May 3rd is National Chocolate Custard Day!
On this day:
May 3, 1715
This animation shows the eclipse path over England and northern Europe.
1715 – A total solar eclipse was visible across northern Europe, and northern Asia, as predicted by Edmond Halley to within 4 minutes accuracy.
A total solar eclipse occurred on 3 May 1715. A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partly obscuring the image of the Sun for a viewer on Earth. A total solar eclipse occurs when the Moon’s apparent diameter is larger than the Sun’s, blocking all direct sunlight, turning day into darkness. Totality occurs in a narrow path across Earth’s surface, with the partial solar eclipse visible over a surrounding region thousands of kilometres wide. This total eclipse was visible across England, northern Europe, and northern Asia.
Observation
This total solar eclipse was observed in England from Cornwall in the south-west to Lincolnshire and Norfolk in the east. It was also observed in Ireland, where large crowds turned out in Dublin to watch it: the weather in Dublin was exceptionally cold and wet, and the eminent judge Joseph Deane caught a fatal chill as a result.
This eclipse is known as Halley’s Eclipse, after Edmond Halley (1656–1742) who predicted this eclipse to within 4 minutes accuracy. Halley observed the eclipse from London where the city of London enjoyed 3 minutes 33 seconds of totality. He also drew a predictive map showing the path of totality across England. The original map was about 20 miles off the observed eclipse path, mainly due to his use of inaccurate lunar ephemeris. After the eclipse, he corrected the eclipse path, and added the path and description of the 1724 total solar eclipse.[1]
Drawing upon lunar tables made by the first Astronomer Royal John Flamsteed, William Whiston produced a more technical predictive eclipse map around the same time as Halley. Both Halley’s and Whiston’s maps were published by John Senex in March 1715.[2][3]
Note: Great Britain didn’t adopt the Gregorian calendar until 1752, so the date was considered 22 April 1715.
Born on on this day:
1695 – Henri Pitot, French physicist and engineer, invented the Pitot tube (d. 1771)
Henri Pitot (May 3, 1695 – December 27, 1771) was a French hydraulic engineer and the inventor of the pitot tube.
In a pitot tube, the height of the fluid column is proportional to the square of the velocity of the fluid at the depth of the inlet to the pitot tube. This relationship was discovered by Henri Pitot in 1732, when he was assigned the task of measuring the flow in the river Seine.
He rose to fame with the design of the Aqueduc de Saint-Clément near Montpellier and the extension of Pont du Gard in Nîmes. In 1724, he became a member of the French Academy of Sciences, and in 1740 a fellow of the Royal Society.
The Pitot theorem of plane geometry is named after him.
Pitot tube
A pitot (/ˈpiːtoʊ/ PEE-toh) tube is a pressure measurement instrument used to measure fluid flow velocity. The pitot tube was invented by the French engineer Henri Pitot in the early 18th century[1] and was modified to its modern form in the mid-19th century by French scientist Henry Darcy.[2] It is widely used to determine the airspeed of an aircraft, water speed of a boat, and to measure liquid, air and gas flow velocities in industrial applications. The pitot tube is used to measure the local flow velocity at a given point in the flow stream and not the average flow velocity in the pipe or conduit.[3]
Theory of operation
The basic pitot tube consists of a tube pointing directly into the fluid flow. As this tube contains fluid, a pressure can be measured; the moving fluid is brought to rest (stagnates) as there is no outlet to allow flow to continue. This pressure is the stagnation pressure of the fluid, also known as the total pressure or (particularly in aviation) the pitot pressure.
The measured stagnation pressure cannot itself be used to determine the fluid flow velocity (airspeed in aviation). However, Bernoulli’s equation states:
Stagnation pressure = static pressure + dynamic pressure
Which can also be written
p t = p s + ( ρ u 2 2 ) {\displaystyle p_{t}=p_{s}+\left({\frac {\rho u^{2}}{2}}\right)} p_{t}=p_{s}+\left({\frac {\rho u^{2}}{2}}\right)
Solving that for flow velocity:
u = 2 ( p t − p s ) ρ {\displaystyle u={\sqrt {\frac {2(p_{t}-p_{s})}{\rho }}}} u={\sqrt {\frac {2(p_{t}-p_{s})}{\rho }}}
NOTE: The above equation applies only to fluids that can be treated as incompressible. Liquids are treated as incompressible under almost all conditions. Gases under certain conditions can be approximated as incompressible. See Compressibility.
where:
u {\displaystyle u} u is flow velocity to be measured in m/s;
p t {\displaystyle p_{t}} p_{t} is stagnation or total pressure in pascals;
p s {\displaystyle p_{s}} p_{s} is static pressure in pascals;
and ρ {\displaystyle \rho } \rho is fluid density in k g / m 3 {\displaystyle kg/m^{3}} kg/m^{3}.
