Your Contact to Sensirion

Umschlag

Contact

Contact our sensor experts:

Contact Form

Distributoren

Distributors

Here you find the nearest distributor in your area:

Our Distributors

Standorte

Locations

Find out where we are located:

Our Locations

toggle menu
  • Home
  • News
  • Sensirion Products
    • SEK-SensorBridge (Evaluation Kit)
    • SEN5x Environmental Sensor Node
    • SCD4x CO2 Sensors
    • SFA30 Formaldehyde Sensor Module
    • SGP41 VOC+NOx Sensor
    • SGP40 VOC Sensor
    • SHT4x Humidity and Temperature Sensors
  • Partner Spotlight
    • Partner Spotlight SparkFun
    • Partner Spotlight MikroElektronika
    • Partner Spotlight Seeed
  • Archive
    • Platforms
      • Environmental Sensor Shield
      • Smart Gadget Development Kit
      • Wearable Development Kit
      • Raspberry Pi
      • Qualcomm Snapdragon
      • Arduino
      • Cypress PSoC Series
      • Generic Linux Support
      • Thingstream MQTT IoT Sensor Starter Kit
    • Applications
      • Sensors for Ventilation
      • CO2 Monitor Reference Design
      • SHTC3 BLE Beacon Development Kit
      • Directional Wind Meter Using SDP3x
      • Smart Speaker
      • Archive
    • Labs
      • #21 - Using an RJ45 Breakout Board Connect with Evaluation Kit Cables
      • #20 - Integrate Flex-PCB Sensors into Prototyping Setups
      • #19 - Smart Factory Particle Monitor Network
      • #18 - Inhalation Application Benefits
      • #17 - Sleep Analysis and Pulse Monitor
      • #16 - Thermal Comfort Sensor
      • #15 - Anti-Fogging for Bathroom Mirrors
      • #14 - Improved Puff Sensing of Inhalation Devices
      • #13 - Spirometer
      • #12 - Breath Detection
      • #11 - Stress, Mood and Emotion Sensing
      • #10 - CO2 Sensing in Car
      • #9 - Milk Fat Concentration Sensor
      • #8 - Presence Detection with IR Sensor
      • #7 - RH Sensor Trigger for BLE Beacon
      • #6 - Smart Inhaler
      • #5 - Windshield Ice Detector
      • >> See All Labs Ideas
    • Tutorials
      • Wearable Development Kit: Build Your Own Device
      • Wearable Development Kit: Logging Data
      • Raspberry Pi: SFM Software Package
      • Raspberry Pi: Alexa with Indoor Air Quality
      • Raspberry Pi: Using Flow Sensors
      • Raspberry Pi: Weather Station
      • Arduino: Create Your Own CO2 Monitor
      • Arduino: Stand-Alone Liquid Flow Meter
      • Arduino: Interface for Liquid Flow Sensors
      • Arduino: Weather Station
      • Arduino: Closed Loop Volume Controller Using Liquid Flow Sensors
      • Arduino: Interface for Multiple SF06 Liquid Flow Sensors
      • Arduino: Interface for SF06 Liquid Flow Sensors with Level Shifter
      • Arduino: CAN Bus Flow Meter
  • Global Website

Contact

search
cart
DOWNLOAD CENTER BUY NOW INVESTORS
Sensirion AG Switzerland
  • Home
  • News
  • Sensirion Products
    • SEK-SensorBridge (Evaluation Kit)
    • SEN5x Environmental Sensor Node
    • SCD4x CO2 Sensors
    • SFA30 Formaldehyde Sensor Module
    • SGP41 VOC+NOx Sensor
    • SGP40 VOC Sensor
    • SHT4x Humidity and Temperature Sensors
    • Sensirion Products

      Start testing our sensors

      Here you can find all information for evaluating and testing Sensirion's latest sensor solutions. From drivers and software to tutorials and quick start guides - we try to offer all tools necessary so that you can kick-start your application!

