Application of Transducers in Biomedical Instrumentation

Application of Piezoelectric Transducer

    • Crystal microphone measures and records the Heart sounds (Phonocardiography). The microphone operates in the flat frequency range of 20 to 1000 Hz, which suppresses the breathing sounds and room noise. Sounds from heart chambers are measured with Catheter-tip piezoelectric sensors.
    • A piezoelectric crystal detects the Korotkoff sounds during systolic and diastolic blood pressure measurement as shown in figure.

application of piezoelectric transducer

  • A piezoelectric sensor senses Radial pulse
  • In ultrasonic scanning devices, piezoelectric transducers are used. When an ultrasonic pulse transmits through the human body, it reflects an echo signal of different frequency. The received is displayed on Cathode Ray Oscilloscope.
  • In hospitals, when medicines are given continuously to a patient, a medical professional should be aware of the amount of medicine given. For this reason the drop – counting is done with a piezoelectric crystal. In the drop – counting technique, a mesh is attached to the piezoelectric transducer. When each drop of medicine falls on the mesh, the piezoelectric transducer generates a pulse.

Application of Thermoresistive Transducer

In metals and semiconductors, the value of resistance changes constantly. This change in resistance is the basis for all Thermoresistive temperature sensors. Temperature coefficient relates temperature and resistance. In positive type, resistance raises with temperature and in negative type resistance decreases with increase in temperature. In metals, temperature coefficient is positive and in semiconductors, temperature coefficient is negative.

  • A thermoresistive transducer measures skin and body temperature.
  • We can measure blood flow in the human body with a Thermoresistive transducer. On the tip of the catheter, the heated thermistor is enclosed. When the catheter passes into the blood vessel, thermistor transfers a portion of its heat to the blood flowing in the body. The cooling effect of the thermistor is directly proportional to the flow of blood. This modifies the value of resistance in the thermistor that relates blood flow.
  • A thermistor measures Respiratory rate. Place a glass bead thermistor directly in the path of nasal airflow. Pass an amount of current through the thermistor. For each cycle of expiration, the cooling effect of nasal airflow causes the resistance of thermistor to increase. Cathode Ray Oscilloscope records the resistance increase.

Application of Photoelectric Transducers

Photoemissive Tube

A photoemissive tube consists of a gas-filled tube with two electrodes, one cathode, and another anode. Cathode acts as one electrode, and it has a specially coated material around it. When light falls on the cathode, it releases electrons. The release in electron produces a current, which is proportional to the light intensity. Usually, antimony, silver, bismuth is employed as coating materials.

Photovoltaic Cell

A photovoltaic cell has an outer layer of selenium, coated with a transparent metal film. The metal film and selenium layer are insulated from each other, and this forms the barrier layer. Light with high intensity illuminates the barrier layer. As the light falls on it, it releases electrons, which causes a potential difference in the barrier layer. After the occurrence of potential difference, the metal film turns into positive, and selenium layer turns into negative. This is an example for the active transducer.


    • We can measure pulsatile blood volume change with a Photodetector. To detect the pulse, we can either use Transmittance or Reflectance techniques as shown in the figure below. In transmittance technique, pulsating blood flow modifies the optical density. In reflectance technique, blood flow changes the intensity of reflected light. The changes in blood flow are seen immediately with these methods.

Applications of photovoltaic cell

    • We can measure changes around the circumference of the chest with a pneumograph that has a photodiode as seen in the figure below. Wrap the chest with a rubber bellow. Inside the bellows, the movable metal bar is attached. When the chest expands during breathing, the amount of light that falls on the photodiode varies due to the metal bar. Calibrate the obtained result to get the respiratory volume.

pneumograph respiration

    • Blood pressure can be measured with Photodetectors as shown in the figure below. At the free end of a bourdon tube between lamp and photodiode, a shade is attached. Bourdon tube is filled using a saline solution. Pressure is created inside the tube due to blood pressure. As the blood pressure increases, the pressure inside the tube displaces the shade. This displacement is proportional to the output from the phototube.

blood pressure

  • To determine the oxygen saturation in the blood (oximetry) photoelectric transducers are used. Measurement of oxygen content is important during open-heart surgery. In the human body, earlobes are rich with vascular beds. So, earlobes are illuminated by a light source. The reflected light is detected with two photovoltaic detectors. The first detector detects the emitted radiation in the red region (640mµ), and another detector detects in the IR region (800mµ). The output from the red channel is related to the oxygen content in blood and the presence of blood and tissue along the optical path. However, the output from the IR spectrum is not proportional to the oxygen saturation. Finally, the difference between the two outputs is proportional to the amount of oxygen present in the blood.

ear-lobe oxymetry

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