Construction of a Compact Low-Cost Radiation Shield for Air-Temperature Sensors in Ecological Field Studies

Summary

With the advent of small, low-cost environmental sensors, it is now possible to deploy high-density networks of sensors to measure hyper localized temperature variation. Here, we provide a detailed methodology for constructing a compact version of a previously described custom-fabricated radiation shield for use with inexpensive thermochrons.

Abstract

Low cost temperature sensors are increasingly used by ecologists to assess climatic variation and change on ecologically relevant scales. Although cost-effective, if not deployed with proper solar radiation shielding, the observations recorded from these sensors will be biased and inaccurate. Manufactured radiation shields are effective at minimizing this bias, but are expensive compared to the cost of these sensors. Here, we provide a detailed methodology for constructing a compact version of a previously described custom fabricated radiation shield, which is more accurate than other published shielding methods that attempt to minimize shield size or construction costs. The method requires very little material: corrugated plastic sheets, aluminum foil duct tape, and cable ties. One 15 cm and two 10 cm squares of corrugated plastic are used for each shield. After cutting, scoring, taping and stapling of the sheets, the 10 cm squares form the bottom two layers of the solar radiation shield, while the 15 cm square forms the top layer. The three sheets are held together with cable ties. This compact solar radiation shield can be suspended, or placed against any flat surface. Care must be taken to ensure that the shield is completely parallel to the ground to prevent direct solar radiation from reaching the sensor, possibly causing increased warm biases in sun-exposed sites in the morning and afternoon relative to the original, larger design. Even so, differences in recorded temperatures between the smaller, compact shield design and the original design were small (mean daytime bias = 0.06 °C). Construction costs are less than half of the original shield design, and the new design results in a less conspicuous instrument that may be advantageous in many field ecology settings.

Introduction

In light of anthropogenic global warming, there has been a growing interest in recording air temperature in a variety of settings to understand and predict ecological responses to climate change^1,2,3. With the advent of small, low-cost environmental data recorders (also referred to as data loggers, thermochrons, or hygrochrons), it is now possible to deploy high-density networks of sensors to measure hyper localized temperature variation, increasing ecologists' ability to more directly observe the ambient environmental conditions experienced by organisms and ecosystems under study. Compared to existing, well-calibrated and rigorously tested—but sparsely distributed—permanent weather stations, such networks present opportunities to assess climatic variation on ecologically relevant scales but may reduce accuracy or comparability among studies if inconsistently or inappropriately deployed.

Near-surface air temperature sensors typically require some type of solar radiation shielding to prevent direct heating of the sensor element, which would result in erroneously warm measurements. Common ways to limit sensor bias include: 1) using existing environmental features such as trees for shading⁴, 2) bias correction and sensor calibration⁵ that derived corrections based on the thermal properties of sensors, and 3) the use of manufactured or custom fabricated shields^6,7. Many researchers choose to use custom fabricated shields because of the low-cost and easy deployment, and necessity in situations where environmental conditions do not provide natural shading. However, a review of the ecological literature indicated that the design of custom fabricated shields varies widely among studies, and individual designs are rarely tested for accuracy. Untested shields can be susceptible to poor choice of materials and design that cause additional heating of the air molecules immediately surrounding the sensor, direct absorption of solar radiation by the sensor itself, or both-leading to average biases of up to 3 °C⁷. On the other hand, simple and cost-effective designs^6,7 are quite effective at shielding sensors (biases of 1 °C or less) and are comparable to commercially manufactured radiation shields.

Here, we provide a detailed methodology for constructing a previously evaluated custom fabricated radiation shield⁷ for use with inexpensive thermochron temperature sensors. The shield design is a modification of one previously described and tested in an open Ponderosa Pine forest setting⁶. In recent tests of several custom-fabricated shield designs, this montane-tested shield resulted in the lowest biases when paired with small thermochrons⁷, but we found it cumbersome and too conspicuous to deploy in the field. The design protocol proposed here reduces the dimensions of the radiation shield by 50%. Such a reduction in size has several benefits: 1) it is less conspicuous and therefore less susceptible to tampering, 2) it can be more feasibly used in a wider variety of ecological settings where space is limited (e.g., on smaller urban street trees), 3) it is more accurate than other published shielding methods that attempt to minimize shield size or construction costs⁷, and 4) it is less expensive than the original, larger design due to the reduced quantity of construction materials required. After describing the construction methods, we explore the effect of the size reduction on sensor accuracy relative to the original shield design using results from a field trial conducted under high downward solar radiation conditions.

