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.