Abstract
Real-time measurement of the relative humidity of air has applications ranging from process control to safety. By using a microfiber form-factor, we demonstrate a miniature and fast-response hygrometer with the shortest-ever response time (3 ms). The sensor head consists of an optical microfiber of 10 µm diameter and 2 mm length configured to form a compact U-shaped probe, and functionalized with a polyelectrolyte multilayer coating of 1.0 bilayer. The sensing mechanism is primarily water-absorption-based optical loss. We have measured a response time of 3 ms and a recovery time of 36 ms. The sensitivity is as high as 0.4%/%RH, and the detection limit is as low as 1.6%RH. The maximum relative humidity is 99%RH, before reaching a recoverable dew-point.
Highlights
Real-time measurement of the relative humidity of air has applications ranging from process control to safety
Existing hygrometers must be faster for applications that require faster temporal response, such as: (a) process control for mass production of chemical compounds[13,14], where humidity is regulated as a part of quality assurance; (b) touchless keypads for information systems, where selectiveness to bare skin leads to less false triggers than capacitive/resistive-touch technologies; (c) water-hazard safety mechanisms for portable electronics and undersea environments (e.g. Eurotunnel); (d) respiratory analyzers for identifying illnesses, where water vapor is less hampered by noise compared to air flow; and (e) atmosphere mapping for more accurate weather forecasts, where high spatial-resolution mapping of the atmospheric relative humidity (RH) via unmanned aerial vehicles can improve weather simulation models
We have demonstrated a new class of humidity sensor that exhibits the shortest-ever response time
Summary
These do not affect the actual response time (i.e. related to humidity rather than system noise) nor the observed response time (i.e. fluctuations are usually not visible in short-duration slopes, even if they are they can be removed with image-processing techniques that do not distort the signal), but the detection limit and accuracy of the sensor. In terms of flow-rate optimization to shorten the response time, it is revealed in Fig. S2 that to attain the shortest possible response time, a minimum threshold of 3.0 LPM must be met This is because faster flow rates can: (a) ascertain a passage of near-pure RH, while slower flow rates allows more significant diffusion and mixing with the ambient air along the way to the sensor head; and (b) ensure the wavefront of moisture propagation is more or less uniform, such that the response time incurred at the light-water interaction stage is minimized. They show resilience to vibrations when handheld, judging by the negligible optical modulation
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