Lafayette Instrument Policy on Venting the Pneumo Sensor

Question Last Updated on: 8/19/2025

What was the original purpose of venting the pneumo sensor?

Older Lafayette polygraph instruments ranging from the analog to the LX4000 computerized polygraph operated with a limited data range, allowing data to more easily reach the system’s thresholds. When using these older instruments, venting pneumos was encouraged to keep the pressure inside the sensor within a range the instrument could use and record. This threshold-exceeding data was commonly found when the pneumo is stretched out or when the sensors were applied too early during the pre-test interview.

In addition, pneumo data in the older systems could also be problematic if the pneumos were attached too tightly. If the pneumo sensors are allowed to fully relax (contract), they will naturally leak down to atmospheric pressure. If they are attached just prior to the acquaintance test and not attached too tightly, it makes no difference whether the sensor is vented.

Do I need to vent the pneumo sensor today?

Beginning with the LX5000, Lafayette introduced a broader data (pressure) range. However, because the industry had grown accustomed to venting older instruments, we continued to provide vented sensors in standard kits. The LX6 and LX7 systems were also designed to employ this widened data range, making venting unnecessary when using current Lafayette polygraph systems. As such, standard sensors will replace vented sensors in all kits while vented sensors will still be available at an additional cost.

What if I feel there is an issue with sensor pressure?

If an examiner sees or feels that there is a problem with one of the pneumos, the sensor can easily be disconnected and reconnected to quickly troubleshoot the issue.

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