LAB FOR MEASUREMENT TECHNOLOGY Prof. Dr. rer. nat. A. Schütze

COST Action TD1105 – EuNetAir WG Meeting: New Sensing Technologies and Methods for Air-Pollution Monitoring

Gas Sensor Systems for Indoor Air Quality Monitoring European Environment Agency, Oct. 3 – 4, 2013, Copenhagen

Andreas Schütze Saarland University, Lab for Measurement Technology

> Outline
Introduction: indoor applications and air quality
Gas measurement systems – more than just sensors





The three “S” Gas measurement systems Signal processing and evaluation Calibration and field testing


Indoor Air Quality monitoring
Target gases and concentrations
Potential sensor solutions


Novel developments
Novel sensors: versatile GasFETs
System self monitoring
Multifunctional multisensor systems


Conclusions 10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Introduction: Indoor Air Quality Why worry about indoor air?
Safety
Gas leak detection (combustible gases, e.g. CH4)
Fire detection (various gases)
Hazardous gas detection (e.g. CO)
Malodor detection (kitchen & bathroom ventilation)
HVAC systems
Reduced air circulation for greatly reduced energy consumption  CO2 monitoring for fresh air
Increased levels of VOCs lead to sick building syndrome  Selective (formaldehyde, benzene etc.) and sensitive (ppb level) detection
Systems have to be adapted to the specific room use scenario

10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Introduction: Indoor Air Quality Sensor requirements
Low cost
Networked systems (in major buildings, but also private homes)
Long lifetime: >10 years without maintenance for private homes Which sensors are used today?
Safety
Gas leak detection: pellistors (ind.), human nose (in Japan: MOS)
Fire detection: various sensors, mostly optical; gas sensor systems under development (EC, MOS, GasFET)
Hazardous gas detection: EC, MOS
Malodor detection: MOS
HVAC systems
CO2 monitoring: NDIR (in major rooms/buildings), EC, GasFET
VOCs: MOS (total VOC), GasFET (emerging) 10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Gas measurement systems – more than sensors The three “S”
Sensitivity
Broad spectrum from ppb (for malodors, ozone, hazardous VOCs) up to 1000 ppm (gas leak, CO2)
Selectivity
False alarms are primary concern for fire detection (ratio 10:1)
VOC detection: hazardous (formaldehyde) vs. neutral (alcohol vapor, cleaning agents) vs. wanted (odorants)
Stability
Industrial applications: maintenance interval < 6 months
Public buildings: annual or bi-annual tests (if that)
Private homes: 10 years lifetime w/o regular maintenance?

10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Gas measurement systems – more than sensors

indoor atmosphere

A generic modular gas sensor system

10/7/2013

r.h.

sensor signal processing and evaluation

chemical and/or IR gas sensors T

display

data interface

electronics

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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Gas measurement systems – more than sensors Temperature Cycled Operation (TCO) – hardware Hardware platform PuMaH for exact temperature control and large dynamic range data acquisition - Pulse-width-modulated Measuring and Heating Unit • Heater temperature control Heater resistor RH(T) controlled for exact temperature control of (micro-)hotplates

• Sensor resistance read-out Gas sensitive layer: RS(gas) – Closed-loop control mode  constant voltage drop across RS – Temperature compensation mode  large dynamic range of 26 bits

• Software controlled • 16 bit PWM outputs used to apply signals to RH and RS  now commercialized “OdorChecker” by 3S GmbH 10/7/2013

Th. Conrad et al., IEEE Sensors Conference 2005

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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Gas measurement systems – more than sensors Temperature Cycled Operation (TCO) – software 30

Benzene

Iso-Pentane

Diethyl Ether

Methyl Tert-Butyl Ether

70% r.h. 30% r.h.

Methyl Alcohol A. Schütze, A. Gramm, T. Rühl IEEE Sensors Journal, Vol. 4, No. 6, 2004

Conductance [a.u.]

