humidity measurement in industrial gases

Humidity Measurement in Industrial Gases Wednesday, 11th October 2006 National Physical Laboratory

Hampton Road, Teddington, Middx TW11 0LW

Programme 09:30 Registration

10:00 Welcome to NPL

10:05 Water Vapour Measurement in HCl Graham Leggett (NPL) Gordon Ferrier (Air Products)

10:35 Humidity Measurements in Compressed Gases Thomas Hübert (BAM)

11:05 Analyser Development for Fast Diagnostics of Plant Conditions Paul Stockwell (IMA)

11:25 Moisture in Medical Gas: The New HTM02 Regulations Nick Malby (Michell Instruments)

11:55 Discussion

12.25 Lunch

13:30 Users View on Humidity Measurement Nick Bates (National Grid, formerly Transco)

14:00 Materials Performance in the Presence of Moisture Alan Turnbull (NPL)

14:30 NPL's Facility for Humidity Calibration at Elevated Pressures and Non-air Gases Stephanie Bell (NPL)

15:00 Break 15:15 Round table discussion on NPL’s three year research plan and meeting industrial needs

16:30 Close

Trace Water Vapour Measurements and Calibration

including Gas Matrix Effects

Tom Gardiner, Graham Leggett, Analytical Science Team,

National Physical Laboratory

Humidity Measurements in Industrial Gases11th October 2006

Summary

• NPL’s Trace Water Vapour Facility• Comparison of trace water vapour sensors• Infared spectroscopic (CRDS) measurements of water

vapour, including matrix effects• Measurements of trace water vapour in HCl.• Conclusions

Requirement for Trace Water Vapour Calibration

• Increasingly challenging purity specifications for gases used in high-tech manufacturing processes.

• Specifications based on gas concentrations (gravimetric traceability) rather than humidity scales (thermal traceability).

• Analytical Science group at NPL has extensive experience in the preparation of static and dynamic gas standards.

• Comparability with humidity standards is a crucial element of facility validation.

NPL Trace Water Calibration Facility

Flow mixing system using mass flow controllers and

critical orifices

Heated getter unit to provide ultra-high purity source gas

Multi-port outlet to enable simultaneous sensor

evaluation

Micro-balance providing on-line weighing of magnetically-suspended permeation tube

Dynamic Flow Dilution System

Temperature control unit

Output valve

Input valve

MFC (diluent)

MFC (WV source)

Vent o/pvalve

WV source outputSonic flow

nozzles

Source control valves

WV source input

Microbalance for On-line Weighing of Permeation Tube

Automated balance 0-20 g load, 3 µg reproducibility

Magnetic suspension of load

Permeation tube mounted in temperature-controlled

cell. Customised EP-stainless steel cell for water

vapour system

Facility Performance

• Zero-offset level measured using spectroscopic techniques to be less than 1 ppbv (limited by spectrometer).

• Flow switching and dynamic dilution systems validated to better than 0.2%.

• Uncertainties in mass emission rate typically 0.3% over one hour.

• Evaluation of the facility using a calibrated frost point hygrometer have demonstrated good (~1%) agreement between frost points of –67oC and –90oC (4 ppmv to 30 ppbv).

Water Sensor Evaluation

• A group of 13 trace moisture instruments have been tested, in order to provide general information on the performance of different sensor types.• The following data shows examples of the tests of response accuracy, time, linearity, and hysteresis.• Other tests were carried out to assess long-term stability, ambient temperature response, and effects of exposure to ultra-dry (sub-10 ppbv) conditions.• The study enables some general conclusions to be drawn for the different measurement methods and their suitability in different applications.

Assessment of Water Vapour Sensors

0

100

200

300

400

500

600

S1 S2 C1 C2 E1 E2 P1 P3 P4 P5 P6 P7

Tim

e / m

inut

es

90% (mins)10% (mins)

Response times for a upward step change of 300 to 850 nmol/mol

Response time, linearity and hysteresis results from water vapour sensor assessment.

Instrument S1 S2 C1 C2 E1 E2 P1 P3 P4 P5 P7

Type CRDS Laser absorption

Hysteresis (at 300

nmol/mol)0.017 0.282 0.009 0.034 0.217 0.2 0.082 0.139 0.27 0.391 0.007

Linearity (R2)

0.9998 0.9829 0.9993 0.9985 0.9513 0.9614 0.9641 0.9877 0.9894 0.9817 0.9875

Frost-point hygrometers Electrolytic Probes Capacitance Sensors

CRDS Measurements of Trace Water Vapour

• A CRDS instrument (Tiger Optics Lasertrace) is being used within NPL’s trace water vapour facility

• CRDS instrument selected as an on-line sensor due to its high sensitivity, fast response and good linearity.

• Currently assessing absolute accuracy of CRDS measurement through comparisons against gravimetric and thermal (frost-point) humidity standards.

• Issues affecting CRDS accuracy include laser stability, absorption and desorption from cell and sample line walls, Tau0 (empty cell) values, gas temperature and pressure, traceability of spectroscopic parameters, and matrix effects.

Infrared Spectroscopic Measurements of Gases

• Infared spectroscopic measurements work by using light to measurement the optical absoprtion ‘fingerprint’ of the target species.

• In the infared region the spectroscopic fingerprint of a particular species depends upon the rotational and vibrational modes of the molecule.

• The shape of the absorption features are effected by the ambient conditions and interactions with surrounding molecules.

