How Does BP Monitor Works? Digital, Mercury & Aneroid Sphygmomanometer

BP monitors are also known as a sphygmomanometer. All these BP monitors are fascinating, and we are sure that the curious boy in you would have tried exploring how they work? The digital ones are pretty basic, and you will find nothing except a circuit board in them. But what about the mercury and the aneroid BP machines. If you are interested in knowing how to do these machines work, this page is for you. Go ahead and check out the working of all the three types of BP machines below.

How Does BP Monitor Works

Working of Mercury BP Machine

Diamond BPDG141 Conventional Mercury Free BP Instrument

In the mercury BP Machines, a column of mercury is connected to a measuring line. Here, you inflate the cuff manually, stopping the blood in the arteries for a brief moment. Once you start deflating the cuff, and once the pressure of the cuff is equal to the pressure of the blood, the blood starts flowing across the cuff. This will create a sound that can be recorded to the corresponding pressure reading. Once you reduce the pressure lower, you will be able to measure diastolic blood pressure similarly. This is the basic working principle of the mercury BP Machines. The principle is also utilized in the digital and Aneroid BP Machines. The difference is that the sensor replaces the stethoscope in digital BP Monitors.

Working of Digital BP Machine

Omron HEM 7120 Fully Automatic Digital BP Monitor

In the digital BP machine, there is a circuit board and a motor to pump the air. The electronic BP monitor comes with a capacitance manometer. The capacitance manometer is a chamber that is divided into two different sections. The part that divides the chamber is a flexible membrane. One side of the chamber is connected to a tube that pumps air in the cuff. The pressure changes cause the flexible membrane to flex. The flex is then converted to electrical capacitance. The electrical signal is then translated into reading with the help of pre-calibrated logic. The reading is then thrown on the screen for you. Some machines also use a valve system for measurement. Since the circuit on the digital BP machine is very compact, the machine also becomes travel friendly.

Working of Aneroid Sphygmomanometer BP Machine

Rossmax GB102 Aneroid Blood Pressure Monitor

Mercury is one of the most toxic metals, and the spillage from the Mercury BP Machines can be hazardous. To deal with this, many brands tried doing away from the Mercury BP machines by launching Aneroid BP Machines. There are two types of Aneroid BP Machines. The first one is designed without mercury. Here, a metal membrane is connected to a spring device further connected to a dial. There is no liquid present in these Aneroid BP Machines, so it is easy to move around. When you pump the air, the needle on the meter moves, and it, in turn, moves the metallic membrane as well. The move-in needle shows you the reading. The second type of Aneroid BP Machine has a mercury column connected to a dial. Once the pressure is the blood is equal to the cuff, the blood starts flowing across the cuff and thus having the same impact.


This was all about the working of the BP Monitors. You would have noticed that the digital BP monitors have the least number of moving parts, and hence they are a lot more reliable when compared with other options. Moreover, the mercury BP machines are being phased out because of the risk associated with the mercury. We hope we have answered your question about the working of the Digital, Mercury & Aneroid Sphygmomanometer. If you still have any questions, please feel free to reach out to us.

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