Aneroid Barometer  Advantages: l No liquid l Easily portable l Low weight l Adaptable to recording  Disadvantages: l Less precise or accurate as a mercury barometer (Normal accuracy about ±0.15 mb) l Needs frequent calibration (1-2 years)

Errors of Aneroid  Temperature: l Use of bimetallic strip or inert gas in changer can reduce effects.  Parallax / Observer Error: l Placement of mirror behind pointer assists.  Hysteresis: l Varies with (1) temperature, (2) range of pressure change, (3) rate of pressure change, (4) material from which the system is constructed.

 Hysteresis is usually unimportant for station pressures do to small changes that occur.  Temperature error is a much more important defect.

B. Indirect Pressure Measuring Instruments  1. Hypsometer: Literally, an instrument for measuring height. l One type utilizes the relationship between atmospheric pressure and the boiling point of a liquid. l The relationship is given by the Clausius-Clapeyron Equation. l Invented by Francisco Jose de Caldas

 Where: L = Heat of Vaporization of Liquid  R = Universal Gas Constant  T = Temperature at boiling ( o K)  Temperature must be very accurately measured.  More accurate at low pressure, high elevation.  Used in some radiosondes.

More Indirect Pressuring Measuring Instruments  2. Piezoelectric Barometer l Electrical characteristics of certain materials change when subject to a change in pressure or force on the material. l An electromagnetic force is produced across the faces of a quartz crystal when a dynamic force is applied. The EMF can be measured and is a measure of the dynamic force exerted.

3. Piezoresistive Barometer  Electrical resistance in a circuit changes when the sensor is subject to a change in pressure. The change is current, produced by a change in resistance, can be easily measured and is a measure of the pressure. l Strain Gauge

Strain Gauge Barometer  Thin wires across a membrane change their resistance due to stretching of the material to which it is bonded according to the following relationship.  Strain Gauge Barometers can be quite small  Accuracy is about ±0.88 mb

4. Variable Capacitance Barometer  4. Change in pressure causes a change in capacitance across the electrodes. l Accurate to ±0.3 mb

ASOS - Automated Surface Observing System  Sensors: l Precipitation Accumulation Sensor l Temperature Sensor l Dew Point Sensor l Precipitation Identification Sensor l Visibility Sensor l Wind Sensor l Wind Direction Sensor l Pressure Sensor l Sky Condition Sensor (Ceilometer) l Freezing Rain Sensor

ASOS Pressure Sensor  Capacitive Type Pressure Sensor  Accuracy ±0.02 inches of mercury  Range is from 16.9 to 31.5 inches of mercury  Resolution is inches and it reports to the nearest inches.  2 to 3 sensors are used at each location.  Sensors are mounted in the Acquisition Control Unit

 Every 10 seconds a measurement is made and used to compute a 1-minute average. Lowest 1- minute sensor pressure value obtained from a pair of sensors whose difference is 0.04 inch or less is the designated pressure to be reported and is used to determine other derived pressure values using various algorithms. l Sea Level Pressure Pressure Altitude l Altimeter Setting Pressure Change l Density Altitude Pressure Tendency