Abstract: A sphygmomanometer with an air reservoir connected above a tube for a mercury column and a selectively openable and closeable valve to the air reservoir. With the valve open, first blood pressure measurements can be made by pressure rising in the mercury reservoir. With the valve closed, the air above the mercury column resists rise in the mercury column but also permits higher pressure measurements using the same mercury column. One scale at the mercury tube measures the pressure with the valve open and another scale at the tube measures the pressure with the valve closed. The measurements with the valve closed are calibrated by appropriately altering the volume of the air reservoir depending upon altitude at barometric pressure. The sphygmomanometer can be used to perform a mercury drop test for determining the altitude at which the sphygmomanometer is located.