Polygraph Test Instrumentation

Instrumentation used in polygraph examinations have developed and kept abreast of technological changes in modern society.

Although the old traditional analogue instrument is still used by a few examiners in South Africa, most of the polygraphists are making use of new state of the art computerised instruments.

Both the analogue and computerised instruments makes use of the same four sensors which register the following:

Abdominal and thoracic breathing (pneumograph tubes)
Cardio-vascular changes (blood pressure cuff)
Electra-dermal activity sensors (finger plates)

Although an analogue in good working condition is very reliable there are certain advantages to using the computerized instrument:

Data Quality 

With the analogue, traditional polygraph data characteristics are typical of technology 30-40 years ago. As ink pens pivot, they distort data position by their curved motion. Trace data is permanently lost at pen stops. In contrast, the computerised instrument has a high sensitivity to physiological responses and the ability to preserve the integrity of the original examination.

Display And Print Characteristics

The computerised instrument provides clarity of tracings, flexibility of review and ease of analysis and a rich set of display options. Polygraph Examiners can adjust chart width, height and colour, line thickness and gridding. Charts can be stacked for comparison purposes, etc. Data can be adjusted during replay without altering the original results. With the analogue instrument, chart tracings are limited to one original set per examination. Chart quality is restricted by analogue technology limitations such as fixed times, fixed gridding and fixed trace centering.


Modern day computerized polygraph systems do not require calibration in the same way that the old analogue systems did. The instrumentation is calibrated at the factory before being shipped to the examiners. On occasion a functionality test should be conducted to ensure that all the instrumentation are in proper working order.

Storage and Retrieval

The computerised instrument provides the facility of storage and retrieval of charts, reports, question lists and other material. This simplifies the transfer of information to supervisors, other examiners, attorneys, etc. Examinations can be stored on diskettes or on a hard drive. All data including charts, reports, questions, etc are immediately accessible and can be transferred instantaneously, using fax/modem or similar technology. An unlimited number of original charts can be printed.

Examination information may be viewed at any time using the computer monitor. Details of examinees and times of tests are recorded automatically on each chart. The Analogue is limited by a total reliance upon the original paper charts or photocopies of them. Original charts must be stored at whatever store facility is available and sent for when needed. Other information such as question lists; reports and other material may be separate from charts, requiring more trips, coordination, etc.

Power Portability

The computerised instrument has the ability to operate for periods of time without external source of electric power. Sensor Box power requirements are supplied through the serial cable and are so insignificant that the computer’s normal battery operating time is virtually unaffected by the Sensor Box requirements. The analogue instrument generally requires access to an external electric power supply.

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For more information specifically related to polygraph examinations, please visit The Polygraph Institute of South Africa.

For more information specifically related to pre-employment screenings, please visit the Pre-Employment Screening Centre.

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