1. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Session 10 / Paper No.2
Maintaining TDL measurement
accuracy in high temperature
applications
Jean-Nic Adami, Mettler-Toledo AG

2. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 2
Outline
 TDL technology benefits
 Path lengths effects
 Influence of process temperature
 External Temperature error
 Spectral Temperature
 Experimental setup
 Conclusion

3. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 3
TDL's now fully established
 High selectivity for the target gas species
 In-situ analysis
 Non-contacting
 Fast response time
 Dynamic measurement of process
 Minimal calibration/validation requirements
 Zero drift free

4. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 4
Cross-stack TDL's
 Originally designed for long
path length applications, i.e.
combustion, DeNOx
 Process Applications tend to
be more challenging
 Installation requirements in
small pipes can be detrimental
to measurement quality
• Challenge to keep alignment
with varying temperatures

5. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 5
Effect of long path lengths
 Concentration measurement over path length is
averaged
 Combustion control: ability to exclude leakage errors
with over-the-burner location
 Higher impact of dust load in process gas
 Beam steering effects can lead to loss of transmission
 But: what is the effective gas temperature over whole
path?
 Temperature sensor is a point measurement
 Location of sensor very relevant for accuracy

6. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 6
Folded path probe design
 Single-port installation
 No alignment user-side
 Suitable for small pipes (2"
and above)
 Concept is expandable to
different process interfaces
 More adapted to control
applications in process

7. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 7
Effect of short effective path lengths
 Wider choice of installation
points
 Collimated beam
 Improved SNR
 Dust tolerant
 Detrimental to detection limit
 Purge flow and process gas
speed must be balanced
 Possible T gradients over pipe
length

8. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 8
Lambert Beer
• I0 : Initial intensity
• I : Signal intensity after crossing the gas
• S : Line strength
• L : Optical path length
• G : Line profile
• n : density  concentration
][exp ),,()( nLGS TPT ×××−= ν
oI
I
=τ

9. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 9
Temperature influence


10. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 10
Temperature influence


11. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 11
Sources of temperature errors
 External temperature
sensor too far away
 Varying process gas
temperature over path
length
 Wall effects on small ID
pipes
 Over the burner location
 Filter on probe type
process adaptions

12. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 12
Probe style TDL with filter
 No process side purging
 Filter for dust prevention
 Automated filter blow-back
 Low beam steering
 Fixed path length

13. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 13
Probe style TDL with filter
Measurement-free zone
(wall thickness)
-100mm (standard)
-200mm
-300mm
Process window
Optical path lengths (effective)
-200mm
-400mm
-800mm
Instrument side
purge
Exchangeable metal filter
-40 microns
-100 microns
-200 microns

14. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 14
Impact of temperature error
(% Relative error on % Oxygen in air reading at
ambient pressure and 1m OPL)
T
∆T=
-3°C
∆T=
-5°C
∆T=
-15°C
∆T=
-30°C
20°C 1.39 2.34 7.22 15.12
100°C 1.24 2.08 6.4 13.38
200°C 1.07 1.79 5.52 11.47
300°C 0.94 1.58 4.83 9.99
500°C 0.76 1.27 3.88 7.96

15. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 15
External temperature error
(Oxygen concentration in air measured in DN100 pipe
with filter probe during temperature ramp to 225°C)
30% error on measurement
 ∆T approx. 40°C

16. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 16
Spectral temperature

S1 S2
( )
( )
( )
( )
( ) 











−−





−=
0
0
2
0
1
02
01
2
1 11
exp
TTk
hc
TS
TS
TS
TS
EE

17. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 17
Spectral temperature measurement


18. Analytical Solutions by the Sea
The 60th Annual Symposium of the Analysis Division
Galveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015
Slide 18
Conclusion
 Best measurement performance with TDL's can be
obtained when effective process gas temperature is
taken into account
 External temperature sensor location
 Expected temperature gradients over long path lengths
 Expected wall effects on short path TDL's
 Best option is to use spectral temperature
measurement to achieve full freedom from above
sources of error