PCM-shielding of heat sources for
reduction of thermal errors
Schalles, M. Fröhlich, T. Röser, M.
Technische Universität Ilmenau
Flügge, J. Köning, R.
Physikalisch Technische Bundesanstalt, PTB
EMRP IND13:
Thermal design and time-dependent dimensional drift
behaviour of sensors, materials and structures
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Research fields
Measurement
dynamics
and signal
processing
Force
measurement
and weighing
technology
Nano-positioning
and
Nano-metrology
Temperature
measurement
Modelling
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Laser metrology
Background
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EMRP IND13: Thermal issues in precision dimensional measurement and
engineering
Thermal optimisation of measuring microscope of Nanometer Comparator at PTB
Sketch of Nanometer Comparator and measuring microscope
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Bridge and measuring microscope (yellow)
Background
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Joule heating due to electrical power of
camera P ~ 4.5W
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Obvious: problem with generated heat
Difference of camera surface temperature
to room temperature ΔTsurf ~ 15 K
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Heat exchange with measuring microscope
and comparator
Non-homogeneous temperature field x
different thermal expansion coefficients →
unknown changes in temperature (and
time) dependent dilatation
Need for thermal improvement/
optimisation
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Camera used in microscope
Thermal image of mounted camera
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Objectives
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Need for thermal improvement/
optimisation
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Camera should be cooled down
Enable stable thermal conditions during
dimensional measurement
Thermal decoupling of camera from
measuring set-up
Provide thermal shield between warm
camera and comparator set-up
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Camera used in microscope
Thermal image of mounted camera
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Cooling approaches
Active Cooling
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Passive Cooling
Actively servo-controlled heat sources
or heat sinks
Control or stabilise the temperature of
a component or instrument
Sources or sinks are peltier elements,
electrical heaters, stirling motors or
heat pumps
In combination with heat exchangers
to temper working media like gases or
fluids
Sometimes coupled directly without
working media
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Constructive changes in instruments to
optimise the paths of heat flow or to
minimise the thermal sensitivity of the
set-up
Parasitic heat guided away to
thermally insensitive components or
environment
Without active controlling or
influencing of heat sources
Sufficient understanding of the heat
transport phenomena or temperature
fields necessary
Gain by temperature measurement or
thermal modelling and simulation
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Advantages and drawbacks of the approaches
Active Cooling
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+
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
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Passive Cooling
Temperature at position of interest can
be controlled directly
Different types of sinks with different
power are available
Power of the exchanged heat can be
adjusted
Sources may contain moving or
rotating parts - mechanical vibrations
in the measurement
Cause dimensional measurement
errors - uncertainties at the nanometer
level
Moving working fluids or even
ventilating air can be a source of
disturbing vibrations either
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+
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Reduced effort (no electronics,
controllers and sensors)
No mechanical vibrations occur
Minor cooling power compared to
active approaches
Resulting temperatures are depending
on the boundary conditions
Temperature deviations which will not
be zero
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Simulation of passive cooling
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Static FEM simulation of temperature field
Temperature stabilised room, Tamb = 20 ºC, vair = 0.2…0.4 m/s
Mounting elements
Bridge
Camera
Starting condition
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Mounting elements Zerodur
Still temperature gradients and heat flow into microscope
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Fin cooler mounted
Cooling by Phase Change and Latent Heat
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Preliminary considerations showed:
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No active cooling system and no active air cooling system should be used (avoiding of
mechanical vibrations)
Cooling power of passive air cooling by heat sinks is too low
Passive cooling and shielding by means of a phase change material (PCM) is chosen
Calibration of thermometers
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PCM-shielding
Cooling by Phase Change and Latent Heat
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Which PCM to use?
Group
Inorganic substances
Organic substances
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Material
Melting temperature/
°C
Latent Heat/
kJ·kg-1
0
29.7646
28.4
15.66
20.48
25.19
29…30
22
17.9
19
333
80
15
n.a.
n.a.
n.a.
170…192
215
199
140
Water
Gallium
Caesium
Ga-In (18 %weight percent In)
Ga-Sn (13,8 %wt Sn)
Ga-Zn (3,7 %wt Zn)
CaCl2∙6H20 (salt hydrate)
n-Heptadecane (paraffine)
Glycerin
Butyl stearate (fatty acid)
Most PCMs based on hydrocarbons or salt hydrates have broad temperature range
of phase transition (unsuitable)
Eutectic alloy GaSn changes phase at ~20.5 ºC
Advantages:
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Phase transition temperature is close to working temperature range of Nanometer
Comparator
Narrow melting temperature range
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Design of Cooling Element – Demonstration Model
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Development of demonstration model with camera dummy
Camera Dummy
Fin Cooler
Cavity filled with PCM
PCM Container
Base plate
Camera Mounting
Construction drawing of Demonstration Model
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Thermal Simulations
0h
0.5 h
8h
9h
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4h
10h
Measurement Setup in Climatic Chamber
PCM Container
Fin Cooler
Camera Aperture
Distributed
Temperature
Sensors
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Measurement in Climatic Chamber
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Temperature stabilises after
~ 0.5 hours
Phase transition lasts 6 to 8
hours (melting)
Temperature at mounting
remains constant within 6
hours
After switching off, material
freezes again
Improvement of plateau
shape and duration of
phase transition
t=0h
Electrical power of camera dummy
switched on (P = 4.5W)
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t=8h
Electrical power
switched off
Summary and Outlook
Summary
• Proof of principle of passive cooling method by means of latent heat
• No moving parts and fluids, no vibrations
• Thermal shield for camera provided
• Stable conditions at camera mounting for 6 h
Outlook
• Further improvement of design (homogenisation of phase transition)
• Development and construction of final design of cooling element which is
appropriable in Nanometer Comparator
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Outlook
Outlook to new design
 Longer duration of phase transition with same amount of PCM
 Radialsymmetric design
 Easy filling of PCM
PCM Container
Camera
PCM Container
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Questions about PCM shielding and cooling?
Acknowledgement
• The EMRP is jointly funded by the EMRP participating countries within EURAMET
and the European Union.
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