You may have noticed some news around these weird sounding devices that measure "arousal". They don't. But they do measure changes in penile shape, and as such, can give users a rough estimate — in a non-invasive way — of how much blood is flowing into the penis during erection.
Image: Diane Kelly
How does a penile plethysmograph work?
Depends on the design. The volumetric plethysmograph consists of a glass tube with a known volume. It has a large opening at one end and a small opening at the other. The large opening goes over the penis, along with an inflatable rubber cuff to seal everything up. As the erection grows, it pressurizes the air in the tube, and the change in pressure tells you (indirectly) how much blood has flowed into the penis. These devices are sensitive to small changes in penile volume, but they're kind of impractical and cumbersome.
The more convenient option is a circumferential plethysmograph like the one pictured above. These come in fully-circular and semi-circular styles, but both work the same way: stick a penis in the center of the device and try for arousal. As the penis grows in diameter, the instrument deforms. Strain gauges on the device measure the deformation and help estimate blood flow.
What is a penile plethysmograph actually good for?
As a non-invasive method of assessing erections, penile plethysmographs can be a helpful part of distinguishing between different types of erectile dysfunction. It's true that the devices were invented to assess sex preferences, and people keep trying to use them for that even though there's no good evidence that it works, and lots of evidence that the results can be faked. But if a guy needs some prostate surgery, some time on the plethysmograph afterwards is an excellent way to make sure his nerve-sparing surgery managed to spare the right nerves.