Spectroscopic Measurement Methods

Direct Absorption Spectroscopy

Source DetectorResonant cavity with HR mirrorsMeasurement volume of length L

( ) ( )( )⎟

⎟⎠

⎞⎜⎜⎝

⎛−=

λλλα

0

logIILN

Cavity Ringdown Spectroscopy (CRDS)

0

0

Time

Sign

al

Expontential signal decay with a period of Tau (the ring down time)

Source Detector

Resonant cavity with HR mirrors

( ) ( ) ( )⎟⎠⎞⎜

⎝⎛ −= λτλτλα 0

111c

N

Absorption lineshapes

• Natural linewidth – due to Heisenburg’s Uncertainty Principle, and driven by the electron lifetime of the initial and final states (∆ti and ∆tf).

• Doppler linewidth – due to the Doppler shifts from the thermal motion of the molecules, determined by temperature (T) and molecular mass (m).

• Pressure linewidth – due to perturbation of the molecule by collisions with other molecules and close encounters with molecular electric fields. Determined by the molecular density (n) and the collision cross-section (σ).

⎟⎠⎞

⎜⎝⎛

∆+∆≈∆fi ttc

112

2

πλλ

mkT

c22λλ =∆

mkTn

c22

πσλλ =∆

Effect of Matrix Gas on Spectroscopic Measurements

• Pressure linewidth dominates for infrared measurements at or above atmospheric pressure.

• Overall pressure linewidth is a combination of the pressure linewidths for each gas present, including the target gas itself (self-broadened linewidth).

• Molecules with significant electric fields (polar molecules such as water vapour) have stronger interactions, leading to broader lines.

• Level of effect is not necessarily the same for each absorption line as it depends on the electron transition involved.

• So the overall lineshape is a complex relationship involving the gas mixture present, the temperature of the system, and the absorption line being measured.

Water Vapour in HCl – Measurements at Air Products Crewe Facility

BIP N

2

VLSI HC

l

P PTiger Optics

MichellHygrometer

Ventto scrubberVacuum

Panel supplied by Air Products

Flow meter

Measurements of Water Vapour in HClCRDS Scan of Water Vapour in Nitrogen

-1.E-02

0.E+00

1.E-02

2.E-02

3.E-02

4.E-02

5.E-02

6.E-02

7177 7179 7181 7183 7185 7187

Wavelength (nm)

modelmeasured

CRDS Scan of Water Vapour in HCl

0.E+00

1.E-02

2.E-02

3.E-02

7177 7179 7181 7183 7185 7187

Wavelength (nm)

modelmeasured

• Spectral scans of several water vapour lines made by changing wavelength of laser in CRDS.

• Comparison of scans in nitrogen and HCl matrices shows the broadening in HCl.

• This is due to the stronger interaction between the HCl and H2O molecules.

CRDS Scan Fitting SensitivitiesLinewidth sensitivity

-3

-2

-1

0

1

2

3

-5 -3 -1 1 3 5

Change in linewidth (%)

Cha

nge

in c

once

ntra

tion

(%)

3.0E-03

3.5E-03

4.0E-03

4.5E-03

5.0E-03

fit R

MS

(arb

. uni

ts)

% change inconcentrationRMS

X-axis stretch sensitivity (tuning rate)

-2

0

2

4

6

8

-5 -4 -3 -2 -1 0 1 2 3 4 5

X-axis stretch (%)

Cha

nge

in c

once

ntra

tion

(%)

0.0E+00

5.0E-03

1.0E-02

1.5E-02

2.0E-02

2.5E-02

Fit R

MS

(arb

. uni

ts)


Shift sensitivity

0.0

0.1

0.2

0.3

0.4

0.5

0.6

-4 -3 -2 -1 0 1 2 3 4Frequency shift (% of Voigt FWHM)

Cha

nge

in c

once

ntra

tion

(%)

3.0E-03

6.0E-03

9.0E-03

1.2E-02

Fit R

MS

(arb

. uni

ts)


TemperatureEffects line strength, line width and gas density

-2.475 Almost linear relationship

Pressure Effects linewidth, line position, gas density 0.526 Almost linear

relationship

Linewidth

Due to spectroscopic errors or uncertainty in matrix effects

-0.475 Slight curvature

Stretch Due to uncertainty in laser tuning rate 0.935 positive

curvature

Shift Due to uncertainty in laser temperature

0.04 for a 1% FWHM shift 0.13 for a 3% FWHM shift

minimum at optimum value

Parameter Notes on possible causes and effects

Sensitivity (%conc./%parameter)

Sensitivity behaviour

Analysis of Water Vapour in HCl Results

• Initial estimate of the relative linewidths was taken from the self-broadened linewidths

• A broadening factor was then applied to all of the linewidths, and optimised to give the best fit to the scan.

• A width parameter of [0.575 (+/-0.005) * self-broadened width] gave the best fit to the HCl scan.

• The width of the strong line at 7181.156 cm-1 increased by a factor of 2.78 when switching between nitrogen and hydrogen chloride matrices.

• This compares very well with the value of 2.76 measured by Vorsaet al.– Quantitative absorption spectroscopy of residual water vapor in high-

purity gases: pressure broadening of the 1.39253-µm H2O transition by N2, HCl, HBr, Cl2, and O2’; V. Vorsa et al; Applied Optics, 44(4), 611-619; Feb 2005

Conclusions – Calibration Facility

• Validation of trace water sensors requires careful design of thecalibration source, with particular attention to gas handling.

• A new trace water vapour calibration facility has been developedcapable of generating controlled amounts of water vapour down to10 ppb (and up to 4 ppm+)

• In addition to evaluating measurement accuracy, the facility can be used to assess other aspects of sensor performance, including– Linearity– Response time– Hysteresis– Long term stability

Conclusions – Spectroscopic Measurements

• Spectroscopic measurements provide an useful, non-contact measurement method for trace water vapour, with high sensitivity, linearity and response time.

