APPARATUS 
Tripod Support 
The top of the tripod support is a 1/8- by 6- by 6-inch 
metal plate to which stub legs approximately 6 inches 
long have been brazed. One of the legs is hinged to make 
the tripod easier to manage in the field. Sections of 
half-inch electrical conduit of suitable length for the 
crop being studied are connected to the stub legs with 
standard threaded conduit couplings. 
To reduce the temperature effects, invar steel tubing® 
(which has an extremely low coefficient of linear 
expansion) was used for the tripod legs during the 
measurement of the growth rate of wheat. However, the 
use of invar was discontinued because of its high cost 
and because of indications that temperature effects 
probably would not limit the overall accuracy of the 
growth measurements. As an example of the error to be 
expected from the temperature effect on the steel 
tripod, assume that the height of the sensor is 5 feet 
above the ground, the thermal coefficient of expansion 
of steel is about 12x10*/° C., and the daily temperature 
range is about 15° C. In this reasonably typical situation, 
the temperature effect on the support tripod is only 
about 0.01 inch of apparent growth per day’s 
temperature range. Part of this effect is compensated for 
by the greater temperature coefficients of expansion of 
the downward pointing aluminum support rod, Plexiglas 
box, and copper wire connecting the sensor to the plant. 
Change in circumference of the Plexiglas axle causes 
some temperature error, but this amounts to only about 
0.0040 inch per 15° C. and is compensated for by the 
simultaneous change in length of a 15-inch segment, or 
about the length of copper connecting wire used. Field 
tests with the sensors attached to 2-foot pine or invar 
steel stakes showed that the error in measured growth 
due to the temperature range of aclear summer day did 
not exceed 0.01 inch. We also determined that rain or 
the condensation of atmospheric moisture as dew had no 
apparent effect on the signal recorded from the sensor. 
The absence of a moisture effect on the apparatus is 
important in observing the effect of dew or the 
beginning of a rain on plant growth. 
Because our studies have been limited to rapid growth 
situations involving young leaves where the measured 
growth is about 1 inch or more per day, errors of the 
magnitude caused by temperature effects on the tripod 
5 Invar tubing available from J. Bishop and Co., Malvern, Pa. 
support have not been considered important. However, 
for measuring during expreme termerature changes, or if 
the plant growth per day is small, it would probably be 
advisable to use invar legs. 
Field installation of the tripod involves making three 
inclined holes 10 to 12 inches deep with either an auger 
or soil probe slightly larger in diameter than the tripod 
legs. The legs are then placed into the holes and are 
forced into the soil an additional 10 to 12 inches when 
soil moisture conditions permit. 
The sensor and its support should be in stable 
equilibrium. Greatest stability is achieved when the 
center of gravity of the sensor box is in the lowest 
possible position, which is when the sensor is located 
over the center of the triangle formed by the base of the 
tripod. Also for stability, the end of the support rod 
opposite the sensor box should point into the prevailing 
wind so that the orientation of sensor and rod is similar 
to that of a wind vane. 
Sensor 
The outside dimensions of the assembled Plexiglas 
box are 3 3/8 by 8 3/16 by 9 3/4 inches (fig. 2). One of 
the sides is hinged at the top to allow access to the 
electrical components. The front, back, and top are cut 
from 3/16-inch stock and the ends and shelf from 
3/8-inch stock. The bonding material used was 
Rez-n-bond.° 
The sensor shown in figure 3 has a disk diameter ot 5 
inches, a slidewire 6 1/8 inches long, and an axle 
diameter of 1.46 inches. Used with a O- to 9-mv. recorder, 
with a calibration constant of 0.20 inch of growth per 
mv., the sensor has a range of 1.8 inches without 
resetting. The range can be increased, with a reduction in 
sensitivity, simply by increasing the diameter of the axle. 
Sensors with a range of 1.6 to 1.8 inches have been used 
satisfactorily to measure the growth of wheat, soybeans, 
and sugarbeets. Sensors used for measuring corn and 
grain sorghum growth have had a range of 3.2 to 3.6 
inches. 
The shaft for the disk and axle is 3/32-inch piano 
wire and turns in ball bearings mounted on the shelf.” 
S Manufactured by Schwartz Chemical Co., Long Island City, 
N.Y. 
7 The shaft bearings, bearing supports, and retaining collars 
were originally intended for building slot racers and are available 
at hobby shops. 
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