Piezo electric transducers:

• The piezoelectric transducers work on the principle of piezoelectric effect.
• When mechanical stress or forces are applied to some materials along certain planes, they produce electric voltage.
• This electric voltage can be measured easily by the voltage measuring instruments, which can be used to measure the stress or force.
• The physical quantities like stress and force cannot be measured directly. In such cases the material exhibiting piezoelectric transducers can be used.
• The stress or the force that has to be measured is applied along certain planes to these materials.
• The voltage output obtained from these materials due to piezoelectric effect is proportional to the applied stress or force.
• The output voltage can be calibrated against the applied stress or the force so that the measured value of the output voltage directly gives the value of the applied stress or force.
• The scale can be marked directly in terms of stress or force to give the values directly.
• The voltage output obtained from the materials due to piezoelectric effect is very small and it has high impedance.
• To measure the output some amplifiers, auxiliary circuit and the connecting cables are required.

(ii) Principle of ionization transducer:

• The principle of ionization transducer is based on the measurement of number of ions deliberately generated from the gas molecules present within the transducer chamber.
• The transducer has two electrodes called anode and cathode. The other terminal, called grid terminal is located in between the anode and the cathode.
• The anode is negative and the grid terminal is positive. When the cathode is heated by means of cathode source, the electrons are emitted and are accelerated towards positive grid terminal.
• Some of the electrons move towards the anode plate and exert repulsive force since the plate is negative. As a consequence, they oscillate in between the grid and the anode plate.
• During the process of oscillation they collide with the gas molecules producing positive ions, which are then attracted by the anode plate and constitute anode current, Ianode.
• The number of positive ions thus formed is proportional to the gas pressure and the number of electrons emitted from the cathode. If the number of electrons releases from the cathode, and the biasing grid current is held constant, then the anode current can be a measure of vacuum.
• Following relationship holds good in case of ionization transducer. Ianode = Sit.Igrid.$\theta$.
  where, Ianode is the anode current, Igrid is the grid current,$ \theta$ is the vacuum or pressure, and Sit is the sensitivity of the ionization transducer. The sensitivity depends on the geometry of the transducer chamber and the type of gas present in it.