Abstract
Two main errors arise from radiation and thermal inertia in the measurement of the air temperature and they interact with each other.
It is too complicated to deal with the radiation errors theoretically, then the author introduces some special conceptions to study this subject.
A thermometer receives heat radiation from the sun, cloud, obstacles and atmospheric water vapor etc. We supposed that those radiation fluxes come from the hypothetically black body which envelopes theromometer. Now being θ* the temperature of this body, φ denotes the ratio of the convective conduction to radiative heat exchange of the therometer, and θ0 is real air temperature thermodynamically defined.
The variation of indication of the thermomter with time may be written;
dθ/dt=-C{φ(θ-θ0)+(θ-θ*)}.
Hence, in equilibrium, the reading of the thermometer is as follows:
Θ=φθ0+θ*/φ+1.
Then, if α is the time constant of the thermometer;
1/α=C(φ+1).
The value of φ was determined by this relationship, after measuring the lag of a thrmometer (Fig. 2). Thus, the error of thermomtry may be expressed by
δ=Θ-θ0=θ*-Θ/φ.
If we keep the accuracy of the thermometry by 0.1°C, the necessary condition is as follows
|δ|=|θ*-θ0|/φ<0.05°C
The application of those formulas to the practical thermometry will be described in a next paper.
