MSL Technical Guide 19 Measuring Atmospheric Pressure with a Barometer Barometric Pressure Units Summary The pascal (Pa) is the metric (SI) unit of pressure This technical guide explains how to use a digital ba- and is defined as a force of 1 newton acting over an area 2 rometer to measure atmospheric pressure in a labora- of 1 square metre so 1 Pa = 1 N/m [1]. One pascal is a tory or industrial setting. It starts with a brief discussion small pressure so measurements are usually reported in of the atmosphere and then talks about barometers, their kPa (1000 Pa) or MPa (1000 kPa). stability and calibration requirements, along with how to Barometric pressure measurements are often re- work out the uncertainty of an air pressure measure- ported in hectopascal (hPa). Hecto means times 100 so ment. 1 hPa = 100 Pa. The prefix hecto is used in barometry as the numerical value of the pressure in hPa is the Introduction same as that measured in millibar (mbar), 1 mbar = 1 hPa = 100 Pa. Many historical meteorological A barometer measures the air pressure of the Earth’s records of atmospheric pressure were recorded in mbar. atmosphere. The atmospheric pressure is generated by There are many other barometric pressure units, the Earth’s gravity acting on the mass of air in the at- some of which are shown in the table below, along with mosphere. The forces involved are surprisingly large, a their symbol and relationship to the pascal. surface pressure of 100 kPa means the force at the base 2 of a column of the atmosphere, cross-section area 1 m , is equivalent to a mass of 10,000 kg. The atmospheric pressure depends on the local environmental conditions Name Symbol Relation to SI such as air temperature, altitude, and weather pattern. pascal (SI unit) Pa N m -2 = kg m -1 s -2 A typical atmospheric pressure measurement, made in New Zealand at sea level, will be in the range 970 hPa hectopascal hPa 1 hPa = 100 Pa to 1040 hPa. The graph below shows the air pressure recorded over a month. bar bar 1 bar = 100 000 Pa millibar mbar 1 mbar = 100 Pa torr torr 1 torr ~ 133.322 Pa 1035 millimetre of mercury 1030 mmHg 1 mmHg ~ 133.322 Pa (conventional) 1025 pounds per square psi 1 psi ~ 6894.757 Pa 1020 inch 1015 1010 1005 Pressure units for liquid manometers, like millimetre Pressure /Pressure hPa 1000 of mercury or inch of water, were commonly used. This type of unit should not now be used, as it is obsolete and 995 can cause confusion. The problem is that “manometric” 990 pressure units can have more than one definition. 0 10 20 30 Time / day Types of Barometer Barometers come in many different types, ranging A barometric pressure measurement is an example from mercury filled glass instruments, mechanical aner- of an absolute pressure measurement. This means a oid instruments, to resonant sensors made using silicon barometer would read zero when measuring a vacuum fabrication technology. We will briefly discuss digital ba- (zero pressure absolute). Most pressure instruments, rometers and end this section with a warning about why such as a car tyre gauge, measure gauge pressures, so mercury-in-glass barometers should not be used. read zero when open to the atmosphere. Barometric pressure measurements are used in a Digital Barometers wide variety of situations such as weather forecasting, air density calculation, measuring changes in altitude All digital barometers have a pressure transducer and in a range of thermodynamic calculations such as that converts the force generated by the pressure into an gas turbine efficiency. electrical signal. The most common type of transducer is a thin metal diaphragm with the atmospheric pressure on one side and a vacuum on the other. The changing at- Measurement Standards Laboratory of New Zealand • fax: +64 (0)4 931 3003 • e-mail: [email protected] • website: http://msl.irl.cri.nz 1 mospheric pressure deflects the diaphragm. This deflec- Atmospheric Pressure Measurement tion is measured either directly or as a change in dia- phragm tension. An atmospheric pressure measurement can be as Currently the best performing barometers are based simple as just recording the reading on the instrument on silicon or quartz resonant sensors whose frequency display, as long as the following conditions are met. The depends on the applied pressure. barometer used for the reading should be correctly in- stalled and have a calibration certificate from an accred- ited calibration laboratory. Performance checks are Mercury Barometers needed to ensure the barometer is working correctly be- MSL does not offer a calibration service for mercury tween calibrations. barometers. Mercury is hazardous to human health and is a difficult fluid to handle and keep clean. Mercury-in- Barometer Installation glass barometers are difficult to use and the readings always require significant correction. We