20

10

0 50

Propylene Oxide

40 30 20 10 0

0

5

10

Signal evaluation: 10/7/2013

15

20 0 5 10 15 20 0 Time in Temperature Cycle [sec]

5

10

15

1. Normalization of the response curves ⇒ reduces sensor drift 2. Generation of secondary features, i.e. levels, slopes etc. 3. Suitable patterns are extracted for further evaluation

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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20

Gas measurement systems – more than sensors Temperature Cycled Operation (TCO) – software Synthetic air, Iso-Pentane, Benzene, Methyl Alcohol, Methyl Tert-Butyl Ether, Diethyl Ether, Prop. Oxide

Evaluation of sensor data based on temperature cycling (example)  Virtual multisensor

Note: the decision tree reflects the chemical composition of the solvents starting with the alkane pentane and the aromatic benzene (both pure CH-compounds), then the alcohol (R-COH) and finally the three ether compounds (R1-O-R2). This indicates that an expansion might be possible to classify many different molecules. 10/7/2013

T-Cycle 1 Iso-Pentane Decision 1 Source: A. Schütze, A. Gramm, T. Rühl IEEE Sensors Journal, Vol. 4, No. 6, 2004

Characteristic features of the curve shapes (i.e. slope at the end of the high temperature phase and curvature during the low temperature phase) are evaluated, to discriminate between different gases in several steps.

clean air & interferents

Benzene Decision 2

Methyl Alcohol

T-Cycle 2

Methyl 4-Butyl Eth.

Decision 3

Diethyl Ether Decision 4

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

Propylene Oxide

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Gas measurement systems – more than sensors Temperature Cycled Operation – system design

Sensor sensors

Messdaten raw data dynamic dynamische meas. Messung

T-Zyklus T-cycle Messhardware electronics

Datennormierung normalization Merkmalsextraktion feature extract.

Unterscheidung/ discrimination/ Trennung separation Klassifizierung classification

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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E. Leonhardt, Thesis project, UdS-LMT, 2007.

Signalverarbeitung Signal processing

10/7/2013

GMA gas test system

EntscheidungsClassification prozess

…and always testing under real application conditions (field testing)!

Static Statische meas. Messung

Many possibilities for optimization: • Sensor selection • Operating mode • Data acquisition • Signal preprocessing • Feature extraction • Separation • Classification

Target appl.

> Gas measurement systems – more than sensors Calibration: novel gas mixing system for VOC testing down to sub ppb-level

10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Gas measurement systems – more than sensors Novel gas mixing system: results of reference measurement (zero air) compound benzene toluene chlorobenzene camphene benzaldehyde phenol benzonitrite octanal benzyl alcohol acetophenone naphthalene bicyclol[2.2.1]heptane,2-chloro2,3,3-trimethyl TVOC 10/7/2013

CAS no 71-43-2 108-88-3 108-90-7 79-92-5 100-52-7 108-95-2 100-47-0 124-13-0 100-51-6 98-86-2 91-20-3 465-30-5

c [µg/m3] 0.17 0.06 0.26 0.29 0.2 0.3 0.61 0.1 0.19 0.62 0.24 16.2

c [ppb] 0.053 0.016 0.056 0.052 0.046 0.06 0.144 0.019 0.043 0.126 0.046 2.6

24.3

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Gas measurement systems – more than sensors Novel gas mixing system: results of first sensor tests Sensor reaction to 1 ppb formaldehyde

10 9,0x10-3

cond. UST 1000 conc. formaldehyde

upper range value of ADC

1

-3

8,0x10

7,0x10-3

0,1

4,1x10

6,0x10-3

-3

40000

time [s]

50000

0,01

5,0x10-3 1E-3

concentration [ppm]

conductance [A/V]

conductance [A/V]

4,2x10 -3

4,0x10-3 1E-4 -3

3,0x10

10000

20000

30000

40000

50000

time [s] 10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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Relevance? Legal limits in France: Formaldehyde 25 ppb in 2015; Benzene 0.6 ppb in 2016 13

> Indoor Air Quality monitoring MNT-ERA.net project VOC-IDS
Volatile Organic Compound Indoor Discrimination Sensor
Scenario specific detection of hazardous VOC
Integration of sensor system into KNX building automation networks

