Buoyancy Correction Note 5/2/03 10:07 am Page 2
Good Practice Guidance Note
Buoyancy Correction and Air Density Measurement
Introduction Determination of Contents This Guidance Note gives Air Density recommendations of good mass Introduction 1 The usual method of determining air measurement practice in calculating density is to measure temperature, air density and buoyancy corrections Need for the Determination 1 pressure and humidity and calculate but should not be considered a of Air Density air density using the equation comprehensive guide. recommended by the Comité What is the Typical Density 1 International des Poids et Mesures of the Air? Need for the (CIPM) (derived by Giacomo [2] and modified by Davis [3]). The equation Determination of Air 1 Determination of is not a perfect model of air density Density and introduces an uncertainty of Air Density approximately 1 part in 104. Air Density Artefacts 2 Air is dense Typical routine measurement capabilities for the air density Variation in Buoyancy 2 A cubic metre of air weighs parameters are indicated as follows Effect with Varying approximately 1.2 kilograms with the best measurement Air Density A 1 kilogram stainless steel capabilities in brackets: weight displaces 150 mg of air When Should I make a 3 Temperature to 0.1 °C (0.01 °C) The density of air can vary between Buoyancy Correction? about 1.1 kg m-3 and 1.3 kg m-3, Pressure to 0.5 mbar (0.05 mbar) How do I make a 3 which is equivalent to a change of Relative Humidity to 5% (0.25 °C 25 mg in the weight of a stainless dew point) Buoyancy Correction? steel kilogram of volume 125 cm3. The above parameters give a typical What do the OIML 3 value on the air density (including Specifications say? the uncertainty from the equation) of What is the Typical 3 4 1 part in 10 (2 parts in 10 best Conventional and 4 Density of the Air? capability) True Mass Standard air density is 1.2 kg m-3 For lower accuracy measurements Related Good Practice 4 the NIST (simplified) air density Guides This is the approximate density of formula can be used: air at 20 °C, 1013.25 mbar and References 4 50% Relative Humidity [(0.348444xP) - h(0.00252t - 0.020582)] AD= For the range of density quoted in (273.15 + t) OIML R111 [1] (1.2 kg m-3 ± 10%) Temperature is 20 °C ± 5 °C Where AD = Air density (kg m-3) Pressure is 1013.25 mbar P = Air pressure (mbar) ± 60 mbar h = Relative humidity of the air (%rh) RH is 50% ± 30% t = Air temperature (°C) For a temperature controlled lab the There is a typical uncertainty of 4 parts in range of air densities experienced will 104 on this equation over the range of air -3 be less (typically 1.2 kg m ±5%) density of 1.2 kg m-3 ± 10%
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Druck Pressure Indicator Protimeter Dew Point Meter Edale model CD Thermometer
Figure 1: Typical Air Density Measurement Equipment
Air Density Artefacts Variation in Buoyancy