Strain gauge plethysmography uses a mercury-filled silicone rubber tube with copper electrodes; what physical change does the gauge measure?

• A. Cuff pressure
• B. Temperature change
• C. Blood flow velocity
• D. Length change of the gauge

Answer: (D)

Strain gauges work by turning mechanical deformation into a change in electrical resistance. In strain gauge plethysmography, the mercury-filled silicone rubber tube acts as the gauge around the limb. When blood flow increases the limb’s volume causes the tube to stretch, increasing the length of the mercury column. The copper electrodes sense the resistance of that mercury path, and since resistance rises with length (while cross-sectional area stays roughly the same), the measured signal reflects how much the gauge has lengthened. This length change of the gauge is what translates into a volume or flow-related measurement. The device isn’t measuring cuff pressure, temperature, or blood flow velocity directly, but the physical deformation of the gauge itself.