10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Indoor Air Quality monitoring VOC-IDS: ppb-level detection of VOCs against background gases? Similar requirements: Fire detection in coal mines
CO/C2H4 mixtures at 10 ppm/100 ppb respectively
Background: up to 1% methane + interfering gases: r.h., CO, H2, NO2 etc. long term stability 400

temperature set point [°C]

P. Reimann, PhD thesis, UdS-LMT, 2011

450

C2H4

387°C

different temperature levels for the target gases ramps between these levels allow max. reproducibility

350

326°C

300

C2H4 and CO

250

feature extraction

CO

200

225°C

185°C

slopes and mean values from these sections

185°C

150 0

10

20

30

40

50

60

time [s] 10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Application examples

6

gas atmosphere

UST 1330; air flow: 500ml/min

P. Reimann, A. Schütze: Sensor Review Vol. 32 Iss: 1, 2012

raw-data feature extraction

1st LDA 30%

50%

70%

discriminant function 2

4 2

2

0 1

-2

50% r. H.

3

-4

70% r. H.

30% r. H.

-6

-8 -20

-10

0

10

20

discriminant function 1

2nd LDA 0.1%

10/7/2013

0.5%

1%


discrimination along function 1
similar for CH4

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Application examples 5

UST 1330 air flow: 500 ml/min

3

0.1%

0.5%

1%

2 discriminant function 2

P. Reimann, A. Schütze: Sensor Review Vol. 32 Iss: 1, 2012

4

3rd

LDA

1 0

F

-2 -3

NF

non-fire CH4 (1 %)

increasing concentration

-1

-4

NF

non-fire CH4+CO (1 % + 4 ppm)

fire CH4+CO+C2H4 (1 % + 4 ppm + ramp 0,06..0,16 ppm)

-5 -100

0

100

200

300

400

discriminant function 1


alarm decision
discrimination along function 1 & 2
additional step(s) for interfering gases Field test: correlation with existing sensors shown, stability needs to be improved 10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Novel developments Novel sensor materials and transducer principles
Nanotechnology for improved gas sensitive layers
Novel transducer principles
Gas ionization (can be electronically controlled!)
GasFETs commercially available

Source: Micronas 10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Novel developments SiC Gas-sensitive Field Effect Sensors (Linköping U, SenSiC)
high temperature operation
allows temperature cycled operation as for MOS
(nano-)porous platinum and iridium gate materials

10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Novel developments SiC GasFETs (Linköping U, SenSiC): high sensitivity

Sensor reaction to 2 ppb benzene 10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Novel developments System integration: temperature and gate bias variation for SiC-GasFETs 240

5% O2 in N2

6

400 ppm CO 40 s ramp

240

40 s ramp

Ugate

200

5

current iDS (µA)

current iDS (µA)

160

120

80

gate bias Ugate (V)

200

160

120

80

4

3

2

1

40

40 0

200°C

0 0

1

2

3

4

200°C

0

5

0

1

2

gate bias UG (V) 300

3

4

5

0

20

40

60

80

100

120

time (s)

gate bias UG (V) 90 5% O2 in N2

5% O2 in N2

120 s ramp

40 s ramp 80

250

200

current IDS [µA]

current iDS [µA]

70

150

100

50 40 30 20

50

150°C

0 0

1

2

3

gate bias UG [V]

10/7/2013

60

4

5

10

200°C 0

1

2

3

4

5

gate bias UG [V]

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

C. Bur et al.: Influence of a Changing Gate Bias on the Sensing Properties of SiC Field Effect Gas Sensors, IMCS 2012 Slide

21

140

> Novel developments • Electrical Impedance Spectroscopy yields similar improvement in selectivity as Temperature Cycled Operation (but time scale is completely different) EIS data

TCO data

shifts caused by 100 ppb ethene in 1% methane

A. Schütze et al.: Improving MOS Virtual Multisensor Systems by Combining Temperature Cycled Operation with Impedance Spectroscopy, ISOEN 2011 10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Novel developments

