Evaporative Liquid Flow Sensor for Wearable Sweat Rate Measurement
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Solution Overview
Problem
Current methods for measuring small liquid flow rates are inaccurate for long-term measurements and are not adaptable to various applications, as they either saturate quickly or require temperature changes that can damage biological samples.
Innovation Solution
A sensor system with an electrical heating element and temperature measurement capability, operating in an evaporative mode to heat the liquid to at least 90% of its boiling point, allowing for precise flow rate determination using parameters like applied power, time, and thermal properties, and optionally using a calibration curve for accurate measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If a closed-chamber humidity sensor is used to measure sweat rate, then the measurement can be performed continuously, but the air inside the chamber quickly saturates with water vapour causing accuracy to drop off rapidly
Solution Approach 1:
The invention changes the operating parameters by heating the sensor surface to elevated temperatures (above ambient temperature), which maintains a temperature gradient that prevents water vapor saturation in the chamber air. This allows continuous measurement over extended periods while maintaining accuracy, directly resolving the contradiction between measurement duration and accuracy.
2Measurement precision
If temperature changes of 1K to 30K are applied to measure liquid volume, then the volume measurement can be performed, but evaporation of the sample occurs and biological samples may be damaged
Solution Approach 1:
The invention extracts the liquid sample from the measurement environment by having it flow across a heated sensor surface rather than containing it in a microwell plate subjected to bulk temperature changes. This allows the sensor to be heated to higher temperatures for measurement without subjecting the liquid sample to damaging temperature changes, thereby resolving the contradiction between measurement precision and sample integrity.
3Ease of operation
If evaporative microfluidic pumps are used to achieve desired flow rates, then the flow rate can be controlled by varying heating power, but the system cannot measure unknown flow rates
Solution Approach 1:
The invention creates a multi-functional system where the heated sensor surface serves both as an evaporative pump (controlling flow rate through heating power) and as a measurement device (determining unknown flow rates through temperature rise analysis). This universal design resolves the contradiction between ease of operation for flow control and adaptability for measurement applications.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables accurate and continuous measurement of unknown flow rates, preventing sensor clogging and allowing for immediate reuse as the liquid evaporates, while maintaining a compact design suitable for wear on the skin or other applications.
Implementation Method 1
The surface is heated by means of the electrical heating element at a known predetermined or measured rate while taking a plurality of temperature measurements of the surface as a function of time... The surface is heated... at an absolute temperature of at least 90% of the boiling point of said liquid
Implementation Method 2
an electrical heating element arranged to heat said surface
Data Source
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Figure 5~6
AI summary
Method and device for liquid measurements comprising: - providing a sensor (1) comprising a surface (3) arranged to receive a liquid (5) thereupon, an electrical heating element (7) arranged to heat said surface (3), and an arrangement (11, 13; 7, 15) for measuring a temperature of said surface (3); - receiving liquid (5) on said sensor surface (3); - heating said surface (3) by means of said electrical heating element (7) at a known rate (P) while taking a plurality of temperature measurements (T) thereof in function of time (t), said heating being at an absolute temperature of at least 90% of the boiling point (Tb) of said liquid (5) under ambient pressure; - deriving a volume or a flow rate measurement related to said liquid (5) from said electrical heating rate (P) and said temperature measurements (T). The sensor system may be used to measure the volume of small amounts of liquid (e.g. dispensed droplets) or to a wearable device, e.g. a sweat rate sensor, where liquid from the skin of a wearer is transported to the heated surface (3) of the sensor (1) for measuring the flow rate.