The dynamic pressure, then, is the difference between the stagnation pressure and the static pressure. The dynamic pressure is then determined using a diaphragm inside an enclosed container. If the air on one side of the diaphragm is at the static pressure, and the other at the stagnation pressure, then the deflection of the diaphragm is proportional to the dynamic pressure.
In aircraft, the static pressure is generally measured using the static ports on the side of the fuselage. The dynamic pressure measured can be used to determine the indicated airspeed of the aircraft. The diaphragm arrangement described above is typically contained within the airspeed indicator, which converts the dynamic pressure to an airspeed reading by means of mechanical levers.
Instead of separate pitot and static ports, a pitot-static tube (also called a Prandtl tube) may be employed, which has a second tube coaxial with the pitot tube with holes on the sides, outside the direct airflow, to measure the static pressure.[4]
If a liquid column manometer is used to measure the pressure difference p t {\displaystyle p_{t}} p_{t} – p s {\displaystyle p_{s}} p_{s}, or Δ p {\displaystyle \Delta p} \Delta p,
Δ h = Δ p ρ l g {\displaystyle \Delta h={\frac {\Delta p}{\rho _{l}g}}} \Delta h={\frac {\Delta p}{\rho _{l}g}}
where:
Δ h {\displaystyle \Delta h} \Delta h is the height difference of the columns in meters.
ρ l {\displaystyle \rho _{l}} \rho _{l} is the density of the liquid in the manometer;
g is the acceleration of gravity in m / s 2 {\displaystyle m/s^{2}} m/s^{2}
Therefore,
V = 2 ( Δ h ∗ ( ρ l g ) ) ρ {\displaystyle V={\sqrt {\frac {2(\Delta h*(\rho _{l}g))}{\rho }}}} V={\sqrt {\frac {2(\Delta h*(\rho _{l}g))}{\rho }}}
Aircraft
Main article: Pitot-static system
A pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft’s airspeed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a static port, and the pitot-static instruments.[5] Errors in pitot-static system readings can be extremely dangerous as the information obtained from the pitot static system, such as airspeed, is potentially safety-critical.
Several commercial airline incidents and accidents have been traced to a failure of the pitot-static system. Examples include Austral Líneas Aéreas Flight 2553, Northwest Airlines Flight 6231, and one of the two X-31s.[6] The French air safety authority BEA said that pitot tube icing was a contributing factor in the crash of Air France Flight 447 into the Atlantic Ocean.[7] In 2008 Air Caraïbes reported two incidents of pitot tube icing malfunctions on its A330s.[8]
Birgenair Flight 301 had a fatal pitot tube failure which investigators suspected was due to insects creating a nest inside the pitot tube; the prime suspect is the Black and yellow mud dauber wasp.
Aeroperú Flight 603 had a pitot-static system failure due to the cleaning crew leaving the static port blocked with tape.
Industry applications
In industry, the flow velocities being measured are often those flowing in ducts and tubing where measurements by an anemometer would be difficult to obtain. In these kinds of measurements, the most practical instrument to use is the pitot tube. The pitot tube can be inserted through a small hole in the duct with the pitot connected to a U-tube water gauge or some other differential pressure gauge for determining the flow velocity inside the ducted wind tunnel. One use of this technique is to determine the volume of air that is being delivered to a conditioned space.
The fluid flow rate in a duct can then be estimated from:
Volume flow rate (cubic feet per minute) = duct area (square feet) × flow velocity (feet per minute)
Volume flow rate (cubic meters per second) = duct area (square meters) × flow velocity (meters per second)
In aviation, airspeed is typically measured in knots.
In weather stations with high wind speeds, the pitot tube is modified to create a special type of anemometer called pitot tube static anemometer.[9]
FYI:
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