Optical Path Length Calibration: A Standard Approach for Use in Absorption Cell-Based IR-Spectrometric Gas Analysis

Table 1

Comparison of path lengths derived from the presented OPC (𝐿_OPC), from mechanical measurements (𝐿_mech), and from gas standard-based retrieval (𝐿_gas), respectively. Given are normalized errors 𝐸_𝑛 referring to 𝐿_OPC.

Cell	Cell type	𝐿OPC/m	𝐿mech/m	|𝐸𝑛(mech)|	𝐿gas/m	|𝐸𝑛(gas)|	Species^a	𝜆/𝜇m
A	MP*	21.87 ± 0.040	—	—	21.26 ± 0.31	0.9	CO2	2.0
B	SP, HP*	0.819 ± 0.0029	0.822 ± 0.0010	0.5	0.805 ± 0.041	0.2	CH4	1.65
C	SP, BW*	0.773 ± 0.0020	0.774 ± 0.0010	0.2	0.773 ± 0.038	0.0	CH4	1.65
C	SP, BW*	0.773 ± 0.0020	0.774 ± 0.0010	0.2	0.774 ± 0.012	0.04	CO2	2.71
D	SP, HT*	0.380 ± 0.0012	0.380 ± 0.0020	0.0	0.380 ± 0.019	0.0	CH4	1.65

aGas standards probed at wavelengths specified in 𝜆 column.
*Cell characteristics: MP multipass White type, SP single pass cell, HP high pressure cell, BW Brewster windows-equipped, and HT high temperature cell.