Abstract
With the application of a recently developed deposition method called initiated chemical vapor deposition (iCVD), responsive hydrogel thin films in the order of a few hundred nanometers were created. When in contact with humid air, the hydrogel layer increases its thickness considerably. The measurement of the thickness change was realized interferometrically with a laser and a broadband light source in two different implementations. The relative change in thickness with respect to humidity can be described with the Flory–Huggins theory. The required Flory–Huggins interaction parameter was determined for the actual hydrogel composition. The setup was designed without electric components in the vicinity of the active sensor layer and is therefore applicable in harsh environments such as explosive or corrosive ones. The implemented sensor prototype delivered reproducible relative humidity () values and the achieved response time for an abrupt change of the humidity s was about three times faster compared to one of the fastest commercially available sensors on the market.
Highlights
Water is involved in many processes in life and technology and plays an important role in the climate system
For the realization of a humidity sensor, the derived hydrogel thickness must be related to the corresponding relative humidity
The analytical model by Flory–Huggins was evaluated initially and the Flory–Huggins interaction parameter χ was derived by calibration measurements for the actual material combination of Poly 2-hydroxyethyl methacrylate (pHEMA) and water
Summary
Water is involved in many processes in life and technology and plays an important role in the climate system. Humidity measurements are a substantial source of information for meteorologists, but there are several other fields in science and industry, where they play an important role as well, ranging from process monitoring to medical investigations [3,4]. Devices for measuring humidity are called hygrometers, which quantify the amount of water vapor in the air. There are several principles ranging from ancient methods based on the elongation of a hygroscopic material (e.g., hair-tension hygrometer), measuring the temperature difference between a dry and wet bulb (psychrometer) to optical detection of the condensation on a chilled surface (dew point hygrometer) [5,6]. A very accurate method is the gravimetric determination of the amount of water, which is quite time consuming and mostly used for standards and Sensors 2019, 19, 999; doi:10.3390/s19050999 www.mdpi.com/journal/sensors
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