Protocol

1. Construction of the Radiation Shield

1. Using a utility knife, cut the corrugated plastic sheets into squares (Figure 1A). One 15 cm square and two 10 cm squares will be needed for each shield.
2. Cuts for the top layer of the small radiation shield (Figure 1B; left image):
   1. On the 15 cm square, measure 4 cm from one edge and draw a line with a pencil. Use a straightedge as a guide to score along the line. (Herein, “scoring” means using a knife to make a cut that goes through only one layer of corrugated plastic sheet, rather than the entire sheet.) Henceforth this edge of the square will be referred to as the “top” (Figure 1B; left image).
   2. Measure 3.8 cm from the edges that are perpendicular to the 4 cm line. Use a straightedge as a guide to score from the bottom up to the 4 cm line (Figure 1B; left image).
   3. Draw a line from both corners above the 4 cm line to the junction of the 4 cm and 3.8 cm lines. Cut along this line (Figure 1B; left image).
3. Cuts for the middle and bottom layers of the small radiation shield (Figure 1B; middle and right images):
   1. Using a straightedge, draw a 6 cm square in the middle of each 10 cm square (Figure 1B; middle and right images).
   2. Score all around the 6 cm square, and from each corner of the 6 cm square to the outer corners of the 10 cm square (Figure 1B; middle and right images).
4. Use aluminum foil tape to completely cover the scored side of the 15 cm square and one of the 10 cm squares, and the un-scored side of the other 10 cm square.
5. Using a 1/4” drill bit, drill holes as shown in Figure 1C, in each of the shield layers.
6. Attach a temperature sensor to the underside of the 10 cm square, which is taped on the scored side and has the two holes drilled into the middle, by running the cable tie through the eyelet of the sensor housing (or its mounting device) and through the holes in the 10 cm square (Figure 1D).
7. Folding the sheets.
   1. Fold the 15 cm sheet along the scored lines. Pressure may be needed in case the tape makes the sides tight and difficult to fold.
   2. Tuck the small triangular flaps on the inside of the larger back flap. When this is done correctly, only taped sides are visible from above. The cut edge of the back flap should be flush with the folded sides.
   3. Use another layer of aluminum tape to secure the folded sides to the back flap. The back flaps could also be stapled together, with a heavy-duty stapler, for added strength.
   4. Take the 10 cm sheets and pinch the sides together along the diagonal scored line. Using a heavy-duty stapler, staple the pinched sides together (Figure 1E). The end product will have a square-bowl shape.
8. Tying the sheets together with 20 cm cable ties.
   1. Beginning with the 10 cm sheet taped on the unscored side, with three holes, place the taped side down. Thread a cable tie through the left back hole of both 10 cm sheets. Leave 2 cm vertical spacing between the two sheets to ensure air flow around the temperature sensor. Repeat this step for the back right hole (Figure 1E; middle and right images).
   2. Take the 15 cm sheet and pass a cable tie through the two side-by-side holes, in the back left (Figure 1E; left image). Attach this tie to the 10 cm sheets, also leaving 2 cm of space between the 15 cm sheet and the top of the upper 10 cm sheet. Repeat this step for the two side-by-side holes in the back right (Figure 1E; left image).
   3. Finally, pass one cable tie through all three holes in the front of the sheets (shown by the arrow; Figure 1E). Tighten the cable tie, ensuring the space is even between all three sheets (Figure 1F).
9. Drill additional holes into the back end of the final assembled product to facilitate mounting, where needed. Wherever the shield is mounted, ensure that the three sheets lay parallel to the ground.

Figure 1: Step-by-step instructions to construct a small radiation shield. (A) 15 cm and 10 cm squares are cut out of the large sheet of corrugated plastic. (B) The 15 cm sheets are then cut and scored, and the 10 cm sheets are scored to allow bending of the shield to the correct shape. (C) Holes are drilled on each sheet. (D) The sensor is tied to one of the 10 cm sheets. (E) The shield is assembled using several cable ties. (F) The final shield is ready for installation. Please click here to view a larger version of this figure.

Results

Representative results using thermochrons outfitted with the new, smaller shield design, the original larger shield design, and the thermochrons with no radiation shield are shown in Figure 2 and Figure 3. These data were recorded at a fully exposed rural location near Raleigh, NC (35.728°N, 78.680°W), and were affixed to a well-calibrated permanent weather station outfitted with a VAISALA platinum resi...

Discussion

The accuracy and repeatability of air temperature measurements depend on the use of an appropriate solar shield that protects the sensor from direct and reflected solar radiation. Here we describe the construction of such a shield that is more compact in size, less expensive, or faster to construct than similar, previously described devices⁶, without sacrificing accuracy. 94% of the recorded temperatures for the thermochrons outfitted with the smaller shield were within 1.0 °C of the best per...

Materials

Name	Company	Catalog Number	Comments
Multipurpose Aluminum Foil Tape	Nashua	1087671	48 mm width
8" cable ties	DTOL	GEN86371	NA
Corrugated plastic sheet	Highway Traffic supply	hts18X24COROW	White sheet 18"L x 24"W, 5-pack
Standard utility knife	NA	NA	NA
Standard Scissors	NA	NA	NA
Heavy duty stapler	Swingline	552277715	NA