• Accurate spectroscopic measurements require a good understand of how the absorption features are effected by the measurement conditions.

• This is particularly true for measurements in different matrix gases, where there can be significant changes to the spectroscopy.

• As with all trace water measurement techniques, the sample handling is a crucial part of the measurement method. This is particularly the case if there is strong interaction between the water vapour and the matrix gas.

Acknowledgements

• Gordon Ferrier; Air Products• Kevin Cleaver and Keith Waterfield; BOC• Wen-Bin Yan and Calvin Krusen; Tiger Optics• Kevin Lehmann; University of Virginia• Stephanie Bell and Marc Stevens; NPL Humidity Group

11/10/06 Humidity Measurements in Compressed Gases 1

HUMIDITY MEASUREMENTS IN COMPRESSED GASES

Thomas Hübert

Bundesanstalt für Materialforschung und -prüfung,

D-12203 Berlin, Germany


What is

?Bundesanstalt fürMaterialforschung und -prüfung

Federal Institute forMaterials Research and Testing

Safety and reliability in chemical and materials technologies

History: 1870 Preußische Königliche Mechan.-Techn. Versuchsanstalt1920 Chemisch-Technische Reichsanstalt1969 Senior Federal authority

Staff: 1579 permanent, temporary, apprentices and trainees Organisation: 9 specialised departments, 35 divisionsBudget: 96.4 Mio€ federal funds

17.7 Mio€ research projects and fees for testing


1. INTRODUCTION MOTIVATION

Water in compressed gasses :

Demands from Standards: • Ph. Eur. “European Pharmakopoe” 4.07/1238.• ISO 8573-1 “Compressed Air – Contaminates and Purity Classes”.• EN 12021 „Respiratory protective devices – Compressed air for breathing apparatus“

Stimulation of corrosionLeaching of lubricants and acceleration of wearDestruction of moving unitsFormation of aggressive substances (acids)Ice formation and disruptionGrowth of fungi and bacteria


1. INTRODUCTION

OUTLINE

1. INTRODUCTION

2. THEORY

3. SENSOR TESTING

4. RESULTS

5. SUMMARY


2. THEORETICAL APPROACH

van der Waals equation 1873

Redlich-Kwong-Soave 1972

Virial equation

TRnnbVV

anp ⋅⋅=−⎟⎟⎠

⎞⎜⎜⎝

⎛+ )(2

2

All formulas are approximations !

interaction (attraction) of gas moleculs

volume of moleculs

Gas mixture (Dalton law):

othersOHNOwetair ppppP +++= 222

Z - compressibilitygas law (real) TRZnVp ⋅⋅⋅=⋅

gas law (ideal) TRnVp ⋅⋅=⋅


2. THEORETICAL APPROACH : HUMIDITY MEASURES

100),('

'⋅=

tpee

w

No. measure symbol unit equations related to e

1 Water vapour pressure of pure phase or air

e, e´ Pa

2 Water vapour saturation pressure above water or ice of pure phase or air

ew, ew’ei, ei´

Pa

3 Dew point or frost point temperature

td, tf °C

4 Relative humidity related to water or ice

Uw, Ui %

5 Water vapour density , absolute humidity

dV kg/m3

6 Mixing ratio r kg/kg

7 Volume content of water vapour wv m3/m3

8 Water vapour molar fraction x mol/mol∑∑ +

=+

=iiv

v

pee

nnnx

''98,621ep

er−

⋅=

100,

⋅⎥⎦

⎤⎢⎣

⎡=

tpvw

vw x

xU

vvv

v xn

npe

VVw ====

∑'

),('' dw tpee =

Te

ZVmd

mix

vv

'121667.0 ⋅⋅==

Humidity measurement in gases: determination of water vapour concentration


4. SENSOR TESTING : CALCULATIONS

No. software supplier name source

1 Thunder Scientific, USA HumiCalc www.thunderscientific.com

2 Michell Instr. UK HumiCalc www.michell.co.uk

3 E+E Electronik, Austria HumCalc www.epluse.at

4 PTB, Dr. Mackrodt hygdat

5 LAB-EL Laboratory Electronics, Poland

LAB-EL Humidity Calculator

www.label.com.pl

6 ThermExel PsychroSi www.thermexcel.com

7 National Weather Service Forecast Weather Calculator www.srh.noaa.gov

8 Australian Bureau of Meteorology Humidity Calculator www.bom.gov.au

rdUvtee d ↔↔↔↔↔ '



⎟⎟⎠

⎞⎜⎜⎝

⎛+⋅

⋅==dw

dwdw tb

tatete exp2.611)()(

-45 > t> +60°Cpure water phase :aw= 17.62, bw = 243.12 K

Magnus

Sonntag (-100.0 < t < +100.0 °C, water)

TTTTTew ln433502.210673952.110711193.2635794.169385.6096)(ln 2521 +⋅⋅+⋅⋅−+⋅−= −−−

TT

TTTTew

log5459673.61014452093.0

1041764768.01048640239.03914993.11058002206.0)(log37

24114

+⋅⋅−

⋅+⋅⋅−+⋅⋅−=−

−−−

Hyland and Wexler

Temperature dependence of water saturation pressure


2. THEORETICAL APPROACH: REAL GAS

⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−+⎟

⎠⎞

⎜⎝⎛ −= 11exp),(

w

w

eP

Peptf βα

∑=

−=4

1

)1(

i

iitAα ∑

=

−=4

1

)1(expi

iitBβ

1+⋅⋅= PTgf h

Concept of enhancement factorGoff 1949

Hebestreit 1988

Water vapour (saturation) pressure depends onTemperatureGas pressureNature of gas

www efe ⋅='

