CONSTRUCTION OF A SENSOR FOR CONTINUOUSLY 
RECORDING THE CHANGE IN HEIGHT OF 
PLANTS IN THE FIELD x 
j 
f 
Wendell C. Johnson and Ronald G. Davis? 
Diurnal fluctuations in growth rate of irrigated 
sorghum and wheat have been reported earlier.°’* To 
obtain the data for these publications, a height sensor. 
was used in conjunction with a strip chart recorder. This 
publication describes the construction of this height- 
sensing instrument. 
Figure 1 shows the basic parts of the height sensor. 
Its one moving part is an integral Plexiglas disk and axle 
that turns on a horizontal axis. The disk and axle are 
housed in a Plexiglas box attached to one leg of a 
supporting tripod by a laboratory support flange, alumi- 
num support rod, and a fixed clamp holder. 
The sensor is connected to the plant with a fine 
copper wire, one end of which is attached to the disk 
near the axle and is wound one turn around the axle. A 
weight cord also is attached to and wound around the 
axle but in the opposite direction. The cord would cause 
the disk to rotate if the disk was not restrained by the 
copper wire attached to the plant. 
The output voltage signal from the sensor is taken 
from a resistance slidewire that lies along a portion of 
the circumference of the plastic disk. A voltage regulator 
! Soil and Water Conservation Research Division, Agricultural 
Research Service, U.S. Department of Agriculture, in coopera- 
tion with the Texas Agricultural Experiment Station, Texas 
A&M University. 
* Research soil scientist and agricultural research technician, 
respectively, USDA, Southwestern Great Plains Research Center, 
Bushland, Tex. 
=. Johnson, Wendell C. Diurnal variation in growth rate of 
grain sorghum. Agron. J. 59: 41-44. 1967. 
2 Johnson, Wendell C. Diurnal variation in growth rate of 
irrigated wheat. Agron. J. 61: 539-542. 1969. 
Conduit 
Coupler 
Laboratory 
Contact 
Aluminum 
Support Rod 
Slidewire 
Plastic 
Figure 1.—A height sensor for obtaining a continuous record 
of plant growth. 
places a known constant voltage across the slidewire, and 
as the plant grows, the disk is allowed to turn. As the 
distance between the upper end of the resistance 
slidewire and the contact point increases, an increasing 
voltage output signal is produced. 
