Tech Note 551 Detail

Proper placement of the weather and humidity sensors.

Affected Products:

PS-2154A PASPORT 6-in-1 Weather Sensor
PS-2124a PASPORT Humidity/Temp/Dew Point Sensor

Problem/Symptom:
How should I mount my weather or humidity sensor?

PASCO Solution:

The PS-2154A PASPORT Weather Sensor contains three physical sensing elements (temperature, barometric pressure, and relative humidity) and the PS-2124(A) Humidity/Temp/Dew Point Sensor contains two physical sensing elements (temperature and relative humidity). All other reported measurements are derived from these three measurements.

To acquire meterological measurements that reasonably agree with local weather stations, requires proper sensor placement. Below are some considerations that are necessary for proper placement:

Shade

Shielding the sensor from solar radiation is necessary to avoid local heating that would make the sensor temperature significantly exceed the local air temperature. The weather sensor should be shielded from direct sunlight by a low-thermal mass, double-walled, high-albedo (glossy white or metallic), louvred shade.

The shading should not come from plants which cool the ambient air temperature via transpiration.

(An additional reason for shading the PS-2154A is that the physical humidity sensor used is very light sensitive.)

Vent

As air has a relatively low thermal conductivity (only 4-5% that of liquid water), obtaining temperature measurements that are close to the specfied accuracy of the sensor, requires that that there be sufficient air flow both to dissipate the heat generated by the weather sensor's electronics and other sources of heat, and to increase the responsiveness of the sensor to changes in air temperature. For the PS-2154A, ambient air flow is sufficient, provided that large unobstructed vents are integrated into the enclosure providing shade. (With the PS-2154(non-A) model, it is recommended that one use a fan to ensure adequate air flow because of the higher heat production of the sensor.)

Shed Precipation

To avoid a microclimate from developing due to evaporative cooling, one does not wish to have precipitated water pool or condense near the sensor. For this reason, the sun shade should be hydrophobic and sufficiently sloped to avoid accumulation of water near the sensor.

Avoid Thermal Mass

The shade should be made of a low thermal mass material, such as plastic, as the heating and cooling of the shade will tend to delay any response of the sensor to ambient air temperature.

Elevate

Due to evaporative cooling and radiant heating, there is typically a strong thermal gradient near ground level. By convention of the World Meteorological Organization the temperature sensor should be elevated to be 1.25 to 2.0 meters above the ground.

Recommended Enclosure

The previous considerations can be combined into a recommendation for an engineered enclosure for meterological measurements with the weather sensor:

  1. Support the sensor in a box with open white plastic slats to shade the sensor from direct sunlight.
  2. Use double-walled open plastic structures on the roof and floor for thermal isolation from radiant heating of these surfaces.
  3. Slope the roof and louvre slats to avoid accumulation of water.
  4. Elevate the sensor 1.25 - 2.0 m from the ground to avoid the intense thermal gradient near ground level.
  5. For the older PS-2154 Weather Sensor, forced-air ventilation is recommended. (For the PS-2154A Weather Sensor it is sufficient to rely on natural ventilation.)

The primary source of this information is the World Meterological Association Guiide to Instruments and Methods of Observation. The guide is recommended for additional information.

Creation Date: 10/20/2005
Last Modified: 12/6/2012
Mod Summary: