# Multiparameter Determination of Thin Liquid Urea-Water Films
## 2 Experimental Setup and Methodology  
Firstly, the broadband spectra of known film parameters are collected with an FTIR spectrometer (Bruker: VERTEX 80v). Spectra in the wavenumber range from around 2000 cm\({}^{-1}\) to 15,000 cm\({}^{-1}\) were recorded with a thermoelectrically cooled Indium Gallium Arsenide (TE InGaAs) detector. A calibration cell [16] was used to enable transmission measurements with predefined parameter variations (see Figure 1). The UWS is placed within the stainless steel spacer ring, which is then clamped between two wedged calcium fluoride (CaF\({}_{2}\)) windows, two O-seals and the top and body plates of the cell. The film thickness is given by the height of the spacer, while the temperature is set through heating cartridges and monitored by a thermocouple. The concentration of the urea in water is known beforehand as it is mixed from solid urea and water.  
To enable a reliable calibration, a comprehensive measurement matrix was used comprising 4 concentration variations, 8 temperature variations and 6 film thickness variations (see Table 1). The minimal concentration was set to 32.5%mass, as this corresponds to the commercially available product. Higher concentrations are likely to occur during the SCR process due to the evaporation of water within the hot gas stream, which is why higher concentrations up to 57.7%mass were also investigated. Film thicknesses  
Figure 1: Calibration cell: between the body (1) and the top plate (2), O-seals (3), wedged CaF\({}_{2}\) windows (4) and a stainless steel spacer ring (5) are clamped to confine the thin liquid film within the spacer of a predefined height. The liquid temperature is set by heating cartridges (6) placed within the body [16].  
as low as 91 \(\upmu\)m could be realized within the calibration cell, while the use of thinner stainless steel foils proved to be unfeasible. Small burrs at the edges of the foils led to an overestimation of the film thickness. The maximum film thickness within the measurement matrix amounted to 310 \(\upmu\)m. The investigated temperatures ranged from room temperature (25 \({}^{\circ}\)C) to 30 \({}^{\circ}\)C and--in steps of ten degrees--up to 90 \({}^{\circ}\)C. All parameter variations were investigated, leading to 192 experiments. However, in some higher temperature cases, bubble formation led to low-quality measurements that had to be discarded. Thus, a total of 160 measurements with different parameter combinations were used for the subsequent evaluation.  
In addition to these transmission measurements, background measurements \(I_{0,v}\) are required, which are taken through the empty calibration cell. Thus, the quantities \(I_{v}\) and \(I_{0,v}\) are known from the experiments, and the respective absorbance spectra can be calculated with Equation (1). For further evaluation, the absorbance spectra are most suitable since these are independent of the measurement conditions and environment or the detector behavior. This way, data from different campaigns and setups or taken with different detectors can be compared, which would not be possible if the attenuated intensity signal were utilized. Comparing the empty cell (background scan) and filled cell (liquid between windows), small changes in the position of the optical assembly of the cell with respect to the light path or uncertainties in the mechanical mounting can lead to intensity shift. To account for such broadband intensity shifts, which are not related to absorption of the liquid, the wavenumber region with no absorption at around 9.398 cm\({}^{-1}\) was taken for the norming of the \(I_{v}\) and \(I_{0,v}\) spectra. Based on these extracted absorbance spectra that are a function of the desired film parameters, the new evaluation and calibration strategy is subsequently developed.