Measurement of the density of moist air using gravimetric artefacts James...
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Measurement of the density of moist air using gravimetric artefacts
James BerryNational Physical Laboratory
March 2007
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Contents
• Background• Air density measurement using artefacts
– Artefact development at NPL– Current generation of NPL artefacts– Air convection effects– Experimental results and uncertainties– Conclusions
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Background
• Primary requirement for precise air density measurement is for the dissemination from Pt-Ir to stainless steel
• An air density uncertainty of 1 part in 105 would give an uncertainty contribution of 1 µg in Pt-Ir / stainless steel comparisons
• Conventional air density measurement is limited by the accuracy with which P, t and DP can be determined
• and ultimately by the uncertainty in the CIPM equation to no better than 5 parts in 105
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Artefact development at NPL
• First generation artefacts – hollow sphere and cup– Developed because no weighing in vacuum facility existed– Artefact mass could only be determined before evacuation and sealing
• Large volume difference –approximately 760 cm3
• Sealing of artefacts proved problematical
• Poor mass stability resulted in little improvement in air density uncertainty
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Current NPL air density artefacts
• One set of large air density artefacts (volume difference approximately 510 cm3)
• One set of small air density artefacts (volume difference approximately 105 cm3)
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Air convection effects
• Previous work by Gläser states that convection effects can occur at a weights surface due to heating or cooling
• Computational fluid dynamics (CFD) models of the air flow over the surface of each shape of air density artefact were constructed
• CFD models indicated that tube shapes are a better design for the small volume artefact than bobbin shapes (confirmed by weighing data)
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Artefact shape effects on weighing stability
1.65
1.652
1.654
1.656
1.658
1.66
1.662
1.664
1.666
1.668
1.67
0 5 10 15 20 25 30 35 40
Time (hours)
mas
s di
ffere
nce
(mg)
-0.295
-0.293
-0.291
-0.289
-0.287
-0.285
-0.283
-0.281
-0.279
-0.277
-0.275
mas
s di
ffere
nce
(mg)
tube
bobbin
19.35
19.4
19.45
19.5
19.55
19.6
19.65
0 5 10 15 20 25 30 35 40
Time (hours)
Tem
pera
ture
(°C
)
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Hollow air density artefact
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Tube shape air density artefact
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Bobbin shape air density artefact
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Experimental results
Air density artefacts vs. CIPM method
1.196 50
1.196 55
1.196 60
1.196 65
1.196 70
1.196 75
1.196 80
29/04/200514:24
30/04/200504:48
30/04/200519:12
01/05/200509:36
02/05/200500:00
02/05/200514:24
Time
Air
dens
ity (k
g/m
3) CIPM
88H-88T
88DH-88DT
88H-88DT
88DH-88T
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Uncertainty budget
1.2 ×10-5
(10.5 ×10-5 )Relative combined uncertainty, uc
(kg m-3)
0.8 ×10-50.004 21Volume uncertainty (cm3)
0.3 ×10-51.8Weighing scheme
Air mass uncertainty (µg)
0.2 ×10-5
0.0 ×10-5
0.5 ×10-5
1.40.003.10
Weighing schemeSorption correctionVacuum mass stability
Vacuum mass uncertainty (µg)
Relative influence,ui (ρ)/ρ
Standard uncertainty, ui
Parameter
Table 8: Uncertainty budget of air density evaluation using the artefact method. All uncertainties are reported at the 1 σlevel. The relative combined uncertainty in brackets is NPLs best uncertainty using the conventional method.
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Conclusions
• Factor of 10 improvement in air density uncertainty compared with conventional (parametric) method
• Good agreement between new method and conventional method corrected for proposed change in Argon content of equation (within uncertainty limits)
• Some small offsets were found between the two methods
• CFD analysis highlighted tubes as a better design of artefact than bobbins