Ecowitt WN31 Multi-Channel Temperature Humidity Sensor

Environmental sensors convert a physical quantity into an electrical reading: Temperature sensors (thermistor or digital probe) report air or surface temperature; a "temperature change" sensor watches for a rapid rise or fall and alerts on it. Humidity sensors measure relative humidity, usually via a capacitive element whose value changes with absorbed moisture. Barometric pressure sensors (e.g. BMP280) measure atmospheric pressure and small changes in it, often alongside temperature. Ambient light sensors report illumination level. Motion / tripwire sensors (passive infrared, or beam-break) flag when something crosses a detection zone. Combination units (the Mel 8704R bundles EMF, temperature and light) simply place several of these sensors in one housing. In paranormal work these readings are logged as context — a sudden draught, a pressure swing, a temperature drop — that can be cross-referenced against anything else observed.

Sensors lag and average. Air-temperature probes respond slowly and read the air immediately around the probe, so a "cold spot" detected in one position may simply be a draught or the probe equilibrating. They measure the cause-agnostic quantity only. A temperature drop is a temperature drop; the instrument cannot attribute it to a source. Placement dominates the reading. Near a window, door, vent, exterior wall or the investigators' own bodies, readings move for ordinary reasons. Motion / tripwire sensors fire on anything in the zone — people, pets, insects, blowing curtains — and PIR units specifically respond to moving heat, including draughts of warm air. Consumer sensors have real tolerances and can drift; two meters in the same room can disagree.

Draughts and HVAC cycling producing temperature and "cold spot" swings. Doors/windows opening, weather fronts and altitude effects moving pressure and humidity. Body heat and breath of the investigators warming or humidifying the local air. Motion sensors tripping on insects, pets, curtains, the team's own movement, or warm-air currents (for PIR). Sensor drift, slow response and unit-to-unit disagreement. Best practice: establish a baseline for each quantity before the session, record where each sensor sits relative to vents/windows/people, expect environmental causes first, and corroborate any change across more than one instrument before treating it as notable. These are standard environmental instruments; none is validated as a detector of anything paranormal.