Blood pressure is an interesting measurement of biological health. Most of us use the term as if it has no relation at all to fluid mechanics, and we might as well call it floxcore gleblu or some other randomly generated name (actual randomly generated word). In pneumatics, 150 psi is considered very high, and most in the hydraulic realm don’t bat an eyelash at 6000 psi, and even 10,000 psi plus has a place in the hydraulic tool industry.
Blood pressure is measured in millimeters of mercury. Most of you will recognize the archaic use of mercury for measuring vacuum. Mercury is a dense, heavy liquid with consistently low viscosity over a wide temperature range. It is used to measure pressure because it’s dense and does not evaporate easily, providing consistent and repeatable measurements. However, in the fluid power realm, we measure using inches of mercury.
I’ll save you from having to Google the conversion of millimeters to inches; there are 25.4 millimeters in an inch. This means if you have a pressure gauge reading 1 inch per mercury, it can be further split up into 25.4 millimeters of mercury. One advantage of mm/hg is that it provides a high level of accuracy. A measurement of 15 mm/hg is just 0.59 in/hg, which is a rather arbitrary method to express pressure.
The second advantage of using millimeters of mercury, is that it provides a solid range of integers to represent a relatively low pressure. If you’re like me, inches of mercury is nearly as meaningless as describing distance in light-minutes. My preferred pressure unit is pounds per square inch. How does a millimeter of mercury translate into psi?
One single psi is equal to 51.7 mm/hg, and this is where it gets interesting. We can all describe blood pressure, and most of you know the range you typically fall within. Every beat of our heart creates a rise and fall of our blood pressure, and the range you see represents the maximum and minimum pressure, which is systolic and diastolic respectively.
The target blood pressure for most healthy adults is 120/80, where 120 is the systolic blood pressure during heart contraction, and 80 is the diastolic blood pressure during heart relaxation. Those of you who beat me to the calculator will have converted these numbers to psi and resulted in 2.32/1.55 as the normal pressure range. You read that right; our normal blood pressure would not even be able to move the needle on a hydraulic or pneumatic pressure gauge.
If you are an unfortunate individual with “high” blood pressure, you might still find it difficult to hit 3 psi of systolic blood pressure, which equates to red-faced 155 mm/hg. That all the oxygen, nutrients, hormones, and other substrates our bodies need to live is circulated by less than 3 psi is absolutely astounding. It also gives us a clue how our bodies can be so fragile in some ways, explaining the warning that air guns should never be blown directly onto your skin. Further to this, it gives you an appreciation for the pressure we’ve been able to achieve in the fluid power industry. And the next time you’re getting your blood pressure checked, don’t complain about the tightness of the cuff … it’s only 3 psi.