Met One 50.5, Руководство по эксплуатации

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50.5-9800 Rev C Page 14
M
LRT
C
=
d
V C
Vd Vn C
t








+
± −
=
2 2
2 2
5. THEORY OF OPERATION
5.1. SPEED OF SOUND IN AIR
The speed of sound in still air can be measured accurately between two points a few centimeters
apart by two ultrasonic transducers set at that distance. The resulting speed of sound is a known
function of the air temperature and composition.
The speed of sound waves in an ideal gas may be written
where R is the universal gas constant (0831.34 mJ/mol K), T is the temperature in Kelvin, M is
the molecular weight (grams/mol) of the gas, and L is the ratio of heat capacities C
p
and C
v ;
C
p
and C
v
are the specific heats at constant pressure and constant volume of the gas, respectively.
5.2. SPEED OF SOUND PRINCIPLE
The transit time of a sound signal traveling from one end of a sound path to the other, separated
by distance d, can be written as follows (Schotland, 1955):
where V is the total velocity, V
d
and V
n
are velocity components in the directions parallel and
normal to the sound path, and C is the velocity of sound in still air. If two transit times, t
1
and t
2
in
opposite directions on the same path are directed, V
d
can be obtained independent from V and
V
n
as follows:
It is this principle that is used to compute the velocity of the air in the path between two opposing
transducers.
5.3. CALCULATION OF THE WIND VELOCITY
In still air, t
1
and t
2
are equal. For a distance 15 cm at 20° C, the transit time is approximately
450 µs. If a 20 m/s wind is in the direction of the sonic pulse, the transit time t
1
will be
approximately 427 µs. If the wind is opposing the sonic pulse, the transit time will be
approximately 482 µs. If these two values are used in the previous equation, the resultant
velocity from equation will be 20 m/s.






− =
2 1
1 1
t t
Vd