it manually.
The three ways to describe humidity
| Type | What it measures | Where you've seen it |
|---|---|---|
| Relative | Water in the air as a percentage of the most the air could hold at that temperature and pressure | The weather forecast |
| Absolute | Mass of water compared with the total mass of air plus water in a volume | Engineering and lab work |
| Specific | Mass of water compared with the mass of dry air, often called water grains (grains per pound of dry air) | Drag racing weather stations |
Relative humidity alone is a bit misleading for engines, because the same percentage means very different amounts of water at different temperatures. Grains is the number drag racers track for that reason.
Wiring and sensor type
Humidity sensors connect to an analog input. Any spare AVI works, and the pullup is normally off. In the Wiring node, set Humidity Type to match the sensor:
- Relative (percentage) for a relative humidity sensor.
- Specific (water grains) for a specific humidity sensor.
- Absolute for an absolute humidity sensor.
The humidity type decides what you'll enter in the calibration, so set it before you calibrate.
Calibration
The main node holds the calibration, pairing the sensor's output voltage with a humidity value. Use the sensor maker's data for these points. If the curve is wrong, everything built on the channel is wrong too, so check the reading against a known reference, such as a local weather station, on a still day.
Settings node
- Calculated Humidity Channels: turn this on and NSP shows all three humidity types, whichever one your sensor actually measures. The conversions need air pressure, so the ECU uses your barometric pressure sensor when one is enabled. Without one, it assumes sea level (101.3 kPa), which will be off at altitude.
- Humidity Channels Update Conditions: humidity sensors can need up to 30 seconds to give an accurate reading. Pick the ECU channel and condition that starts the timer, so the channels only update once the sensor has had time to settle.
Display and diagnostics
Display Maximum is where the humidity gauges and bars top out, and it's also the highest value you can use in settings and tables. In the Diagnostics node you can enable high and low voltage DTCs plus a Maximum Humidity DTC. Each gets a severity, which ties it to an engine protection level.
Mount the sensor where it reads ambient air, away from radiator and engine heat. Pair it with an ambient air temperature sensor so you're seeing the full weather picture in your logs.
FAQ
Do I need a humidity sensor for a street car?
Rarely. It pays off on race cars chasing consistency and on anyone comparing runs across different days.
Which humidity type should I display?
Whatever you're used to. With calculated channels on, you can log all three and use grains for tuning decisions.
