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Labs Idea #4: Pulse Monitor with Differential Pressure Sensor

  • Motivation
  • Approach
  • Implementation

Author: Sensirion Innovations Team
November 2017


Motivation

Wearables measuring heart rate is a growing trend. Mostly the pulse is measured with photoplethysmography (PPG) using LEDs and photo diodes. However, dirt on the optics, dark skin or tatoos can make PPG based pulse measurement on the wrist difficult.

A very sensitive differential pressure sensor can detect the air wave generated by the pulse wave in the artery or carotid. Thus, it is possible to detect the heart rate independent of skin color or tattoos.


Approach

Sensirion's tiny SDP3x differential pressure sensor is a semiconductor MEMS version of the traditional hot wire anemometer. The sensing principle is based on thermal conductivity through the gas flowing over the sensor surface (figure 1).

Figure 1: Thermal measurement principle

In opposition to diaphragm based differential pressure sensors, Sensirion's CMOSens differential pressure sensor characteristic shows high sensitivity especially at low differential pressures (figure 2). This allows to reliably measure ultra low differential pressures.

Figure 2: Sensirion flow sensors are more sensitive at low differential pressures than diaphragm type pressure sensors

Sensirion's differential pressure sensors (SDP8xx or SDP3x) use the flow through principle explained in figure 1, but are calibrated for differential pressure output in Pascal (1 mbar = 100 Pa = 1/2 in. water column) instead of flow output. Whenever air flow is generated a pressure difference exists. Therefore, a differential pressure sensor can also detect air flow.

A Sensirion differential pressure sensor (SDP8xx or SDP3x) is capable of detecting pressure differences even below one Pascal. In order to detect the air wave from the pulse wave moving through the wrist or carotid artery, a set-up is required, which forces the air pressure wave to enter the differential pressure sensor through one port and leave the sensor through the other port. Therefore, one port of the differential pressure sensor is attached to a small cavity, which is positioned over the artery (see figure 3-4). Similar results are achieved using a stethoscope as cavity (see figure 5).

Figure 3: One port of Sensirion's tiny SDP3x differential pressure sensor is connected to a cavity positioned over the artery
Figure 4: A small cavity collects the pulse signal and directs the air pressure wave to the SDP3x differential pressure sensor
Figure 5: A stethoscope is used to collect the pulse signal

The pulse is detected by the high peak of the generated air pressure wave (figure 6). For this measurement the Sensirion SDP3x EK-P4 Evaluation kit was used.

Figure 6: the pulse wave generates a clear pressure peak with the small cavity
Figure 7: the pulse wave of the carotid measured with the stethoscope as cavity
Figure 8: the pulse wave of the heart measured with the stehoscope as cavity

The measurement results demonstrate that Sensirion's SDP3x differential pressure sensors can measure the heart rate by detecting smallest pressure waves generated by the pulse wave in an artery.

Conclusion

  • Sensirion's differential pressure sensor can detect the pulse wave in the artery or carotid
  • The heart rate can be derived from frequency of the pulse wave
  • Good signal-to-noise in resting position

Implementation

This paragraph should give in a how to style how to build the solution for the own purposes.

To build a heart rate monitor a Sensirion differential pressure sensor SDP8xx or SDP3x series (figure 9) can be sourced from distributors listed on the Sensirion website.

Figure 9: Sensirion SDP8xx and SDP3x series of differential pressure sensors

Communicate with the digital sensor over I2C according to the explanations in the SDP800 or SDP3x series data sheet or use the appropriate evaluation kit.


Patent Information

No 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.


Related Sensors to This Article

Sensirion Differential Pressure Sensors SDP800, SDP810

Differential Pressure Sensors SDP800 Series

Differential pressure sensors of the SDP800 series are the reliable solution for precise air flow measurement in most demanding, but cost sensitive HVAC applications.

Sensirion Differential Pressure Sensors SDP3x

Differential Pressure Sensor SDP3x

The SDP3x differential pressure sensor measures just 5mm x 8mm x 5mm, opening up new dimensions of integration and applications possibilities. It features excellent accuracy and long-term stability.


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