)(),('),(

TZtpZ

xxtpf

w

wgasw ⋅=

calculation of f (Greenspan, Wylie):



Enhancement factors

t/°C p/MPa p/MPa p/MPa

0,1 1 10

0 1.008 1.076 1.761

20 1.005 1.055 1.548

40 1.004 1.040 1.403

t/°C p/MPa p/MPa p/MPa

0,1 1 10

0 1.005 1.005 1.511

20 1.004 1.039 1.392

40 1.003 1.031 1,306

air methane

Calculated according from uncertainty ~10%

Hebestreit 1988

1+⋅⋅= pTgf h


2. THEORETICAL APPROACHGas pressure influence on water vapor amount

Water solubility in air is nearly constant up to 300 bar, pressure increase results in condensation

isochorisobar

“A wet sponge being squeezed”



Pressure dew point (PDP)

derived from the Magnus equation:

Gas pressure influence on water vapor amount

aaaaddd P

PPtftePtfte 1,111 ),()(),()( ⋅=

aww

dw

aw

dwd

d

pp

batb

pp

atbt

t1

1

1

ln1

ln

⋅⋅+

−

⋅+

+=



Compression from normal pressure to 8 bar results in increase of dew pointfrom -8 to 20 °C.

Expansion of air of dew point of 10 °C form 36 bar to 5 bar results in decrease

of dew point to - 23 °C.

Gas pressure influence on dew point

2

1

1

2


3. SENSOR TESTING: Calibration

Definition: The set of operations which establish, under specific conditions, the relationship between values indicated by a measuring instrument, measuring system or material measure, and the corresponding known values of a measurand. (VIM-6.13)

Calibration function

Analytic function

)(SGc =

)(cFS =

Output y Measuring Instrument Input x


3. SENSOR TESTING : TEST FACILIY


3. SENSOR TESTING : TEST FACILITY


3. SENSOR TESTING : EXPERIMENTAL RANGE

humidity range range of measurement uncertainty conditions

ambient 1 ≤U ≤ 99 % (U - rel. humidity)-40 ≤ td ≤ +40 °C (td –dew point temperature)

0.5 to 2 %≥0.2 °C (td)

-40 to +100 °C, ±0.3 K

trace 0.02≤ v ≤ 2000 µl/l (ppmv)v – volume amount-100 ≤ tf ≤ -14 °C(tf –frost point temperature)

0.02 to 35 µl/l

≥ 0.2 to 2 °C

1 bar, room temperature

high 50 ≤ f ≤ 300 g/m3

40 ≤ td ≤ 80 °C

5.5 to 10 g/m3

≥0.3°C

70 to 180°C0.3 K1 bar

pressure -60 ≤ td ≤ +60 °C 0.1 ≤ f ≤ 120 g/m3

0.2 to 1 °C0.005 to 3 g/m3

1 to 350 bar


• Accreditation

competence of a lab for testing and calibration (DIN EN ISO 17025).

• Certification

conformity of a product(DIN EN 17025, DIN ISO 9000).

3. SENSOR TESTING : QUALIFICATION


4. RESULTS: METHODS FOR HUMIDIY MEASUREMENTS

No Method Principle Measured signal

Gas pressure range

Humidity range

Manufacturer(selection)

1 Two-pressure generator

Mixing of gas streams Pressure/volume

1…10 bar td -90…+90°C NPL, NIST, PTBE+E electronics

2 Mechanical Change of length length ? r.h. 10…100 %. Galltec, Brown Boveri-Kent

3 Psychrometer Cooling of wet air stream

temperature ? td -20…100 °C r.h. 5…100 %

BARTEC

4 Gravimetric Absorption of water Mass/ gas volume

? td -60 …+60 °Cr.h. 0…100 %

NPL

5 Condensation Water of ice formation on a mirror

Light intensity/temperature

1…300 bar td -90…+60 °C Michell Instr., MBW, General Eastern,Mini: CIS, IL-Metronic

6 Conductivity Change of conductivity

Voltage, impedance

r.h. 0…100 % Novasina

7a Capacity Change of permittivity of polymer

capacitance 1-15 bar r.h. 0…100 %td -60…40 °C

TESTO, CS-Messtechnik,Vaisala, Rotronic

7b Capacity Change of permittivity of oxide (alumina)

capacitance 1…350 bar -100…+20 °C Panametrics, Endres& Hauser, Michell Instr., Alphamoisture

8 Acoustic Wave Adsorption changes wave expansion

frequency ? td -100… Du Pont, Beckmann, AAA

9 Spectroscopic CRD,Absorption of water

absorption 1..250 bar 0.2 ppb…5 ppm-80…+30 °C

Tiger Optics, BARTEC

10 Electrolytic Water electrolyse current 1…200 bar 1…5000 ppm MEECO, DKS


4. RESULTS

Comparison of precision dew-point hygrometers

frost point S4000TRS (°C, ist) -53,19

corrected frost point S4000TRS

(°C)*-53,12

frost point MBW 373 (°C) -53,09

difference +0,03 K

frost point S4000TRScalc. from 1 bar

-39,13 °C (6,035 bar)

frost point MBW 373 -40,00°C (6,035 bar)

