Environmentally Robust CarbonMonoxide Sensor for Fire Detection

Dr. Neils R.S. Hansen& Dr. Ann M. Harvey

Objective“ To develop a quality CO sensor made via a

robust automated manufacturing process.”

Portable Gas Detection

Flammable Gas Detection

Toxic Gas Detection (Semiconductor)

BRAND NAMES

Residential Gas Detection

Sensor Supplier

UK’s No. 1 manufacturer & supplier of residential carbon monoxide (CO) alarms, 1.5 M units sold since 1996.

UK Headquarters, Poole, England

We manufacture both CO detectors & sensors to pass BSI, UL & CSA standards

Approximately two million cells sold since 1996. Only CO cell used presently for fire detection.

World’s largest gas detection company.

DESIGN STRATEGY

Critical analysis of existing products and processes

Improve performance to meet more stringent certification approvals introduced since 1996

Design for automation

Failure Modes and Effects Analysis

Failure rate in field use is < 0.05 %

Process : poor sealing due to manual errors

Effects Zero CO sensitivity ( filter blockage ) False alarms ( corrosion currents )

Complete customer satisfaction demands the effective elimination of these failure modes from the ECO-Sure(TM) cell design.

Design : electrolyte leakage in extreme R.H.

Analysis of CO Cells ( End-of-Life after ~ 5 yrs in field use )

0

10

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50

60

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80

90

100

0 1 2 3 4 5

CO Cell Lifetime (years in field use)

Nor

mal

ised

Sen

sitiv

ity (

% o

f ini

tial)

Predicted Sensitivity DecayMeasured Decay of Old ProductPredicted Ecosure Decay

Performance Requirements

Improved baseline stability

Wider operational humidity rangeHigh R.H. = no electrolyte leakageLow R.H. = improved span stability

Increased physical and chemical filtration capacity

Reduced cost of work in progress

Design for Automation

Patented design has fewer components for automated manufacture

Efficient use of all components Charcoal cloth acts as a filter, an electrode support and also an efficient

gassing plenum Wick acts as both an electrolyte reservoir and compresses the electrode

stack Separator preferentially draws electrolyte from the wick and electrically

isolates electrodes

Materials have all been proven historically over five years

Previous Design

Pt Working Electrode

Plastic HousingImpregnated CharcoalCloth Filter (2 layer)

Wick

Pt Counter Electrode

Push-fit Pins

Electrolyte

ECO-Sure(TM) DesignPlastic Housing

Impregnated CharcoalCloth Filter (5 layer)

Pt Working Electrode

Wick

Pt Counter Electrode

Insert-moulded Pins

Separator Electrolyte

Theoretical Electrolyte Volume vs Relative Humidity

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10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100

Relative Humidity ( % at 20 oC )

Elec

trol

yte

Volu

me

( % o

f cel

l fre

e vo

lum

e )

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1

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8

9

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11

12

Con

cent

ratio

n (m

ol d

m-3

)

ECO-Sure Electrolyte Volume Electrolyte Concentration

maximum capacity

Equilibrium humidity of injected electrolyte

ECO-Sure(TM) Cells During High Humidity Storage

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110

0 5 10 15 20

Time ( weeks )

Nor

mal

ised

Sen

sitiv

ity (

% )

0.0

0.1

0.2

0.3

0.4

0.5

Wei

ght C

hang

e ( g

)

ECO-Sure cell sensitivity

Mean ECO-Sure weight change

48 ECO-Sure(TM) cells stored at 96% R.H and 20oC

UL TestFailure

ECO-Sure(TM) Cells During Low Humidity Storage

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0 5 10 15 20 25

Time ( weeks )

Nor

mal

ised

Sen

sitiv

ity (

% )

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

Mea

n W

eigh

t Cha

nge

( g )

ECO-Sure cell sensitivity

Mean ECO-Sure weight change

48 ECO-Sure(TM) cells stored at 11% R.H and 50oC

UL TestFailure

More efficient use of electrode allows a 16 % reduction in the area of the cap hole

ECO-Sure(TM) Cap Assembly

Old Design Cap Assembly

2 layer charcoal cloth

5 layer charcoal cloth

X-sections

Cross Sensitivity Comparison ( Old Design vs ECO-Sure(TM) )

90

60

4

15

40.5 0.53

0

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90

100

800 ppm NO2 ( VdS ) 2000 ppm Ethanol(EN50291)

50 ppm SO2 ( VdS ) 200 ppm IPA ( UL2034)

Gas Applied

Res

pons

e ( C

O p

pm e

quiv

alen

t )

Old designECO-Sure

PASS criteria is < 36 ppm CO equivalent

Improved Baseline Temperature Stability Effect : Positive Baseline Drift at elevated temperature

Effect : Negative Baseline Drift at elevated temperatures

Comparative Baseline Drift PerformanceOld Design = + / - 8 ppm CO equivalent(acceptable for residential CO monitoring)ECO-Sure(TM) < + / - 2 ppm CO equivalent(required for fire detection applications)

Cause : As manufactured organic contamination de-sorbs from the filter.Solution : Condition cells at elevated temperature prior to release

Cause : VOCs out-gassing from potting compound

Solution : Improved curing regime

Baseline Temperature Drift of ECO-Sure(TM) Cells

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-1

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6

0 5 10 15 20 25 30

Time ( hours )

Res

pons

e ( C

O p

pm e

quiv

alen

t )

-20

-10

0

10

20

30

40

50

60

Tem

pera

ture

( C

elsi

us )

Temperature

Mean Baseline DataMaximumMinimum

Baseline Temperature Shock of ECO-Sure(TM) Cells

-2

-1

0

1

2

3

0 1 2 3 4 5 6 7 8 9 10

Time ( hours )

Bas

elin

e D

rift (

CO

ppm

)

-40

-30

-20

-10

0

10

20

30

40

50

60

Tem

pera

ture

( C

elsi

us )

Mean Baseline DriftMaximum Drift ( Mean + 2 x s.d. )Minimum Drift ( Mean + 2 x s.d. )Temperature

Baseline Temperature Shock of ECO-Sure(TM) Cells

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5

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40

0 1 2 3 4 5 6 7 8 9 10

Time ( hours )

Bas

elin

e D

rift (

CO

ppm

) Threshold Alarm Gas Concentration

ECO-Sure(TM) Product SummarySemi-automated production since March 2001 Two stages of process FMEA fed into automated production line Yields are recorded for continual process improvement

ECO-Sure(TM) cells have been approved in instruments tested to : BS, LPC, VdS & UL2034 (due April, 2002)

UL Component Recognition Advanced recognition scheme, monthly linearity test, maximum

concentration measurement and continuous 15 ppm CO exposure Provisional recognition granted (Feb 2002) achieved on first submission

Automated Manufacture

Quality Improvements ( Components ) Cleanliness Dimensional tolerance Placement

Quality Improvements ( Process ) Thermal welding of electrode followed by 100 % leak testing Electric welding and continuity testing of every current collector Every critical operation has a minimum of one verification check SCADA system monitors and reports production status

Consistent high quality of manufacture

Product Summary

Product currently supplied in volume to multi-national fire and residential CO detector manufacturers

Significant investment in automation to improve manufacturing process and increase throughput

Significant improvement in cell performance to significantly exceed all current standards