# Fiber-Coupled Multipass NIR Sensor for In Situ, Real-Time Water Vapor Outgassing Monitoring

## TL;DR

A new compact sensor uses near-infrared light to monitor water vapor release from materials in real time, validated with silicone samples and simulations.

## Contribution

A fiber-coupled multipass NIR sensor design for real-time water vapor monitoring with a compact footprint and high dynamic range.

## Key findings

- The sensor successfully measured water vapor desorption from Sylgard-184 samples in real time.
- Sensor data aligned with numerical simulations using a triple-mode diffusion–sorption model.
- The compact design enables in situ monitoring with high optical path length.

## Abstract

This work presents the recent development of a fiber-coupled multipass near-infrared (NIR) gas sensor used to monitor water vapor desorption of small material coupons. The gas sensor design employs a White cell topology to maximize the optical path length over a compact, hand-size footprint. Water vapor concentrations are quantified over a large dynamic range by simultaneously applying wavelength modulation and tunable diode laser absorption spectroscopy techniques. A custom headspace optimized for material desorption experiments is assembled using commercially available vacuum chamber components. We provide in situ measurements of water vapor desorption from two geometries of the industrially important silicone elastomer Sylgard-184 as a case study for sensor viability. To corroborate the results, the gas sensor data are compared to numerical simulations based on a triple-mode diffusion–sorption model, consisting of Henry, Langmuir, and Pooling modes.