difference -0,87 K

1 bar

6.035 bar


4. RESULTS : TESTING OF NEW HUMIDITY SENORS

CCO, CCC and LiCl dew-point sensors


3. RESULTS : HUMIDITY SENORS

Detection principle of CCO, CCC and LiCl dew-point sensors

Saturation vapor pressure above water and LiCl solution

Pres

sure

in P

a

Temperature in °C

LiCl

water


4. RESULTS : DEW POINT SENSOR

Deviation from Reference <0.5 K

CCO Sensor



Deviation from Reference <1 K

CCC Sensor




LiCl Sensor



Polymer and Oxide Sensor

semiconductingn-type Fe2O3

Polymer Sensor

Pt100



Polymer and Oxide Sensor

beaeZ −⋅=)(

0 20 40 60 80 100

1300

1350

1400

1450

1500

1550

1600

1650

1700 H2OK2SO4K2NO3

(NH4)2SO4

NaClKJ

K2CO3

t=25°C

CaCl2

LiCl

Polymer Sensor 1006

increasing humidity decreasing humidity linear Fit

sens

or c

apac

ity (p

F) b

ei 1

0 kH

z

relative humidity (%)

exponential decreaselinear increase

.).%( hrbapF ⋅+=


4. RESULTSOxide Sensor

0 2 4 6 8 10

2

-10-5

05 10152025

Sen

sor S

igna

l (V)

Dew-Point Temperature (°C

)

Gas Pressure (MPa)

nitrogen


4. RESULTSOxide Sensor

methane


4. RESULTS

Gas pressure influence on oxide sensor signal

0 500 1000 1500 2000 2500 3000 3500 4000

1,0

1,5

2,0

2,5

3,0

3,5

4,0 MPa

0,1 1 4 7 10

sens

or s

igna

l (V)

water vapor pressure e*f (Pa)

0 2 4 6 8 10

0,0002

0,0004

0,0006

0,0008

0,0010

0,0012

0,0014

B

deka_sensor_CH4a_graph5, 21.3.06

sens

itivi

ty (V

/Pa)

gas pressure (MPa)

∑=

+= n

iii

E

PK1

1

1θ

Langmuir Isotherm for adsorption of several gases on surface

fraction of empty sites: Decrease in adsorption sites for water

1E-4 1E-3

1,0

1,5

2,0

2,5

3,0

3,5

0,1 MPa 10 MPa

Sens

or s

igna

l (V

)

Molanteil


4. RESULTSPolymer Sensor



4. RESULTS

Gas pressure influence on polymer sensor signal

pRHbpRHb

RHRHpRHRHp

oooop )(1

)(),(+

⋅∆−=∆−= ∞

formal fit with Langmuir-like mode of gas sorptionLuijten 1998


4. RESULTS

Example for calibration at increased pressure

Frost point S4000TRS (°C)* -64,80 -19,78 -14,65

Frost point at 7 bar absolute pressure (°C)** -50,36 + 2,35 + 9,31

Reading frost point sensor (°C) - 56

Deviation of the sensor from calculated value of S4000TRS

(K / 7bar abs. pressure)

- 5,64

Adjustment of reading of sensor (K) + 6

Reading Frost point of Sensors (°C)

after correction - 50 + 3 + 10

Deviation of sensor after adjustment (K) + 0,36 + 0,65 + 0,69

*NPL- Calibration Certificate No. 44436 (02.10.03)** Software „HumiCalc“, Copyright 1993 Thunder Scientific Corporation, Vers. 1.21w

Polymer sensorsource: TESTO


4. RESULTS

References

http/www.drucklufttechnik.de/english

[1] Ph. Eur. “European Pharmakopoe” 4.07/1238.[2] ISO 8573-1 “Compressed Air – Contaminates and Purity Classes”.[3] EN 12021 „Respiratory protective devices – Compressed air for breathing apparatus“, 1998.[4] ASTM D 1142-95, “Standard Test Method fort he Water Vapor Content of Gaseous fuels by

Measurement of Dew-Point Temperature“, ASTM International, USA 2000.[5] St. Bell, St. Boyes, “An assessment of experimental data that underpin formulae

for water vapour enhancement factor”, NPL 2001 online.[6] J. A. Goff, “Standardization of Thermodynamic Properties of Moist Air”,

Heating, Piping and Air Cond. 21 (1949), S. 118.[7] L. Greenspan, “Functional Equations for the Enhancement Factors for CO2-free Moist Air”,

J. Research NBS, A. Physics and Chemistry, vol. 80A, 41-44, 1976. [8] R. G. Wylie and R. Fisher, “Molecular Interaction of Water Vapor and Air”,

J. Chem. Eng. Data 41, p. 133-142, 1996.[9] A. Hebestreit, „Messung der Wasserdampftaupunkt-temperatur in Hochdruckgasleitungen“,

msr, 31, 403-405, 1988..[10] C.C.M. Luijten, M.E.H. Dongen, L.E. Stormbom, “Pressure influence in capacitance humidity

measurement”, Sens. & Act. B 49, 279-282, 1998.


5. SUMMARY

5 Sensors (CCO, CCC, LiCl, Polymer, MOX) tested at -15 (-60) to 20°C

Deviations from reference <2 K

Application in compressed gases up to 10 MPa.

