Barometer Calibration

Barometers are instruments that determine absolute pressure. Absolute pressure is defined as the compression force exerted by the weight of air molecules in the surrounding atmosphere. Absolute pressure readings are always corrected with the sea level (altitude), as altitude changes affect the air density. Absolute pressure decreases with the increasing altitude due to the lower air density, and increases when closer to earth. Accounting for these altitude corrections, modern Barometers have altimeters integrated, to avoid revisions in the absolute pressure reading.

Similar to Absolute pressure, Gauge and Differential pressures are also considered standard pressure units. Gauge pressure is the default parameter across industries and is defined as the difference between Absolute and Atmospheric pressures. Gauge pressure is referred to while monitoring the tire pressure, blood pressure, pump capacity, or specifications of any industrial equipment. Differential pressure , on the other hand, is the gauge pressure difference measured between two points. Differential pressure gauges consist of two inlet ports, each connected to two different volumes. The movement of fluid in the mercury column of the barometer indicates high and low-pressure zones.

Types of Barometer:

Hydrostatic-based barometers use hydrostatic fluids (commonly water and mercury) to determine atmospheric pressure. These barometers consist of a transparent columnar tube, usually made of solid glass and filled with a static, in-compressible liquid. The columnar section is sealed one end to hold the vacuum and partially filled with the incompressible liquid. The other end is open to the atmosphere and makes an interface with the surrounding environment. As the atmospheric pressure changes, the pressure exerted by the atmosphere on the liquid reservoir exposed to the atmosphere changes and causes the liquid to move. Gradations are marked on the tube to record the fluid level (above the base-point), converted as Absolute pressure using the formula “ρgh,” where ρ, g, and h represent the density, acceleration due to gravity, and the level of mercury (above the base-point) respectively. While water as an incompressible liquid is less hazardous than mercury, mercury is often a better choice for fabricating accurate hydrostatic barometers due to its higher density, yielding better accuracy.

Aneroid or Electronic barometers consist of a tiny, flexible sealed metal box called an aneroid cell made from beryllium-copper alloy. Changes in external air pressure cause the aneroid cell to expand or contract. Mechanical mechanisms amplify this expansion and contraction to give a pressure reading. Modern pressure measuring devices are designed to convert the absolute pressure to provide gauge pressure readings. Aneroid barometers are more practical than hydrostatic barometers because of their robustness and accuracy. While the aneroid barometer is the underlying mechanism behind modern absolute pressure measuring devices, pressure can also be measured using simpler and sophisticated methods.

Barometer Applications:

1. Meteorology: Weather stations, data buoys, GPS, and other environmental applications to estimate the precipitable water vapor.
2. Hydrology and Agrology: Condensation and vapor pressures, Monitoring the gas and liquid media, maintained in extreme environments.
3. Industrial applications: Vacuum pumps, the pressure maintained in packaging machines
4. Pressure-sensitive applications: Aviation, Exhaust gas analysis, Laser interferometers, lithography, etc.
5. Labs and Calibration applications: Monitoring absolute pressure reading to ensure optimal environmental standards during calibration.

Barometer Selection:

Barometers are selected based on the application, level of accuracy, and electrical integration requirements. Following are the barometer categories, grouped with their features:

1. Digital and Analog barometers: Digital barometers are modern, high-end barometers with resolutions up to 0.001 millibars (hPa). Analog (dial) barometers are robust, with high precision and data reliability.
2. Gauge and Portable barometers: Gauge barometers use the Hydrostatic principle with a U-Shaped glass tubing and heavy metal (Hg, Mercury) fluid inside the column. Portable barometers are handheld, electronic devices working on the Aneroid mechanism.
3. Multi-Parameter Barometers: These barometers are integrated with Thermo, Hydro and Altimeters, to correct absolute pressure reading with temperature, dew-point, and altitude variations.
4. Laboratory barometers: In addition to monitoring atmospheric air pressure, these barometers also monitor the quality of environmental air and are suitable for labs and calibration facilities.
5. Data logging and Analog/Digital output capability: Modern barometers can save the pressure readings locally and transfer them via an mV (mill volt) or mA (mill amp) output. These barometers are suited for automated control systems and similar applications.

Correction Factors:

Certain corrections are applied to ensure that the observed barometer reading is rendered accurate. Correction factors account for instrument errors and atmospheric corrections and are broadly classified as:

1. Instrument correction or Calibration correction: Instrument correction is the mean difference between the readings of a given barometer and that of a standard instrument. This correction factor is derived from the Calibration chart and applied for each absolute pressure reading. The correction factor can be plus or minus.
2. Temperature Correction: This factor accounts for the difference between the coefficient of thermal expansions of incompressible fluid (in a Hydrostatic barometer) or beryllium-copper alloy (in an Aneroid barometer) to that of the thermal expansion of scale.
3. Gravity correction: This factor accounts for the changes in Acceleration due to gravity (g) at various locations and depends on the latitude and longitude.
4. Removal correction: This correction factor is applied to account for the barometer elevation, which differs from the adopted station elevation or climatological station elevation.

Barometer Calibration – Why is it essential:

Following errors set in within a Barometer due to prolonged usage at various operating conditions. Operators should measure and eliminated these errors using calibration and resetting:

1. Capillarity defects: These defects are often noticed in hydrostatic barometers, set in due to the interaction of mercury with the surrounding environment. The movement of fluid is restricted due to its density errors, resulting in an incorrect reading.
2. Misalignment: Misalignment of U-Shaped tubes results in varied set point reading.
3. Imperfect vacuum: Broken seals due to improper maintenance is a common phenomenon, leading to incorrect pressure readings
4. Scale errors: Varied expansion between the beryllium-copper alloy and the scale, leading to scaling errors

Cumulative defects in a barometer often result in operational uncertainties leading to maintenance loss. Calibration avoids these errors, ensures timers’ accuracy for sustained and repeated use, and enhances service life. Following a regular and timely calibration schedule ensures accuracy of measurement and enhances process accuracy.

e2b calibration offers industry-leading ISO-certified Barometer calibration services. Our labs are ISO/IEC 17025 accredited and operated by a team of qualified calibration experts to test and calibrate your barometers. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for all your equipment calibration needs.