strongly rec- Follow the manufacturer’s recommendations but also ommend replacing mercury-in-glass instruments with take into account how the instrument was calibrated. digital barometers. Mercury barometers must be dis- Factors to consider include: posed of carefully through an approved waste disposal • Instrument orientation. Some barometers are sensi- company. tive to tilt and are normally oriented upright and level. Ensure that the barometer is calibrated in the orientation in which it is used. Barometer Selection • Measurement conditions. Drafts and flow of air Factors to consider, when selecting a barometer, in- across the pressure port can cause small meas- clude the required measurement uncertainty, stability urement errors. Temperature sensitivity may be a (drift rate), ease and frequency of calibration and fea- factor for barometers mounted out-of –doors. tures such as a computer interface. • Head correction. Remember that the barometric pressure changes with altitude, it decreases by Measurement Uncertainty ~1.2 hPa for a 10 m increase in attitude (near sea level). Liquid trapped in the pressure port or tubing Most digital barometers will easily achieve meas- will also cause errors, a 10 mm height of water will urement uncertainties of < 1 hPa. This will be adequate produce an error of ~1 hPa. for most applications; for example weather monitoring You may need to apply the calibration corrections to and forecasting need to resolve pressure changes of the barometer reading although often these corrections less than 1 hPa while air density calculations require un- are small enough that they can be ignored. The correc- certainties of 10 hPa or less [2]. Here at MSL we can tions in a MSL calibration report are simply added to the provide barometer calibrations with expanded uncertain- displayed reading. For example if the barometer is dis- ties as low as 0.02 hPa, providing the barometer is ca- playing a pressure reading of 980.25 hPa and the cali- pable of performing at this level. bration report correction at 980 hPa is –0.1 hPa then the corrected pressure reading is Barometer Stability Barometer stability is the drift in measured pressure p =980.25 +− ( 0.1) = 980.15 hPa . with time. Stabilities of < 0.1 hPa per year are achiev- able although this will depend on the barometer. Some barometers that we have calibrated over several years, Measurement Uncertainty show drift rates as low as 0.02 hPa/yr. Select a barome- Calculating the uncertainty of an air pressure meas- ter with an annual drift rate that is less than your re- urement is straightforward. There are only two or three quired measurement uncertainty. uncertainty terms that need to be considered, most of which are known before any measurement is made. The Cost of Calibration first two terms arise from the barometer performance; they are the calibration uncertainty and hysteresis. The Barometer recalibration intervals range between six third term is the measurement repeatability that arises months and two years depending on the instrument, the from the barometer performance and from the stability of required uncertainty and the conditions of use. Barome- the pressure being measured. See MSL Technical Guide ters with a lower drift rate generally have a longer recali- 13 for more discussion of pressure gauge uncertainty bration interval so have a smaller operating cost. [1]. Choose a barometer with a pressure port, i.e. a tube allowing a pressure tight connection. Instruments without a pressure port need to be placed in a chamber, which Calibration Uncertainty complicates the calibration and increases the cost. The calibration uncertainty is taken directly from the barometer calibration report. A typical MSL calibration Computer Interface report uncertainty statement gives the expanded uncer- tainty of the corrections Ucal at a 95 % confidence level, Choose a barometer with a computer or instrument refer GUM [3]. We can convert from expanded to stan- interface, such as USB, LAN, RS232, etc. An interface dard uncertainty by dividing by the coverage factor k. For allows the atmospheric pressure to be read automati- example, if Ucal = 0.06 hPa and k = 2.2 then cally and logged by computer. This is particularly useful when measuring changing air pressures. U 0.06 u =cal = = 0.027 hPa . cal k 2.2 Measurement Standards Laboratory of New Zealand • fax: +64 (0)4 931 3003 • e-mail: [email protected] • website: http://msl.irl.cri.nz 2 Here the uppercase symbol U refers to expanded uncer- tainty while u refers to the standard uncertainty. Time / s Reading / hPa Spreadsheet Function 1 1015.25 Hysteresis 2 1015.10 3 1015.25 The hysteresis uncertainty component can be esti- 4 1015.25 mated from the difference between the calibration report 5 1015.30 rising and falling pressure corrections, at the same 6 1015.30 nominal pressure.