1. Capabilities and challenges of optical measurements
Jane Hodgkinson
Centre for Engineering Photonics
School of Engineering
Cranfield University

2. Why use optical absorption?
gas
light source
detector
window

3. Non-dispersive infra-red (NDIR) sensing
Broadband light source is optically filtered to target particular gases
Commercially available components; filters can be specified
Small footprint (20mm D x 16mm H)
Typically configured for CO2 *
20 mm
16.5 mm 3
3.2
3.4
3.6
3.8 4
4.2 20 40 60
wavelength / μm
gas absorption / cm-1
filter transmission / %
CH4 absorption band
measurement filter
reference filter
* Hodgkinson J, Smith R, Ho WO, Saffell JR, Tatam RP. Sensor. Actuat. B 186, 580– 588, 2013.

4. Potential for discriminating alkanes60
methane
ethane 40
propane
butane
absorption / cm-1
pentane 20
hexane
3.1
3.2
3.3
3.4
3.5
3.6
wavelength / μm 6
Lower hydroacrbons can be discriminated (C1-3)
Higher hydrocarbons cannot so easily
Total absorption corresponds to C-H bonds - good correlation with CV for C2+
2 point measurement would give CV
Full spectrum would also give full information 4
Total absorption / arb 2 1 2 3 4 5
CV / 103 kJ mol-1

5. Tunable diode laser spectroscopy (TDLS)
Laser emission wavelength may be tuned over a narrow range, enabling the gas spectrum to be measured in a very small region
Near IR region uses telecomms components (lowest cost)
1.62
1.64
1.66
1.68
1.7 50
100
wavelength / μm
transmission / %
This gives:
Gas specificity
Immunity to baseline drifts
Calibration stability
Configurable for accuracy or sensitivity

6. Potential for discriminating C1-C3
Methane + ethane measurement previously implemented in this region[1], accuracy for %LEL measurement of NG was as predicted from gas compositions (± 3%) [2]
Methane, ethane, propane demonstrated in principle by Siemens [3]
Width of single laser scan
methane
ethane
0.2
propane
absorption / cm-1
butane
pentane
0.1
hexane
1.680
1.685
1.690
wavelength / μm
[1] Hennig O, Strzoda R, Magori E, Chemisky E, Trump C, Fleischer M, Meixner H, Eisele I. Sensor. Actuat. B 95, , 2003
[2] Hodgkinson J, Pride R D. Meas. Sci. Technol. 21, (10pp), 2010.
[3] Fleischer M, Strzoda R, Magori E, Meixner H, Hennig O. EP , 2000 (ceased 2007)
Bachmaier G, Hennig O, Magori E, Meixner H, Strzoda R, Tump C. EP , 2004 (withdrawn 2010)

7. Commercial implementation
Geotech – ppm methane detection in landfill gas (high CO2 matrix)
Use of long pathlength cell and 1.65μm laser, lod ~ 1ppm
Kannath A, Hodgkinson J, Gillard RG, Riley RJ and Tatam RP Proc SPIE 7952, , 2011.
GMI / Strathclyde NDIR with 3.3μm LED
Longer path (12 cm) gives lower limit of detection than needed for this application (100ppm)
Massie C, Stewart G, McGregor G and Gilchrist JR. Sensor. Actuat. B –836, 2006.

8. Summary – capabilities and challenges
Detection capabilities
Good signal to noise ratios, long-term reliability, field proven technology
Possible to predict and model response to gas mixtures
Specific measurement of C1-C3 using laser spectroscopy
With NDIR, inferential measurement of CV
NDIR can also measure CO2 levels
Challenges for gas quality measurement
How to measure total inert gases (N2)
How to predict / validate performance if compositional variation is not known or specified (this is particularly a UK issue)

9. Way forward? Test measurement strategies on paper
Requires good data on potential gas compositions
Industry-wide consensus on required performance
Needs engagement from all stakeholders (safety, regulation, operators)
Translation of errors in CV, Wobbe index etc into errors in measurands, via knowledge of compositional variation
Key step to enable instrumentation companies to invest
Approach has been used previously in UK for %LEL accuracy
Methane – only measurement found to be inadequate
Goal-setting company standard produced, test compositions identified
Performance of optical technologies predicted and validated experimentally; methane + ethane gave required performance