1224 
compensate for the heating at the entrance to the hous- 
ing and at the thermometer bulb. The most promis- 
ing is that which uses a vortex chamber for the required 
cooling, placing the thermometer at the center of the 
vortex (see ne 1). By proper construction of the 
ataan aes 
TPP erry Re per 
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TERRIER RRR oc) 
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THERMISTOR: 8 
SUPPORT oo 
Co} aan of a “vortex thermometer” as designed 
by P. od. Harney for measuring true air temperature from an 
eulane, (Courtesy of Signal Corps Weather Radar Research at 
M.1.T.) 
housing and controlled throttling of the flow, the ther- 
mometer can be made to indicate free-air temperature 
correctly over a wide range of air speeds [51]. If 
another thermometer element is mounted on the same 
aircraft so as to receive the full adiabatic heating effect, 
the difference between the two will be a measure of 
true air speed. The vortex-type housing also provides 
good mechanical protection for the element. All factors 
considered, it appears to be the most promising recent 
development in aircraft thermometry. 
Radiation effects, particularly radiation from the 
sun, may be another source of error. However, with a 
well designed radiation shield, and with the high ven- 
tilation speeds available on aircraft, radiation effects 
may be made negligible. 
The most serious problem is accurate measurement 
in the presence of liquid water. The effects of liquid 
water are various. First, any housing so far devised 
which provides adequate ventilation will allow the 
thermometer element to become wet, and it will re- 
main wet for a time after emergence into dry air. (First 
tests of the vortex thermometer in clouds indicate that 
either its element is wetted to some extent, or water 
present when the pressure is high causes evaporational 
cooling “upstream”’ from the element.) The thermome- 
ter will then read some sort of wet-bulb temperature. 
Tf it indicated the true wet-bulb temperature, it would 
be a useful quantity, but it might not be completely 
wetted and would undoubtedly be affected by the tem- 
perature of the water, as well as that of the air. One 
simple, but only partially adequate, solution is to use 
two elements, one coated with a hydrophobic substance, 
METEOROLOGICAL INSTRUMENTS 
the other with a hydrophilic substance, and both ex- 
posed to the direct air flow [7]. In air of low liquid- 
water content, the hydrophobic bulb is then assumed 
to indicate the dry-bulb temperature (with appropriate 
corrections). In air of high water content, the hydro- 
philic element is assumed to measure the wet-bulb 
temperature (again with appropriate corrections). (See 
Fig. 3.) The difficulties are (1) obvious uncertainties 
in the case of intermediate water contents, (2) uncer- 
tainties in the appropriate corrections to use, particu- 
larly for the wet-bulb temperature, and (8) uncertain- 
ties regarding the effects of the temperature of the water 
striking the thermometer. This general method might 
be slightly improved by the use of a constantly wetted 
wick instead of a hydrophilic coating, but it would re- 
main inadequate. 
Second, we must consider the effects of water on the 
temperature of the air before it reaches the bulb, as 
well as the effects of water on the bulb itself. All heat- 
transfer-type thermometers must have adiabatic heat- 
ing effects at the thermometer element or upstream 
from it. If there is liquid water present, there will be 
some evaporation which may or may not be at the 
moist-adiabatic rate. The vortex thermometer corrects 
for dry-adiabatic heating by subsequent cooling, but 
it is not yet clearly established that the cooling process 
will introduce the right correction in the presence of 
liquid water. 
In spite of these heating and cooling uncertainties, 
it seems worth while to make a further study of means 
of keeping the thermometer element dry. This would 
not seem impossible, although the usual means of re- 
moving water from the air mechanically requires de- 
flecting surfaces, upstream from the element, which 
themselves become wet and consequently lower the 
temperature by evaporation. Even when the air is 
originally saturated, the temperature rise at the deflect- 
ing surfaces due to dynamic effects will lower the rela- 
tive humidity and permit evaporation. Possibly some 
modification of the vortex thermometer with porous 
capillary surfaces to remove the water would be worth 
trying. 
This general problem of temperature measurement 
in the presence of liquid water has been dealt with at 
some length because it presents such a challenge. Even 
if one succeeds in conquering it at temperatures above 
freezing, then a whole new set of difficulties arises 
under icing conditions. (Dry snow appears to cause no 
trouble.) All these difficulties naturally make one turn 
away from thermometers operating on conductive heat 
transfer from the air. At least two entirely different 
principles have been used. In one the speed of sound 
[1, 49], which is a function of temperature and humidity, 
is measured. If one defines temperature in terms of 
molecular motion, the velocity of sound is a more 
direct measure of that temperature than observing the 
temperature of some secondary material, as one does 
with a conventional thermometer. Several very ingen- 
ious variations of this sonic-velocity idea have been 
devised, some of which are suitable for aircraft use. 
The presence of liquid water, even under icing condi- 
