Barometry The art or science of barometric observa3on
History Giovanni Ba3sta Baliani observed that syphon pumps could not pump water higher than ~ 34 feet Galileo - proposed it was due to a vacuum Gasparo Ber3 created the first working barometer some3me between 1640 and 1643 Evangelista Torricelli credited with inven3ng the barometer in 1643
Atmospheric Pressure The atmosphere exerts a pressure on the surface of the Earth equal to the weight of a ver3cal column of air Omni- direc3onal force exerted upon an area Eqn: Common Units: Mm Hg hpa mb
Equa3on of State What other units is pressure related to? What is the equa3on that relates them?
Units of Pressure SI unit of pressure is the pascal (Pa) kg m - 1 s - 2 Preferred unit in meteorology is the millibar 1 mb = 1hPa = 100Pa 1 in. Hg (@ 273.15 K) = 33.86 hpa 1 standard atmosphere (sea level) = 1013.25hPa
Barometric Height Formula What does the measurement of pressure depend on?
Three Types of Pressure Measurements Absolute total sta3c pressure (ie barometric pressure) Gauge pressure rela3ve to ambient atmospheric pressure Differen3al pressure rela3ve to some other pressure
Sta3c versus Dynamic Pressure Sta3c pressure is actual air pressure Dynamic pressure is pressure exerted by wind flow Dynamic pressure can produce errors in sta3c pressure measurement
Direct (In- situ) Measurement of Pressure Balancing the force of the atmosphere against the weight of a column of fluid (For3n barometer) Balancing the force of the atmosphere against the force of a spring (Aneroid barometer)
Mercury Barometers Manometer is the simplest form of a mercury barometer Can be either open- ended (Differen3al pressure measurement) or close- ended (Absolute pressure measurement) Can be awkward to use since manual measurements of fluid height in each arm and the difference calculated to get the raw output
Manometer Measurements Difference in height is related to the hydrosta3c equa3on
Manometer Measurements (a) How are pressure difference and height related?
Manometer Measurements (b) How are pressure difference and height related?
For3n Barometer Improved version of the manometer offering high accuracy and easy calibra3on. Somewhat portable. Excellent long- term stability A column of mercury is enclosed in a glass tube that is sealed at the top with a reservoir of mercury at the bobom. A vacuum is created at the top of the tube
For3n Barometer Height of the column of mercury is determined using the abached scale The height of the mercury in the reservoir must be adjusted upwards using an adjus3ng screw to the fiducial (reference) point The measurement is then taken from the abached scale
For3n Barometer Measurements Open the case and immediately read the temperature Use the bobom screw to adjust the height of the mercury to the fiducial point. Adjust the scale index to the top of the mercury column. Keep your eye level with the mercury meniscus in the tube. Read the pressure using the vernier Lower the level of the mercury in the cistern
Why Use Mercury? Has a high density (14x heavier than water) leading to a column that is of reasonable length Low vapor pressure Easily purified and chemically stable Remains liquid for a wide range of temperatures (- 38.87C 356.58C)
Sources of Error for Mercury Barometers Dynamic wind pressure can produce (posi3ve and nega3ve) errors on the sta3c measurement on the order of several millibars Density of mercury is a func3on of temperature so temperature effects must be compensated for
Sources of Error for Mercury Barometers Since the force of the atmosphere is balanced against the weight of the mercury in a column, local gravity must be known and a gravita3onal correc3on calculated The presence of gas (other than mercury vapors) in the vacuum por3on of the tube will cause an error in measurement
Sources of Error for Mercury Barometers The surface tension of mercury will cause a depression in the mercury column of smaller- bore tubes. The correc3on for this is usually incorporated into the index correc3on (calibra3on). The barometer must be kept ver3cal Impuri3es affect the density and measurement
Temperature Correc3on The temperature correc3on for a mercury barometer
Volume and Linear Expansion Coefficients Mercury (β): 1.818 x 10-4 K - 1 Aluminum (α): 23.0 x 10-6 K- 1 Brass (α): 18.9 x 10-6 K - 1 Steel (α): 13.2 x 10-6 K - 1 Iron (α): 11.4 x 10-6 K - 1
Local Gravity Correc3on for La3tude
Local Gravity Correc3on for Eleva3on
Final Gravity Correc3on The correc3on factor for local gravity is given as:
Corrected Sta3on Pressure The raw barometer reading is converted to sta3on pressure by:
Aneroid Barometers Consists of an evacuated (vacuum) chamber with a flexible diaphragm that moves in response to an applied pressure. The restoring force is a spring that may be part of the diaphragm Aneroid without fluid Two types Metallic- diaphragm and silicon- diaphragm
Calibra3on Equa3on for Aneroid Barometers
Bourdon Aneroid Barometer Consists of a flabened tube with round ends bent in a circular pabern. The tube is open to the ambient pressure but is enclosed in an evacuated box. As pressure increases, the tube tries to assume a circular form, causing it to straighten out. This movement can be correlated to pressure.
Sources of Error for Aneroid Barometers Exposure Errors Temperature- induced errors Hysteresis Effects Dril
Indirect Measurement of Pressure Indirect measurement of a variable other than pressure that is a func3on of pressure Boiling point of a liquid Boiling point of pure water at standard sea- level pressure is 373.15 K Decreases with increasing height
Hypsometer Height Meter Contains a flask of fluid, heated to maintain con3nuous boiling with a temperature sensor Need to know the rela3onship between vapor pressure and temperature to derive pressure
Vapor Pressure The pressure of a vapor in equilibrium with its non- vapor phases Dependent on temperature Related through the Clausius- Clapeyron Equa3on
Hypsometer Pressure Calcula3on
Comparison of Barometer Types Mercury Barometers (For3n) Simple physical concept Require no calibra3on Difficult to automate and transport Must be kept ver3cal Needs temperature and gravity correc3ons Mercury vapor is toxic Improper handling may introduce bubbles into mercury column Height of column cannot be changed
Comparison of Barometer Types Aneroid Barometers Very small size, easily portable Easily automated Insensi3ve to orienta3on, mo3on and shock No gravity correc3on needed No toxic chemicals Concept is simple, but calibra3on is always required Temperature sensi3vity is high, no simple or predictable correc3on Subject to unpredictable dril
Comparison of Barometer Types Hypsometer Small size, reasonable portable Easily automated Sensi3ve to orienta3on No gravity or temperature correc3on needed No dril or hysteresis Concept is simple, no calibra3on required
Exposure Error Barometers are designed to measure sta3c pressure Need to be isolated from dynamic effects Imprac3cal for barometers to be inside buildings unless equipped with a sta3c port Sta3c port needs to extend beyond the pressure field of the building
Exposure Error Pressure field of building can be 2.5x building height ver3cally and 10x height of the building horizontally S3ll need sta3c port for moun3ng outside on towers Ver3cal sta3c port s3ll doesn t completely reduce the dynamic error, but keeps it to a minimum as long as the sta3c port is kept ver3cal
Exposure Error Can be very problema3c for pressure measurements on buoys