Calibration in pressure range of 0.1 to 10MPa

Gas pressure influence due to adsorption model

1. INTRODUCTION humidity in compressed gases

2. THEORY real gas behaviour: f(p,t)

3. SENSOR TESTING dewpoint and capacitive hygrometer

4. RESULTS under pressure uncertainty ~ 2 K

Comparison of 2 precision hygrometer with deviation 0.03 K (1 bar)


5. SUMMARY

Over two thirds of the earth’s surface is covered with water, 97.2% of which is contained in the five oceans. The Antartic ice shild, containing 90% of all fresh water on the planet, is visible at the bottom. Atmospheric water vapor can be seen as clouds, contributing to the earth's albedo.

tanks to my co-workers:

Ulrich BanachHeidi LorenzKarin KeilGerd HenningAndreas LorekDirk KleinJürgen Raesch

Improving gas analysis

Simulating Plant Conditions

An examination of instrument response to brief moisture ingress


Plant conditions

Most process applications require switching between gas supplies of various qualities. When using gases in some processes it is important that components like catalysts are not exposed to greater than 2 PPMv


Plant conditions

Industrial process with using gas switching systems there are lengths of pipeline that contain stationary gas at some point during the process

Stationary gas will wet up


Plant conditions

Moisture levels will increase in gases within pipe systems with zero flow– Dalton's Law of Partial Pressures explains

the reason that “dry” high pressure gases have a lower water vapour pressure than the surrounding ambient air at low pressure

– Some pipe walls are porous or will out-gas moisture content

– Seals and joints leak


Consider Partial Pressures

Atmospheric

N2 = 78% = 0.78 BarA

O2 = 21% = 0.21 BarA

H2O = +6 oC DP = 10 mBarA

Compressed Gas

N2 = 78% = 7.8 BarA

O2 = 21% = 2.1BarA

H2O = -60 oC DP = 0.01 mBar


Plant conditions

It is possible that brief “slugs” of moist gas will enter the system as valves are opened or when purging the static gas from the pipework system

This would lead to a rapid wetting of the system followed by a rapid drying once a dry gas flow is re-established


Gas Rig

What would be the response of moisture analyser systems under these rapidly changing conditions?

A rig was designed to reproduce the "worst case" situation and challenge two types of moisture analyser


Gas Rig


Test Procedure

To simulate the sample system a 2m length of ¼ tube was installed between the moisture generator and the analysers on testTwo systems were tested– An aluminium oxide transmitter– A G6 Tunable diode laser system


Test Procedure

The analysers were installed in series to avoid preferential flows. – The laser did not change response when

placed in front of the aluminium oxide sensor

– The aluminium oxide performed slightly better when placed in ahead of the laser

A flow rate of 2.5 l/min was used


Test Procedure

To simulate a brief "slug" of moisture into a pipework system a 0.5 Sec. exposure to moist gas was used

A flow rate of 2.5 L/Min

After the initial dry-down the test was repeated every 10 minutes


Dry Down

The aluminium oxide sensor and system showed good dry down response reaching 90% in 5 min

The laser analyser and system had a 90% step change in 38 Sec

0

50

100

150

200

250

300

350

400

450

14:30 15:00 15:30 16:00 16:30 17:00

G6

ALU


ResultsReproducing plant failure

0

50

100

150

200

250

300

350

400

450

14:4

0:00

14:5

0:00

15:0

0:00

15:1

0:00

15:2

0:00

15:3

0:00

15:4

0:00

15:5

0:00

16:0

0:00

16:1

0:00

16:2

0:00

16:3

0:00

16:4

0:00

Time

PPM

v

G6 Laser Aluminium Oxide


Conclusion

Things CAN happen quickly in dry gas systemsIf this was a real system the user would not be aware that there is an issue to be addressedThe speed of response of the laser system allows better visibility of process conditions


Further Work

What is the effect of adding a particular component to the response of the pressure reduction sample system?– Regulators– Valves– Filters– Flow meters

Does temperature play a role?

Materials Performance in the Presence of Moisture

Alan TurnbullCorrosion and Electrochemistry Group

What does moisture do to materials

Corrosion and corrosion assisted fracture of metals

Accelerated fatigue cracking

Polymer/composite/glass degradation

Degradation of stone work, concrete

Cracking of wood

Condensing media for electrochemical catalytic activity in fuel cells

In-Plane TensionUnidirectional Laminates

Continuous UD glass fibre-reinforced laminate

Typical tensile failure

Stress RuptureStress RuptureEE--glass Fibresglass Fibres

E-glass fibres are sensitive to moisture and handlingUTS = 3.5 GPa (virgin) and 2.0 GPa (post-processing)

Effect of moisture on glass fibres: polymer composites &optical fibres

Leaching of alkali oxides (sodium and potassium oxide) from the fibre surface.

— Si —O —R + H2O → — Si —OH + R+ + OH-

Formation of surface micro-cracks ⇒ stress concentrators.

Permanent loss of strength (even after drying).

Accelerated metal fatigue

Moisture can significantly enhance the crack growth rate associated with cyclic loading of metals (fatigue)

For “water-free” fatigue testingdewpoint temperature can bevery low, e.g for some Al alloys itcan be below –50 °C.

(Courtesy of J Petit)

da/d

N

∆Keff

Hydro

gen

assista

nce

Intrinsic

Stag

e II

Adso

rptio

n As

sist

ance

Intrinsic

Stag

eI-

Like

(da/dN)cr

da/d

N

∆Keff

Hydro

gen

assista

nce

Intrinsic

Stag

e II

Adso

rptio

n As

sist

ance

Intrinsic

Stag

eI-

Like

∆Keff

Hydro

gen

assista

nce

Intrinsic

Stag

e II

Adso

rptio

n As

sist

ance

Intrinsic

Stag

eI-

Like

∆Keff

Hydro

gen

assista

nce

Intrinsic

Stag

e II

Adso

rptio

n As

sist

ance

Intrinsic

Stag

eI-

Like

(da/dN)cr

Fatigue of silicon MEMS in humid environment

High stresses induce a thickening of the amorphous surface SiO2oxide layer at stress concentrations such as notches: stress/moisture-assisted cracking of this oxide layer results in fracture of the beam

Corrosion in condensing moisture

Corrosion occurs when liquid layer or droplets form on the surface to enable electrochemical reaction – wetting of surface enhanced by hygroscopic salts; corrosion rate influenced by material and by composition of atmosphere.

