Where to place climate sensors

The single most common reason a climate record turns out to be worthless is not the instrument. It is that the instrument was measuring something other than what the person reading the data believes it was measuring.

What a sensor actually measures

A logger reports the air within a few centimetres of itself. If that air is in a stagnant pocket behind a cupboard, that is what you get. If it is in the plume of warm air rising off a radiator, that is what you get. The room average, which is what everyone assumes they are reading, is a construct that no single sensor ever measures.

Placements that ruin a record

On an external wall. The wall surface is colder than the room in winter and lags it by hours or days. A sensor mounted flat against it reads a boundary layer, systematically shows higher relative humidity than the room, and will make a perfectly adequate space look like a mould risk.

Above or beside a heat source. Radiators, pipe runs, lighting transformers, and the top of a display case with lamps inside. All of these produce a local hot spot and therefore a local dry spot.

In direct sun, at any time of day. A sunbeam that crosses a shelf for forty minutes each morning will put a spike in the record every single day, and it is a spike in the sensor, not in the collection.

In an airflow. Directly under a diffuser, beside a door that opens frequently, or in front of a fan coil. The readings will be real but they describe the supply air, not the space.

On the floor. Cold air pools, floors are cold, and floor level is the wettest part of most rooms. This is a legitimate place to measure if the collection is on the floor, and misleading otherwise.

Inside a case, described as a room reading. A sealed case has its own climate by design. Logging inside it is often exactly right, but the record has to be labelled as case data or someone will later average it with room data.

The working rules

• Place the sensor at the height of the objects, in free air, at least a hand’s width from any surface.
• Keep it away from external walls, heat sources, direct sun and supply air.
• One sensor per climate zone. Two rooms that differ in heating, orientation or use are two zones.
• One sensor outdoors, shaded and ventilated, for the whole site.
• Inside sealed cases, one sensor each for any case holding sensitive material, logged and interpreted separately.

Write it down, to the shelf

A record that says “Store 2” is nearly useless in five years. The note should say which bay, which shelf, and what is around it: “Store 2, bay 7, shelf 3, 1.4 m, 300 mm clear of internal wall, no heat source within 2 m.”

This sounds fussy until the first time a record shows an unexplained shift and nobody can establish whether the sensor was moved. When a sensor does get moved, record the date, because the discontinuity in the data is otherwise indistinguishable from a real change in the building.

Verifying a placement

Before committing a sensor for a year, park a second logger somewhere obviously different in the same room for a fortnight, and compare. If the two agree closely, the room is well mixed and one sensor represents it. If they diverge by several degrees or ten points of humidity, you have two zones and you need to decide which one you actually care about. That fortnight of work will save an argument later. Reading climate data covers how to compare the two series without fooling yourself.