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Convection in high res probesPete Gierth, Bruker UK
Worldwide application meeting 2015
Convection
• Wikipedia says:
• Convection is the concerted, collective movement of groups or aggregates of molecules within fluids (e.g., liquids, gases) and rheids, either through advection or through diffusion or as a combination of both of them.
• We normally mean simply advection, and treat diffusion separately
• Source is density gradient, arising from temperature gradient• Extent is affected by viscosity amongst other things
Convection
• The main problems caused by convection:
• Distortions of diffusion measurements
• Losses in gradient coherence selected experiments
• Distortion of gradient echo profiles in gradient shimming
D=4*10-10 m2/s
0.05 mm/s0 mm/s
0.1 mm/s
0.2 mm/s
0.5 mm/s
Effect of convection on diffusion measurements
Effect of convection on diffusion measurements
Rayleigh–Bénard Convection
http://commons.wikimedia.org/wiki/File:ConvectionCells.svg
Onset of convection - Rayleigh NumberCase of infinitely long cylinder
Acceleration due to gravity
Thermal expansion coefficient
Density
Specific heat capacity
Tube radius
Viscosity
Thermal conductivity
𝑅𝑎=𝑔 𝛽𝜌2𝑐𝑝𝑟
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𝜂𝜅𝑑𝑇𝑑𝑧
Onset of convection – critical temperature gradient Case of infinitely long cylinder
𝑅𝑎=𝑔 𝛽𝜌2𝑐𝑝𝑟
4
𝜂𝜅𝑑𝑇𝑑𝑧
dT/dZ (CHCL3) ~ 0.1-0.3K/cm
dT/dZ (DMSO) ~ 0.6-1.8K/cm
dT/dZ (H2O) ~ 2.4-7.2K/cm
Horizontal temperature gradientsHadley convection
450l/hr
450l/hr
200l/hr 1.5Wdec.
1.5W
6W320K5mm/s
3.3mm/s
Temperature profile in the X-Y planeNominal temperature 320KStrong gradient along Y axisTemperature range +/- 0.14 = 0.7K/cmTheoretical vmax = 2.1mm/s in CDCl3
Horizontal temperature variation
~1K/cm
Data on Bruker probesFitting
V=0.06mm/s
V=0.2mm/s
V=1.2mm/s
Data on various Bruker probesChloroform sample
• Older TXI, dual flow, 400l/hr • Older SEI, dual flow, 400l/hr• No chiller
Data on various Bruker probesChloroform
• TXI, made 2012?• BCU-I• 535 l/hr• Shim temp ~306k
Data on various Bruker probesChloroform
• BBFO (not Smart)• 535 l/hr
Data on various Bruker probesChloroform
• BBFO (not Smart)• 400 l/hr• T gradient <0.2K /cm @ 323K
Data on various Bruker probesChloroform
• BBFO (not Smart)• No VT gas at all
0.012mm/s
0.02mm/s
Data on various Bruker probesChloroform
• SmartProbe, 3mm tube• 400 l/hr• T gradient about 0.1K/cm @ 323K as measured in 5mm tube• Vmax <<0.1mm/s around room temp
• Vmax = 0.1mm/s @323K
Data on various Bruker probesChloroform
• SmartProbe• 500 l/hr, no chiller (400 MHz), 535l/hr, BCU-I (500MHz)• 5mm tube
Data on various Bruker probesChloroform
Prodigy298K, 535l/hrVmax = 0.24mm/s
“Cryoprobe”simulated by imposing artificial T gradient200 l/hr gasflow
Slice selective experiment on MeODEach row refereced using CD2H peakGradient of OH peak position shows T gradientZ shim correction on water sample = 24 units
Chloroform2.2mm/s
D2O0.14mm/s
“Cryoprobe”0.3K/cm temperature gradient
Chloroform – 3mm tube0.08mm/s
Conclusions
• Convection is indeed ubiquitous• Extent is rather probe dependent• Under most conditions significant enough to
affect measured D values• Use 3mm or sapphire tubes if necessary • If in doubt, measure!
Acknowledgements
Aitor Moreno
Jerome Coutant
www.bruker.com
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