Polygraph Instrument

Components, accessories of the polygraph, lie detector

Polygraph Instrument

The polygraph instrument has different components, accessories that allows the system to collect, register psychophysiological data from an individual.

Polygraph instrument components, parts, accessories

A normal polygraph instrument used for the detection of deception has at least the following sensors:

  • Pneumographs
  • Elector-dermal sensors
  • Sphygmomanometer

Normally a polygraph is composed of the following parts:

  • 1 Data Acquisition System
  • 2 Pneumographs
  • 1 Galvanometer / Electro-dermal sensor
  • 1 Sphygmomanometer
  • 1 Movement sensor
  • Polygraph software

What do the components of the polygraph instrument measure?

The DAS (Data Acquisition System) is the heart/brain of the polygraph. This is where all the information is gathered and transformed into readable information.


All the other components are attached to the DAS that transforms and transmits the psychophysiological information to a software program, normally installed in a computer.

The different components will measure:

  • Pneumograph – Respiratory activity
  • Galvanometer – Galvanic Skin Conductance, Elector-dermal activity
  • Sphygmomanometer – Cardiovascular activity (Blood pressure, Pulse, Heart rate)
  • Movement sensor – Any possible movement that can distort psychophysiological readings

What causes the psychophysiological activity?

The polygraph registers a very specific physiological activity. It measures physiological changes that are caused by the Autonomic Nervous System.

For a better understanding of the Autonomic Nervous System and polygraph, please view the following video:

This activity is involuntary, independent, automatic and makes part of the natural defense system that every human has. This is the activity that the instrument will register.

So, it is important to understand that the polygraph device is foremost a device that helps the polygraph examiner to interpret emotions. To be able to qualify these emotions as deception or truth the expert needs to apply proven and validated methodologies.

This is why it is important to understand that the polygraph instrument is a tool to an examiner. For more information on the reliability of the polygraph apparatus, please check the following page on validity & reliability of a polygraph exam.

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