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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

4

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