saw sensors 23

8/8/2019 Saw Sensors 23

http://slidepdf.com/reader/full/saw-sensors-23 1/27

1

Improved dew point measurements

based on a SAW sensor



2

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 s0.2oC

± poor performance at dew points below -70oC



3

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 Qg cm-2

temperature resolution of 0.003oC

discontinuous measurements of dew point



4

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



5

Theory

Dew 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



6

Nucleation tends to be governed by high

energy location on the condensing surface

The condensing surface or surfacecontamination affect the dew point

measurement

Change in drop size occur via twoadditional processes: ripening and

coalescence

Ripening =(diffusion of vapor molecules

from small to large drops)/((t caused by

vapor pressurelarge drops - vapor pressuresmall

drops )



7

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



8

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



9

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 beingrequired for the same reflectance change

Kelvin effect: an increase of the vapor

pressure caused surface tension of smalldroplets

Kelvin effect decreases the measured dew

point



10

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



11

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



12

SAW condensation sensor

The condensation is approximated as anideal(non-viscous) fluid layer

oo

21

2221

f

f

V

V

fh)k k (V

V

)

2

(

V

4fhk fh)k k (

V

V

JJ!!

Vd!

dPd

dPdd Vd!



13

Experimental

An Al mirror detects condensation usingoptical reflection

Substrate: ST quartz

Operating frequency: 80 MHz

Cooling the sensor to the dew point p

water condenses on surface of the substrate

p SAW phase shift

LED reflects IR light off the mirror p

condensation changes mirror¶s reflectance

pdetect with a phototransistor



14

Temperature is measured with an RTD

Study the effects of surface contamination:dual delay line YZ LiNbO3 incorporated in

an oscillator circuit



15

R esults and analysis

SAW sensor measurements of condensation

density

The detection limit is roughly double than

that was found by Wylie



16

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 fluidlayer



17

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 12h by controlling it with SAW phase



18

Optical sensor measurements of condensation

density

The reflection voltage is not proportional tocondensation density

The condensation density drift affects

control system stability, and results in theneed for a dry-off cycle when using the

optical technique



19

Effects of surface contamination on SAW and

optical measurements



20

Fig. 5(a): 60 m change in optical

reflection voltage: 5.5 Qg cm-2 of

condensation measured with SAW sensor

Fig. 5(b): 60 m change in optical

reflection voltage: 30 Qg 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



21

Surface contamination

experiment: pass tobacco

smoke across surfaceduring dew measurement

at 50% RH and RT p

repeat 5 timesp

clean thesensor

The dew point

measurement error of the

SAW sensor varies by

s0.5oC



22

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 thatsurface contamination appears to have no

effect on SAW dew point measurement

accuracy



23

Frost point transition

Detection of the frost point transition is at -23oC

As the dew deposit froze the SAW velocityincreases to its value when no condensation is

present

This increase in SAW velocity may be due to thecoalesced state of the frost deposit



24

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 reflectionvoltage or SAW velocity

SAW amplitude could be used to maintain

constant condensation density during thefrost transition period providing continuous

dew point measurements



25

SAW performance comparisons with a

commercial instrument

Resolution of the SAW sensor is s0.025oC

Resolution of the commercial optical sensor

is s0.2oC



26

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)



27

When the condensation density> 0.8 Qg 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 density< 0.1

Qg cm-2

Teflon AF coating offers two advantages:

± It could be used to improved sensor response

time at extremely low dew points(<-70oC) ± It reduce he need to periodically clean the

sensor

saw sensors 23

Documents