Critical relative humidity

Humidity at which no corrosion of metal in question takes placeAffected by capillary condensation and by nature of salts formed on surface

RH OF CONDENSATION ON A POLLUTED SURFACE AT 20ºC

Polluting salt

Na2SO4 (NH4)2SO4

NaCl CaCl2

Calcium chloride is a component of sea salt.

RH of condensation

93% 81% 78% 35%

So, a surface polluted with sea salt is wet at all RH > 35%

Topical issue - nuclear waste containment

Electronic components

D

BA

C

• 400 x 200µm pattern• AuNi finish

Contaminants

Contaminant Chemistry ConcentrationAnionic Surfactant Laurybenzolsulfonsaeure (LABS) 1%

Solvent flux Adipic, succinic, glutaric acid & rosin

1.7%

Exposure to moisture at temperature

More corroded

Water base Acrylic (2)

Epoxy Water base Acrylic (1)-Spray

Water base Acrylic (1)

Polyurethane Fluoroacrylate SiliconeSolvent base Acrylic

Moisture in gases – corrosion and fracture

Gas pipelineo Top-of-the-line corrosiono Bottom-of-the-line corrosion

Anhydrous ammonia cargo tank

Gas cylinders

Top of line corrosion in wet gas pipelines(courtesy of IFE)

Water chemistry in condensing water will be very different from the bulk water phase:

Condensing water will have low pH (from CO2) and high corrosivityCorrosion inhibitors will not be present in the condensing waterSalts from formation water not present in top of lineCorrosion products accumulate rapidly in the condensing water

TLC problems in the field(courtesy of IFE)

Corrosion when water condensation rate is above 0.15 to 0.25 g/m2s

Presence of organic acid (e.g. acetic) in the gas

Top of line corrosion reported in several gas pipelines in South East Asia

Excessive cooling – no insulation, flowing river water

Moisture in natural gas pipeline:bottom-of-the-line

At 5:26 a.m. on August 19, 2000, an explosion occurred on one ofthree adjacent large natural gas pipelines near Carlsbad, New Mexico. Southern California. Twelve people, including five children, died as a result of the explosion. The explosion left an 86 feet long crater.

Moisture in natural gas pipeline

Moisture in natural gas pipeline

$2.52m civil penalty: “…….This included failure to communicate to appropriate personnel when excessive water content was in the gas stream……”

Low gas velocity allowed water to separate from gas and collect in pipeline

Dip in pipeline at location → stagnant pool

Micro-organisms accumulated and with CO2, H2S, O2 and Cl- - very aggressive combination for corrosion

Configuration at location prevented use of pigs to “sweep” liquid away

No internal corrosion checks done at susceptible location

Fracture occurred due to localised wall thinning and exceedance of critical stress for fracture at that location

Moisture in anhydrous ammonia – a beneficial effect at the right level?

On August 22, 2003, anhydrous ammonia was being transferred from the storage tank to thecargo tank

While the cargo tank was still being loaded, its front head cracked open, releasing vapour

Moisture in anhydrous ammonia – a beneficial effect at the right level

Through-wall crack caused by stress corrosion cracking – not a new problem forstorage of anhydrous ammonia. Reason for failure was that production of theammonia had become so improved that water levels were below recommendedsafe level – 0.2% by weight. Procedures require addition of water for Q&T steelbut not followed

Internal corrosion of gas cylinders

Explosions of CO2 gas cylinders due to corrosion and stresscorrosion cracking is major problem (in UK, 10 explosions in 18 months to May 2002).

Solely due to moisture contamination – liquid backfeed and rainwater ingress main problem

Teaspoonful of water (5 g) is enough to destroy a cylinder

Internal corrosion of steel gas cylinders

CO2 + H2O = H+ + HCO3-

Acidic pH, high stress, susceptible steel→corrosion and cracking

Residual pressure valves can limit ingress of water but in absence of these, reliable method of detecting small amounts of water required.

Internal corrosion of gas cylinders

Other failures include HF, HBr….. – combination of corrosion and high pressure

Summary

Materials exposed to humid environments, whether general atmospheric or industrial gases, can degrade by a range of mechanisms depending on the material and its functionality

Failures can be catastrophic

Awareness of the potential for a problem supported by reliable measurement and monitoring with rigorously imposed protocols is essential

Complacency is a major concern

Humidity Calibration for Elevated Pressures and Non-air Gases

Stephanie Bell (NPL)

THERMAL MEASUREMENT AWARENESS NETWORK (TMAN)Humidity Measurement in Industrial Gases

NPL, 11 October 2006

The issues with gases

The issues with humidity calibration and sensors in different gas environments: some are “physical”– Thermal properties of a gas affected by pressure

(number density)– Heat capacity– Thermal conductivity, convective and other heat

transfer– If heat exchange with sensors is significant, differs

between calibration and use, how valid is the calibration?

– Does water vapour diffusion vary with carrier gas conditions?

The issues with gases

Plus a set of more “chemical” issues:– Some gases corrode or permanently degrade

sensors– (we don’t tend to want to calibrate in those gases!)– Other gases temporarily affect sensors

(interferences)– Some gases react with water vapour, making

humidity measurements “ambiguous”

• We can begin to judge whether these effects are significant for users if we– Have a capability to make tests/calibrations under

“conditions of use”– Contrast these with performance in “traditional”

calibration conditions– Try to infer something for extremes (what happens at

10 bar → what at 100 bar)– Compare unalike sensing methods against each

other

Our study• We looked at user needs

– partly via contacts – partly speaking direct to

users

• Who wants what?

