Why log humidity at all?
Water vapor takes up room in the intake charge that would otherwise be oxygen. On a dry day the engine gets a little more usable air per cycle than on a humid one, so the same map can feel different from season to season. A humidity input lets you see that in your logs, compare dyno sessions fairly, and build corrections if you ever need them.
You don't even need a physical sensor to use the function. If you know the humidity, you can set up the input so it's driven by a dial and type the value in manually.
Three ways to describe humidity
| Type | What it measures | Where you see it |
|---|---|---|
| Relative | How much water the air holds as a percentage of the most it could hold at that temperature and pressure | Weather apps, most off-the-shelf sensors |
| 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 dry air alone, often quoted as grains of water per pound of dry air | Dyno weather stations |
In the Wiring tab, set Humidity Type to whatever your sensor actually outputs: Relative (percentage), Specific (water grains) or Absolute.
Wiring the sensor
Pick the input under Connections. An analog voltage input (AVI) is the preferred choice. A synchronized pulsed input (SPI) works too when you've run out of AVIs.
| Input | Setting | Our advice |
|---|---|---|
| AVI | Pull up: switches in a 1k resistor to 5V | On for a two-wire sensor. Off if the sensor has its own power supply |
| SPI | Sensor Type | Set to Hall Effect whenever it is reading an analog signal |
| SPI | Pull up: a weaker 10k resistor to 5V | Same rule as the AVI pullup |
If the humidity sensor is also read by another ECU, such as a factory ECU in a piggyback install, that ECU usually supplies the pullup already. Turn the Haltech pullup off or you'll pull the reading off calibration.
Settings tab
Enable Calculated Humidity Channels gives you all three humidity channels at once, whichever type your sensor reads. The conversion needs air pressure, so the ECU uses your barometric pressure sensor if one is enabled. Without one, it falls back to standard sea-level pressure, 14.7 psi (101.3 kPa), which is fine near the coast and less accurate at altitude.
Update conditions
Humidity sensors are slow. They can need up to 30 seconds to settle on a true reading, so the humidity channels only update when a condition you set is met. Choose an ECU channel with Select, a comparison under Maths (equal, not equal, greater, less, or the "or equal to" versions), and the Value it has to meet. That condition starts a timer, and the channels refresh from there. A sensible approach is to tie the update to a stable condition, for example engine running, so you're not logging a half-settled number from right after key-on.
Calibration, display and diagnostics
The Calibration tab is where you load or save a sensor calibration. ESP ships with a number already installed. Open one with the yellow folder icon, or build your own from the manufacturer's voltage data and save it with the blue disk icon so you can reuse it on the next car. Display Maximum on the Display tab just sets the top of the gauge in ESP.
| Diagnostic | Triggers when | DTC |
|---|---|---|
| Raw Min | Sensor voltage drops below your limit, usually a short or open circuit | P00F2 |
| Raw Max | Sensor voltage rises above your limit, usually a short or open circuit | P00F3 |
| Operating Max | Humidity jumps suddenly past a plausible level | P00F5 |
All three are off by default. If Engine Protection is active, each can be given a DTC severity. For a sensor that's mostly for logging, we set the raw limits to catch wiring faults and keep the severity low. The DTC index lists the codes if you see one pop up.
Troubleshooting
| Symptom | Usually |
|---|---|
| Reading pinned at zero or maximum | Open or shorted wiring, or the pullup set wrong for the sensor type |
| Value never changes | Update condition never true, so the timer never starts |
| Numbers look off at altitude | No baro sensor, so the calculation is assuming sea-level pressure |
| Reading slowly drifts after start-up | Normal settling. Allow the sensor its time before trusting it |
Humidity sensor FAQ
Which humidity type should I log?
Whatever your sensor reads natively, then enable the calculated channels. Relative humidity is easiest to sanity-check against a weather report; specific humidity is the most useful for comparing runs.
Do I need an air temperature sensor too?
For relative humidity to mean much, yes. Pair it with an ambient air temperature sensor so you have the full weather picture in your logs. For other sensor setup, see generic sensors.
