Every PIR datasheet states a mounting height, usually as a range: 1.8 to 2.4 metres, or 2.2 to 3 metres for the long-range models. It reads like an installation tolerance. It is not. The number is where the lens projection was designed to land, and moving the sensor outside the range reshapes the coverage pattern in ways that no sensitivity setting can recover — a sensor mounted 40 centimetres too low delivers a different product than the one that was ordered.
Why height rewrites the pattern
A PIR sensor does not detect a room; it projects a lens-defined pattern of segments into it, and the pattern's geometry assumes the element sits at the design height. Tilt the sensor down from 2.4 metres and the near segments — designed to cover the approach floor — fold back under the unit, creating a dead zone right where people walk. Mount it low and the far segments overshoot the back wall, putting detection depth outside the room while the near coverage bunches up and starts seeing the backs of chairs, radiators, and anything else warm the pattern was never meant to meet.
The compensation instinct is to raise sensitivity. That restores trigger counts but also restores everything the pattern was excluding: the draft, the window glare, the radiator cycle. The height was doing discrimination work, and a slider cannot do that work.
What the range on the datasheet encodes
A 2.2-to-3-metre range on a long-range unit is not generosity; it is the span across which the lens pattern stays inside its stated depth and angle. Within the range, the trade is explicit: lower mounting buys a wider, shallower pattern suited to corridors and rooms where the approach crosses the pattern; higher mounting buys reach, at the cost of a dead zone under the unit that grows with height. Outside the range, the manufacturer is no longer describing the product the buyer receives.
For wholesale buyers the practical question to ask is not the maximum height but which pattern variant exists at which height — some platforms ship alternate lenses for high-mount and corridor use precisely because one projection cannot serve both. A supplier who offers the same housing with a swap-in lens is offering a different product line disguised as an accessory.
The ceiling-versus-corner decision inherits the problem
Mounting position is usually a style argument — corner mounts look intrusive, ceiling mounts disappear. Optically it is a geometry argument. A corner mount at 2.3 metres looks across the room diagonally, which suits a dome's fan and gives the longest unobstructed segments. A ceiling mount projects a cone downward; its value is 360-degree coverage of the space under it, and its weakness is a detection boundary that shrinks with ceiling height — the stated coverage diameter is usually quoted at 2.4 metres ceilings, and drops roughly proportionally above that. A ceiling unit spec'd for 2.4 metres and mounted at 3.6 metres covers a third less floor than the datasheet circle suggests.
Neither position is wrong. What is wrong is choosing the position after ordering the lens, when the coverage math is already fixed.
What belongs in the specification
Three lines keep the geometry honest. First, the mounting height and position per room, chosen against the lens variant, not after it. Second, for ceiling models, the coverage diameter restated at the actual ceiling height of the project. Third, confirmation that the installation manual's height range matches the lens shipped — the mismatch between a high-mount lens and a low mount plate is exactly the kind of error that survives sample review and surfaces at the first site.
XingLiao manufactures PIR motion sensors with height-specific lens variants across shared platforms, specified per deployment as OEM/ODM programmes. Buyers can send the mounting survey and receive the lens and height pairing for each room type.
The short version
The height on the datasheet is part of the optical design, not an installation suggestion. Mount within the range, restate ceiling-mount coverage at the real ceiling height, and pick the lens variant for the height and route — then the sensitivity slider can go back to doing its actual job, which is fine-tuning, not rescuing geometry.
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