A penile plethysmograph is a device used to measure blood flow to the penis. Because good blood flow to the penis is important for erections, penile plethysmography can be especially helpful for doctors evaluating a man’s erectile function.

The procedure can be done in a few different ways.

Volumetric plethysmography

With this method, a glass tube is placed over the penis and an erection is induced. Doctors learn about blood flow by measuring changes in air volume between the flaccid (non-erect) and erect states.

Circumferential plethysmography

This version involves a sensor called a strain gauge that is placed around the penis.

There are two types of strain gauges:

An electromechanical strain gauge is made of lightweight metal that wraps around the penis. When an erection occurs, the metal changes shape. Comparing the flaccid and erect measurements of the gauge can indicate changes in blood flow.

As the name implies, a mercury-in-rubber strain gauge is a rubber ring filled with mercury. When the penis goes from a flaccid to an erect state, the ring circumference changes and alters the distribution of the mercury, which can determine blood flow differences.

Penile plethysmography may help distinguish between physiological and psychological erectile dysfunction (ED).