  • Partner Spotlight
    • Partner Spotlight SparkFun
    • Partner Spotlight MikroElektronika
    • Partner Spotlight Seeed
  • Archive
    • Platforms
      • Environmental Sensor Shield
      • Smart Gadget Development Kit
      • Wearable Development Kit
      • Raspberry Pi
      • Qualcomm Snapdragon
      • Arduino
      • Cypress PSoC Series
      • Generic Linux Support
      • Thingstream MQTT IoT Sensor Starter Kit
    • Applications
      • Sensors for Ventilation
      • CO2 Monitor Reference Design
      • SHTC3 BLE Beacon Development Kit
      • Directional Wind Meter Using SDP3x
      • Smart Speaker
      • Archive
        • On/Off Body Detection
        • Perspiration
        • Antifogging in VR Goggles
    • Labs
      • #21 - Using an RJ45 Breakout Board Connect with Evaluation Kit Cables
      • #20 - Integrate Flex-PCB Sensors into Prototyping Setups
      • #19 - Smart Factory Particle Monitor Network
      • #18 - Inhalation Application Benefits
      • #17 - Sleep Analysis and Pulse Monitor
      • #16 - Thermal Comfort Sensor
      • #15 - Anti-Fogging for Bathroom Mirrors
      • #14 - Improved Puff Sensing of Inhalation Devices
      • #13 - Spirometer
      • #12 - Breath Detection
      • #11 - Stress, Mood and Emotion Sensing
      • #10 - CO2 Sensing in Car
      • #9 - Milk Fat Concentration Sensor
      • #8 - Presence Detection with IR Sensor
      • #7 - RH Sensor Trigger for BLE Beacon
      • #6 - Smart Inhaler
      • #5 - Windshield Ice Detector
      • >> See All Labs Ideas
    • Tutorials
      • Wearable Development Kit: Build Your Own Device
      • Wearable Development Kit: Logging Data
      • Raspberry Pi: SFM Software Package
      • Raspberry Pi: Alexa with Indoor Air Quality
      • Raspberry Pi: Using Flow Sensors
      • Raspberry Pi: Weather Station
      • Arduino: Create Your Own CO2 Monitor
      • Arduino: Stand-Alone Liquid Flow Meter
      • Arduino: Interface for Liquid Flow Sensors
      • Arduino: Weather Station
      • Arduino: Closed Loop Volume Controller Using Liquid Flow Sensors
      • Arduino: Interface for Multiple SF06 Liquid Flow Sensors
      • Arduino: Interface for SF06 Liquid Flow Sensors with Level Shifter
      • Arduino: CAN Bus Flow Meter
Sensirion AG Switzerland
  • Perspiration

     

     

  • Sensirion Developers
  • Archive
  • Applications
  • Archive
  • Perspiration

Perspiration

Through perspiration we are constantly losing water over the surface of our skin. This has an influence on the state of hydration on our body.

The lost water needs to be replaced by drinking. Especially elderly people are prone to dehydration because the water content in the human body is decreased with increasing age. Based on research papers about hospitalizations due to dehydration, more than a half a million cases per year were registered. The potential savings if those hospitalizations and the associate costs could be avoided are more than a billion USD. One possible way to address this problem is to measure the perspiration rate.

This is where our humidity sensor SHTW2 comes in. With two SHTW2 sensors arranged in the right way, it is possible to measure the perspiration rate accurately. The constant transpiration, which is nothing else than evaporation of water through our skin creates a gradient of absolute humidity. This gradient is actually proportional to the perspiration rate. What we need to measure are two sensors which are placed in some kind of channel.

Advantages of Using a Humidity Sensor for Perspiration Measurement

The implementation of a humidity sensor allows further features:

  • Hydration monitoring
  • Activity tracking / monitoring
  • Mood monitoring (stress level, anxiety)
  • Relax / sleep monitoring
  • Skin hydration
  • “Cold feeling” alarm


Recommended sensor: Humidity sensor SHTW2


Perspiration Measurement - Tutorial

Motivation

How much water do we loose through our skin over a day or during exercise? Does the rate of water loss (perspiration/transpiration) correlate with my performance? And if it does how can I measure it? If you have ever thought about one or more of the questions above there is an answer to each of them! These answers are delivered by a device we call "Perspiration Device". As of now (November, 2016) there are no devices on the market which can measure perspiration rates continuously or even in a small form factor. Our "Perspiration Device" resolves this and allows for a fast and precise measurement of the perspiration rate.

