Wearable Sweat Rate Sensors for Human Thermal Comfort Monitoring

Abstract

We propose watch-type sweat rate sensors capable of automatic natural ventilation by integrating miniaturized thermo-pneumatic actuators, and experimentally verify their performances and applicability. Previous sensors using natural ventilation require manual ventilation process or high-power bulky thermo-pneumatic actuators to lift sweat rate detection chambers above skin for continuous measurement. The proposed watch-type sweat rate sensors reduce operation power by minimizing expansion fluid volume to 0.4 ml through heat circuit modeling. The proposed sensors reduce operation power to 12.8% and weight to 47.6% compared to previous portable sensors, operating for 4 hours at 6 V batteries. Human experiment for thermal comfort monitoring is performed by using the proposed sensors having sensitivity of 0.039 (pF/s)/(g/m²h) and linearity of 97.9% in human sweat rate range. Average sweat rate difference for each thermal status measured in three subjects shows (32.06 ± 27.19) g/m²h in thermal statuses including 'comfortable', 'slightly warm', 'warm', and 'hot'. The proposed sensors thereby can discriminate and compare four stages of thermal status. Sweat rate measurement error of the proposed sensors is less than 10% under air velocity of 1.5 m/s corresponding to human walking speed. The proposed sensors are applicable for wearable and portable use, having potentials for daily thermal comfort monitoring applications.

Figure 1

The watch-type sweat rate sensor: (a) overall view; (b) bottom view; (c) cross-sectional view along A-A’ in (b); (d) enlarged bottom view of the humidity chamber with capacitive humidity sensor.

Figure 2

Operation of the watch-type sweat rate sensor: (a) operation procedure; (b) sweat rate detected by capacitance rising rate; (c) humidity chamber movements and capacitance profiles.

Figure 3

The fabricated watch-type sweat rate sensor.

Figure 4

Characterization of the thermo-pneumatic actuation of the humidity chamber: (a) experimental setup; (b) the input power train, composed of the heater power of 1.6 W for 30 sec and 0 W for 90 sec, for the one cycle humidity chamber movement; (c) time-dependent humidity chamber position for the input power train of Fig. 4b; (d) time-dependent humidity chamber position and temperature inside the humidity chamber for the ten cycle of the operation of Fig. 4c.

Figure 5

Artificial skin experiment: (a) experimental setup for the watch-type sweat rate sensor characterization; (b) time-dependent relative capacitance in the five different sweat rate conditions during three measurement cycles; (c) capacitance rising rate depending on the sweat rate ranging from 3.76 g/m²h to 137.68 g/m²h; (d) capacitance rising rate depending on the sweat rate, in the three different air velocity conditions in the ranges of 0~0.5 m/s, 0.5~1.0 m/s, and 1.0~1.5 m/s.

Figure 6

Human experiment: (a) watch-type sweat rate sensor in the wrist with 1 mm gap between the human skin and the humidity chamber; (b) the thermal status control using three different conditions and the survey used for thermal status evaluation; (c) typical profile of capacitance depending on time, where the capacitance is measured by the humidity sensor from subject 1 during the thermal status control; (d–f) sweat rate depending on thermal status for the three different subjects (subject 1–3).