Gas Sub-atmospheric

Near 1 bar >1 bar to 20 bar

20-40 bar

40-70 bar 70-200 bar

Air / Nitrogen

Environmental “altitude” testing

Calibrations available

Large interest Compressed air

Natural Gas

Some applications

Large number of applications

Some applications

CO2 Power industry

Argon Military applications – leak testing

Military applications – leak testing

SF6 Switchgear

H2 Hydrogen fuel cells

HCl Some process applications

User priorities

• Compressed air up to 10 bar- - dew points (at pressure) -75 °C to +20 °C

• Natural gas- initially near 1 bar – key threshold near 50 ppm water content (~dew/frost point near –45 °C )

• Inert or less reactive (e.g CO2)- - initially near 1 bar, later to 20 or 40 bar. Frost point range down to about –75 °C (~ 1 ppm).

• Devices to be calibrated- mainly capacitive, condensation,spectroscopic types

• But sometimes users want to reduce a process gas to atmosphere then interpret for process pressure– Relies on calculations– Humidity pressure conversion in air relatively easy– Conversion in natural gas less so (gas non ideality)

e.g. conversion of units in natural gas ASTM 1142, ISO 18453

• Project will give some attention to this

What other NMIs do• Germany – BAM – work up to 3 MPa (30 bar)

– ISHM 2006 poster (dependencies on pressure for some sensor types)

• Netherlands NMi/VSL – to look at pressure and/or non-air gas (natural gas) …at early stage

• Austria E+E Elektronik (appointed national humidity standard) calibration at pressures up to 10 bar

• “Two-pressure” generators in many calibration labs but don’t provide calibrations “at pressure”.

Others in UK – past – e.g.– Domnick Hunter humidity generator – to 17 bar– Michell/Air products work 1998 (nitrogen &

tetrafluoromethane)– Sira humidity work at pressure– Any others we should know about currently ?!

• “Standard”… “facility”… or “capability” at NPL?• Prior recommendation (NMS Thermal Working Party)

Objectives

• Construct a facility for humidity calibration in air at pressures initially up to 10 bar (perhaps higher later)

• Next, adapt this for non-air gases according to user priorities

• Range nominally 1% water vapour down to at least 50_ppm, possibly to 1 ppm (i.e. dew point +20 °C to at least –50 °C, perhaps –75 °C)

• Uncertainty less critical to most users – target 5 to 10 times that of established humidity standards at NPL

• For rapid and cost-effective development, facility initially based on modified commercial two-pressure generator

• Gas blending approach, in certain cases

• Resulting realisation will be either dew point or other units (e.g. mole fraction) as appropriate

• Also caters for nominally ambient-pressure instruments that need high input pressures

• Sub-atmospheric pressures possibly in future

• An outline design exists • gas supply, compressor, filter and dryer stages;

two-pressure generator; pressure and flow controls; and test instrument manifold

• Considerations of materials compatibility; gas purification and (e.g. for natural gas) composition; gas disposal/recovery and safety

Steps

• Consultation/design study - completed• Initial development - pressure capability – to Mar 07• Initial verification - pressure capability – to Mar 07• In 2007-10 programme

– Complete the verification of facility for calibrations of humidity sensors at elevated pressures

– Extend to enable calibrations in gases other than air

Generator

• Thunder 3900 as a starting point

• Modified with valve on outlet to control pressure

• Work on flow-pressure control configurations

Collaboration - instruments

New team member

• Wepawadee Pothinual - PhD student from Brunel University and NIMT (Thai national metrology institute)

Gas pressure issues

If thermal properties (heat exchange) an issue• Obviously consider gas flow• If pressure ↑

– heat capacity per volume ↑– (heat capacity per mole unchanged)– thermal conduction ↑

If water vapour diffusion an issue• If pressure ↑, vapour diffusion rate ↓

Experimental approach

Natural gas composition

Component Range (mole %)

Methane 87.0 - 96.0Ethane 1.8 - 5.1Propane 0.1 - 1.5iso – Butane 0.01 - 0.3normal – Butane 0.01 - 0.3iso – Pentane trace - 0.14normal – Pentane trace - 0.04Hexanes plus trace - 0.06Nitrogen 1.3 - 5.6Carbon Dioxide 0.1 - 1.0Oxygen 0.01 - 0.1Hydrogen trace - 0.02

(and in “sour gas” H2S,)

http://www.uniongas.com/aboutus/aboutng/composition.asp

Natural gas composition

Component Range (mole %)

Methane 87.0 - 96.0Ethane 1.8 - 5.1Propane 0.1 - 1.5iso – Butane 0.01 - 0.3normal – Butane 0.01 - 0.3iso – Pentane trace - 0.14normal – Pentane trace - 0.04Hexanes plus trace - 0.06Nitrogen 1.3 - 5.6Carbon Dioxide 0.1 - 1.0Oxygen 0.01 - 0.1Hydrogen trace - 0.02

(and in “sour gas” H2S CO2)

http://www.uniongas.com/aboutus/aboutng/composition.asp

Where matrix gas components can condense, saturation/condensation humidity processes need caution

More on gases data

Approximate molar heat capacities at constant pressure as Cp/R (near ambient T)

Ar 2.5 CH4 4.3Cl2 4.1 CO2 4.5H2 3.4 H2O 4.0He 2.5 NH3 4.3N2 3.5 HCl 3.5O2 3.5 C2H6 (ethane) 6.3

R = 8.314 51 J · K −1· mol −1

http://www.kayelaby.npl.co.uk/chemistry/3_10/3_10_1.html

Gas non-ideality

humidity measurement in industrial gases

Documents