The jogging was done at cool conditions (ca. 10C) with a moderate pace, therefore the perspiration rate was rather low during the activity in the cool environment.

We built the following two devices: the bare "Perspiration Device" and a demonstrator with the "Perspiration Device" connected to the "Sensirion Wearable Development Kit".

Approach

The underlying physical law for perspiration measurement is the diffusion equation that states how a collective of micro-particles behaves based on the random motion of each micro-particle (Wikipedia - Diffusion).

We can put a hollow cylinder on the skin and measure the concentration gradient in order to get the perspiration rate (see figure below). The skin defines the lower boundary condition of the differential equation with a given rate of perspiration (Neumann boundary condition). Whereas at the upper end of the tube the concentration equals to the environment.

Now the concept of our device is to measure the absolute humidity (AH which represents the concentration of H2O) at two separate locations spaced with a fixed width. With the model above we can calculate the perspiration rate as follows (with D being the diffusion coefficient):

However above calculation is not exact (not considering any transient effects or any surface effects) there needs to be a calibration factor, that we are able to determine precisely with a dedicated calibration setup, please contact us for more details. In addition, the process is temperature dependant, this can be addressed as well.

Apparently this measurement principle saturates as soon as there is liquid sweat on the skin or more precisely at the body facing opening of the device. This happens obviously during phases of very intense sweating, but as can be seen in the plot, also when the surrounding absolute humidity is so high, that with the current perspiration rate the relative humidity reaches 100% at the body facing opening of the device.

Implementation

We implemented above principle the following way:

Three PCB's were stacked together to form a channel like depicted in the schematics below. On top of this channel - visible for the sensor through a tiny hole (see top layer photos) - two SHTW2 sensors were placed to measure the AH gradient. However above principle is possible with a lot of different design-ins, this one is just one proposal.

Sketch: humidity flow depicted with arrows

"PCB stack": Top view and bottom view

Top layer: SHTW2 sensors (sensing membrane on the bottom of the sensor) mounted above tiny hole

Middle layer: forms the actual channel

Bottom layer

Detail: Sensor above hole, looking into channel, sensor needs to be sealed towards environment (white material on top of sensor)

The code for computing the absolute humidity from the sensor values (relative humidity and temperature) and the perspiration rate is depicted below. In order to have a better signal we filter the raw temperature signal with a first order low pass filter (also see code below).

Code for Perspiration Rate Calculation


static float absoluteHumidity(float temperature, float relativeHumidity)

{

    static const float A = 6.112f; // hPa

    static const float Tn = 243.12f; // °C

    static const float m = 17.62f;

    return 216.7f * (relativeHumidity * 0.01f * A * std::exp(m * temperature / (Tn + temperature)) / (273.15f + temperature));

}

 

void perspirationEngine(unsigned long timeStampMs,

                        float temperatureAmbientSensor, float relativeHumidityAmbientSensor,

                        float temperatureSkinSensor, float relativeHumiditySkinSensor,

                        float *perspiration)

{

    /* compute the time delta from the timestamp, the time delta will slightly vary but this is desireable for the filter computation */

    static unsigned long timeStampMsLastCall = 0;

    static float delta_t = int(timeStampMs - timeStampMsLastCall) / 1000.0f;

    timeStampMsLastCall = timeStampMs;

 

    /* relative humidity offset of ambient sensor */

    static const float RH_OFFSET = 0.0;

    /* temperature offset of ambient sensor */

    static const float T_OFFSET = 0.0;

    /* distance between sensors in m */

    static const float D_SHT = 0.008f;

    /* diffusion coefficient @ 25°C in m^2/s */

    static const float DIFF_COEF_25_DEGREES = 2.6e-5f;

