Improved dew point measurements based on a SAW sensor

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Improved dew point measurements based on a SAW
sensor
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Introduction

• More accurate and less expensive methods for
  measuring water vapor concentration
• Relative humidity sensors and chilled surface dew
  point sensors
• Drawbacks of optical dew point hygrometer
• high cost
• frequent mirror contamination
• instability under continuous use
• inability to detect the frost-point transition
• accuracy of ?0.2oC
• poor performance at dew points below -70oC

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• Dew point hygrometers based on direct mass
  measurements of condensation
• Inamatsu
• measure condensation with frequency of a bulk
  quartz device and measured temperature with RTD
• continuous tracking of the dew point
• resolution of 0.1oC
• Ziegler
• measure condensation with attenuation of a bulk
  quartz oscillator and measured temperature with
  the frequency of oscillation
• LOD 2 ?g cm-2
• temperature resolution of 0.003oC
• discontinuous measurements of dew point

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• Dew point hygrometer using a SAW sensor
• Kuisma, 1983
• detect condensation with SAW attenuation and
  measure temperature with an RTD
• no apparent advantage over optical dew point
  hygrometers
• Galioeau, 1995
• detect condensation surface density and dew point
  using SAW velocity
• LOD of condensation surface density 3 ng cm-2

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TheoryDew point measurement theory

• Dew point temperature at which the partial
  pressure of water vapor the vapor pressure of
  water
• Dew point measurement cool a surface until
  condensation forms and measure the temperature of
  that surface
• When condensation forms on a surface, it results
  in a reduction of the overall energy of that
  surface

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• Nucleation tends to be governed by high energy
  location on the condensing surface
• The condensing surface or surface contamination
  affect the dew point measurement
• Change in drop size occur via two additional
  processes ripening and coalescence
• Ripening (diffusion of vapor molecules from
  small to large drops)/(?t caused by vapor
  pressurelarge drops - vapor pressuresmall drops )

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• Coalescence process of two or more small drops
  combining to form a single large drop when they
  come in contact with each other
• Coalescence and ripening result in previous
  covered nucleation sites become available for new
  drop formation

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Dew point measurement errors

• Raoult effect a lowering of the vapor pressure
  of the condensate due to the pressure of
  extremely small amounts of water solute matter
  (contaminates) on the condensing surface
• The Raoult effect raises the measured dew point
• The Raoult effect error is difficult to prevent

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• If the amount of surface condensation is high
  enough, the dew point error due to the Raoult
  effect can be minimized
• Surface contamination alters the reflectance of
  the mirror in optical sensors resulting in much
  larger condensation density being required for
  the same reflectance change
• Kelvin effect an increase of the vapor pressure
  caused surface tension of small droplets
• Kelvin effect decreases the measured dew point

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• Increasing the amount of condensation reduces
  errors due to the Kelvin effect
• Since Raoult and Kelvin effect errors are
  opposite in sign it has suggested the two effect
  cancel each other out at relatively large
  condensation densities
• The accurate dew point measurement can be
  determined by increasing the condensation density
  until there is no variation in the dew point
  measurement

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• A high energy surface produces a larger
  measurement error than a low energy surface when
  the condensation density is kept below a certain
  level
• The effect of the frost point transition

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SAW condensation sensor

• The condensation is approximated as an
  ideal(non-viscous) fluid layer

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Experimental

• An Al mirror detects condensation using optical
  reflection
• Substrate ST quartz
• Operating frequency 80 MHz
• Cooling the sensor to the dew point ? water
  condenses on surface of the substrate ? SAW phase
  shift
• LED reflects IR light off the mirror ?
  condensation changes mirrors reflectance ?detect
  with a phototransistor

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• Temperature is measured with an RTD
• Study the effects of surface contamination dual
  delay line YZ LiNbO3 incorporated in an
  oscillator circuit

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Results and analysisSAW sensor measurements of
condensation density

• The detection limit is roughly double than that
  was found by Wylie

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• The measured values are slightly less than the
  theoretical values
• The SAW frequency change is linearly proportional
  to condensation density
• The lower measured values are attributed to the
  approximation of dew as a uniform fluid layer

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• The sensitivity of SAW sensor to condensation
  density is 0.74 ng cm-2 per Hz
• Minimum mass resolution is 18.5 ng cm-2
• SAW system could maintain a constant level of
  condensation density for at least 12 h by
  controlling it with SAW phase

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Optical sensor measurements of condensation
density

• The reflection voltage is not proportional to
  condensation density
• The condensation density drift affects control
  system stability, and results in the need for a
  dry-off cycle when using the optical technique

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Effects of surface contamination on SAW and
optical measurements
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• Fig. 5(a) 60 mV change in optical reflection
  voltage 5.5 ?g cm-2 of condensation measured
  with SAW sensor
• Fig. 5(b) 60 mV change in optical reflection
  voltage 30 ?g cm-2 of condensation measured with
  SAW sensor
• A SAW dew point sensor provides more stable and
  accurate dew point measurements when surface
  contamination is present

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• Surface contamination experiment pass tobacco
  smoke across surface during dew measurement at
  50 RH and RT ? repeat 5 times ? clean the sensor
• The dew point measurement error of the SAW sensor
  varies by ?0.5oC

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• The error is attributed to variations in
  instrument measurement resolution and the series
  placement of the sensors in the gas stream
• The surface contamination can be precisely
  measured by SAW microsensor and that surface
  contamination appears to have no effect on SAW
  dew point measurement accuracy

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Frost point transition

• Detection of the frost point transition is at
  -23oC
• As the dew deposit froze the SAW velocity
  increases to its value when no condensation is
  present
• This increase in SAW velocity may be due to the
  coalesced state of the frost deposit

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• The phase change from water to ice could account
  for an increase SAW velocity
• The significant instability of dew point
  measurements occurred during the frost point
  transition when controlling condensation density
  with optical reflection voltage or SAW velocity
• SAW amplitude could be used to maintain constant
  condensation density during the frost transition
  period providing continuous dew point
  measurements

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SAW performance comparisons with a commercial
instrument

• Resolution of the SAW sensor is ?0.025oC
• Resolution of the commercial optical sensor is
  ?0.2oC

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Effect of surface coatings

• The amount of condensation above the dew point is
  greater for hydrophilic material (Dupont 5878)
  than the hydrophobic material (Teflon AF)

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• When the condensation densitygt 0.8 ?g cm-2 the
  temperature of the SAW sensor is at the dew point
  for all surfaces
• Teflon AF coated SAW sensor can measure the dew
  point at condensation densitylt 0.1 ?g cm-2
• Teflon AF coating offers two advantages
• It could be used to improved sensor response time
  at extremely low dew points(lt-70oC)
• It reduce he need to periodically clean the sensor