EIS

feature extraction

feature extraction diagnostic

discrimination yes

classification

hardware for TCO operation available at reasonable cost 10/7/2013

discrimination

difference no output

classification

hardware for EIS -poor mobility -high cost -long acquisition time

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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A. Schütze et al.: Improving MOS Virtual Multisensor Systems by Combining Temperature Cycled Operation with Impedance Spectroscopy, ISOEN 2011

T-cycle

decision process

decision process

Sensor self-monitoring with combination of TCO and EIS

> Novel developments




Implementation using an FPGA (field programmable gate array) MLS signal refined with variable amplification using dedicated circuit Data acquisition using high speed ADC (sample rate 200 MHz)

sensor stimulation 10/7/2013

data acquisition

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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A. Schütze et al.: Improving MOS Virtual Multisensor Systems by Combining Temperature Cycled Operation with Impedance Spectroscopy, ISOEN 2011

Low cost EIS hardware realization: general approach

> Novel developments MOS-IR measurement system:
Gas filled chamber (9 cm length)
MOS gas sensor (MICS 5131, e2v)
Transmission: Thermopile (HIS A21 F4.26, Heimann Sensors)
Ambient condition monitoring (p, r.h./T)
Electronics controlled by a microcontroller
Configuration settings set by a GUI (LabVIEW)
Data evaluation offline using Matlab

K. Kühn et al.: Investigations on a MOX Gas Sensor as an Infrared Source for an IR-based Gas Sensing System, IMCS 2012 10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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> Novel developments

1.05 125 ppm CO 120 ppm C 2 H4

1.00 0.95 0.90 0.15 0.10 0.05 0.00 0

1

2

Time in temperature cycle (s)

900

K. Kühn et al.: IMCS 2012

Max. cyc. val. electrical conductance (a.u.)

Normalized electrical conductance (a.u.)

MOS signal: Raw data DC resistance → Shape of the response curve from temperature cycle CO (25, 75, 125 ppm) C2H4 (20, 70, 120 ppm)

800 700 600 500 400 300

air

0

gas

air

500

gas

air

1,000

gas

1,500

2,000

Time (s)

Transmission signal: Raw signal (thermopile) → DFT analysis of raw data (90 ON/OFF cycles) → |Y(f=fA)|2(c)

2

600 500 400 300

synth. air 2,000 ppm 5,000 ppm 40,000 ppm

4M 3M 2M 1M 0

200 0

1

2 Time (s)

10/7/2013

3

4

0.48

0.49

0.50

0.51

0.52

DFT magnitude |Y(f=fA)|2 (a.u.)

synth. air 40,000 ppm CO2

DFT magnitude |Y(f)| (a.u.)

Signal intensity (a.u.)

700

CO2

4M

3M

2M 0

10k

Time (s)

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

20k

30k

Concentration (ppm)

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40k

> Novel developments Transmission signal, fA = 6 Hz square wave mod. of the MOS gas sensor (MICS 5131, SGX)

10/7/2013

1M

800k 75 % r.h. RT

600k CO2

400 300 200 100 0 0

5,000

C2H4

NH3

NO

NO 2

CO

25k 20k 15k 10k 5k 0

10,000 15,000 20,000 25,000 30,000 Time (s)

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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K. Kühn et al.: Investigations on a MOX Gas Sensor as an Infrared Source for an IR-based Gas Sensing System, IMCS 2012

1M 1.2M

CO2 [ppm]

Carrier gas [ppm] DFT magnitude |Y(fA)|² (a.u.)

Transmission signal CO2 (Thermopile detector)

> Conclusion
Indoor applications are of increasing interest, especially for improving energy efficiency and health
Gas measurement systems are more than just sensors
Multifunctional, intelligent multisensor systems can address emerging applications
Application specific development still required
Field testing is an absolute must for any chemical sensor system  Field tests of VOC-IDS sensor systems are starting now
Chemical sensor systems can become ubiquitous in modern building environments and a key to Indoor Air Quality

10/7/2013

Gas Sensors for Indoor Air Quality Monitoring – EuNetAir-Meeting – Copenhagen, Oct. 3-4, 2013

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LAB FOR MEASUREMENT TECHNOLOGY Prof. Dr. rer. nat. A. Schütze

Thank you for your attention.