METEOROLOGICAL EQUIPMENT FOR SHORT WAVE 227 
Temperature and Humidity 
Resistance Elements 
TEMPERATURE 
Temperature-sensitive resistors are satisfactory both 
with regard to accuracy and the absence of lag. The 
British have used platinum resistance thermometers 
very successfully in stationary installations. In the 
United States electrolytic or ceramic resistance elo- 
ments are commonly used. The latter can be made to 
change their resistance several fold over a relatively 
narrow temperature interval. Their accuracy is there- 
fore limited, not so much by the accuracy of the cur- 
rent measurement as by their intrinsic stability after 
calibration, proper radiation shielding, etc. 
The electrolytic element developed for the Bureau 
of Standards radiosonde’? has a time-lag constant 
(time required to attain the fraction (1—e™*) = 0.63 
of: the total change) of 8 sec at an airspeed of 3 m per 
second, of 14 sec at an airspeed of 1 m per second, and 
of 40 sec in still air 
Recently the ceramic Sanborn element® has come 
into use; it has about the same lag characteristics as 
the electrolytic element but is practically free from 
°Manufactured by Paul H. Sanborn, 2602 Riverview Drive, 
Parkersburg, W. Va. 
ARH 
METER 
SONDE 
ELEMENTS 
RH 
aging. The following time-lag constants have been 
measured :* 8 sec at an airspeed of 3 m per second and 
12 sec at an airspeed of 1 m per second. Another 
source reports 20 sec at an airspeed of 5 m per second 
(this value seems too large in comparison with the 
others) and 42 sec in still air. 
MoIsTURE 
The Bureau of Standards resistance element as well 
as the Gregory humidiometer (a British development) 
uses a dilute solution of lithium chloride. 
In the Bureau of Standards element the lithium 
chloride film is deposited on the surface of a thin cyl- 
inder on which there is a bifilar winding of two thin 
wires. The stability and aging characteristics of this 
element are described in the literature.¥? An average 
actual accuracy of 5 per cent relative humidity is 
claimed for the ordinary radiosonde when used under 
routine conditions. Higher accuracy (1 per cent RH) 
is claimed, at least at temperatures above freezing, 
when used with captive balloon equipment,™ partially 
because the current is frequently reversed to cut down 
polarization effects and partially because the calibra- 
tion can be more closely watched. Tests? show that at 
an airspeed of 2.5 m per second the time lag constant 
is 3 sec at 24 C and 11 sec at 0 C. 
0-S0 
MICROAMP 
os 
c 
| l5Vv 
0-50 
M IC ROAMP 
6 voc 
Figure 1. Wired sonde circuit. 
The potentiometer P applies a constant voltage (0.36 v at low and 0.18 v at high RH) to both of the independent circuits of the sonde proper. 
The currents, determined by the resistances of the relative humidity and temperature elements respectively, are read on the RH meter and 
T meter. S3 commutates these currents at half-second intervals; Si and S2, actuated simultaneously with S3, maintain constant polarity at the 
meters. The 1,000-nf condensers CC smooth the currents through the meters. S1S2S3 are contained in the pile-up of a single relay which is actuated 
by @ miniature worm-geared motor as shown. The 10,000-ohm protective resistance F is shorted out during measurement. Connections to the 
ground end of the cable are made through slip rings (not shown) mounted on the cable Teel. All components, excepting the sonde, cable, and 
6-v storage battery, are housed in a single case 20x9x7 in. 
227 
