INTRODUCTION
Properties of materials and nearly all biological, chemical, and physical processes are temperature dependent.
Temperature is also a fundamental weather variable. As a result, temperature is perhaps the most widely
measured environmental variable.
Fan-aspirated radiation shields are designed to shield temperature and humidity sensors from incident solar
radiation while maintaining equilibrium with ambient air through active aspiration by a fan. Typical applications of
fan-aspirated radiation shields include air temperature and humidity measurements in weather networks, often
for weather forecasting. In addition, air temperature and humidity are critical input variables for calculation of
evapotranspiration and irrigation scheduling. Fan-aspirated shields are also important in the precise measurement
of air temperature and humidity gradients above the land surface.
Apogee Instruments model TS-100 fan-aspirated radiation shields are made from high-quality injection-molded
plastic with low thermal conductivity, maximum weather resistance, and a gloss white finish for high reflectivity.
The small size makes it easy to work with and reduces the surface area exposed to incoming radiation. The shield is
designed as a lightweight, low power, and low cost alternative to other fan-aspirated and naturally-aspirated
(passive) shields for air temperature and humidity measurements. The efficient, low-power fan draws ambient air
into the shield using a rocket nozzle contour to maximize airflow around the internal sensors. The curved inlet
helps draw air into the shield during high cross winds. These unique properties make the TS-100 ideal for use in
remote locations with limited power supply.
The TS-100 does not come packaged with any other sensors to accommodate users using other probes, however,
TS-100 shields are designed to optionally accommodate PRTs with a custom adapter. Apogee strongly
recommends using the ST-110, and recommends the TS-110 fan-aspirated radiation shield package, which comes
with a precision air temperature sensor (model ST-110, thermistor). ST-110 air temperature sensors are
weatherproof, have excellent long-term stability, and are designed for continuous air temperature measurement
when housed in the TS-100. ST-110 thermistors offer improved accuracy over thermocouples because they do not
require accurate measurement of datalogger panel temperature. The advantages of thermistors over platinum
resistance thermometers (PRTs) are high signal-to-noise ratio, requirement of only a single-ended channel for
measurement, and lower cost. Accelerated aging tests indicate that the epoxy-encased ST-110 thermistor has a
long-term stability equal to more expensive reference PRTs.
In 2019, Apogee added two additional sensor packages. The TS-120 package includes a TS-100 and an EE08 air
temperature/RH probe manufactured by E + E Elektronik in Austria. The version sold by Apogee Instruments
(Apogee model number EE08-SS) includes a M12 stainless steel connector and custom cable with a ninety degree
connector that optimizes the fit of the probe inside the Apogee TS-100 fan-aspirated radiation shield. The EE08-SS
offered by Apogee includes the proprietary coating from E + E Elektronik for the relative humidity sensing element
that provides maximum long-term stability. Additionally, the temperature and RH sensing elements are protected
by a stainless steel filter for maximum long-term stability. The TS-130 package includes a TS-100, EE08-SS, and ST-
110.
Measurement errors due to thermal conduction of heat to the sensor are minimized by reflective white heat shrink
tubing, and shading of lead wire on the north side of the shield (in the northern hemisphere; south side of shield in
southern hemisphere). Thermal conductivity to the precision thermistor is further minimized by using constantan
wire, which has twenty-fold lower thermal conductivity than copper wire.