    /* emperical constant, slope of temperature vs calibration factor */

    static const float T_CORRECTION_FACTOR = 0.04f;

    /* diffusion coefficient temperature dependent */

    float DIFF_COEF = DIFF_COEF_25_DEGREES * T_CORRECTION_FACTOR * temperatureSkinSensor;

    float CONVERSION_FACTOR = 1.0f / D_SHT * DIFF_COEF * 3600.0f;

    /* calibration factor of device */

    static const float CALIBRATION_FACTOR = 1.5f;

 

    /* define time constant for low pass filter and apply filtering on both temperature values */

    static const float TAU = 30;

    temperatureAmbientSensor = firstOrder_lowPassFilter(temperatureAmbientSensor, TAU, delta_t);

    temperatureSkinSensor = firstOrder_lowPassFilter(temperatureSkinSensor, TAU, delta_t);

 

    /* use adjusted temperature and relative humidity to calculate absolute humidity */

    temperatureAmbientSensor += T_OFFSET;

    relativeHumidityAmbientSensor += RH_OFFSET;

    float absoluteHumidityAmbientSensor = absoluteHumidity(temperatureAmbientSensor, relativeHumidityAmbientSensor);

    float absoluteHumiditySkinSensor = absoluteHumidity(temperatureSkinSensor, relativeHumiditySkinSensor);

 

    if (absoluteHumiditySkinSensor > absoluteHumidityAmbientSensor) {

        *perspiration = CALIBRATION_FACTOR * CONVERSION_FACTOR * (absoluteHumiditySkinSensor - absoluteHumidityAmbientSensor);

    } else {

        *perspiration = 0.0f;

    }

}

Low Pass Filter


static CircularBuffer<float, 2> firstOrder_lowPassBuffer;

 

float firstOrder_lowPassFilter(float x, float tau, float delta_t)

{

    static float alpha = delta_t / (tau + delta_t);

    float y = alpha * x + (1 - alpha) * firstOrder_lowPassBuffer.get();

    firstOrder_lowPassBuffer.add(y);

    return y;

}

Gerber Files

  • Gerber.zip
  • Gerber_layout.zip

Related Information

  • Use Case "On/Off-Body Detection"

Patent Information

Aspects of this application are protected by Sensirion patent(s) US 8,838,037 B2, JP 5963481 B2 and/or other patents pending. For commercial licenses, please contact Sensirion AG.

For the sake of clarity, no rights are granted for uses without a Sensirion sensor. Neither the grant of a license under the patent(s) mentioned nor any other information or recommendation presented on this website do absolve you from the obligation of investigating the possibility of infringement of third parties’ rights and, if necessary, clarifying the position.


Follow Us

Sensirion Products

  • CO2 Sensors
  • Formaldehyde Sensors
  • VOC Sensors
  • Humidity and Temperature Sensors

Partner Spotlight

  • Sparkfun
  • MikroElektronika

Archive

  • Labs
  • Applications
  • Tutorials
  • Platforms

Further Information

  • Developer News
  • Download Center
  • FAQs
  • Contact
  • Locations

Your Contact to Sensirion

Umschlag

Contact

Contact our sensor experts:

Contact Form

Distributoren

Distributors

Here you find the nearest distributor in your area:

Our Distributors

Standorte

Locations

Find out where we are located:

Our Locations

Please Find Here Various Support Topics

Buy Our Products at Your Local Distributor

About Sensirion

Environmental Sensors

Flow Sensors

Media/Newsroom

Investors

Useful Links

Sensirion Automotive Solutions

Career

Support Center

Partner Access

General

Terms and Conditions

Terms and Conditions for US Customers

Supplier Social Responsibility

Quality, Environment and Ethics

Sensirion AG

Laubisruetistrasse 50

8712 Staefa ZH, Switzerland

Tel. +41 44 306 40 00

infosensirioncom

2022 © Sensirion AG Switzerland
  • Home
  • Sitemap
  • Privacy Policy and Cookies
  • Imprint
  • Login
  • Disclaimer