Linearity of the electromechanical penile plethysmograph's output at large expansions

Despite the widespread use of the Barlow‐type electromechanical plethysmograph for monitoring changes in penile circumference, doubts have arisen about its sensitivity at large expansions. This study investigated the linearity of both a used and unused gauge over a wide range of standardized expansions. For both gauges, the relationship between electrical output and physical expansion was almost linear. A logarithmic 10 transformation on the physical expansion data did not for either gauge achieve perfect linearity. Contrary to suggestions in the literature, therefore, such a transformation is not required when using this gauge.