Physical Standards and Conventions

There are accepted standards in this world. This goes directly to the definition of, "convention". A convention is an agreement among people such that, "This will be what we mean when we refer to that". For instance, the conventional flow of electricity is from positive to negative. That method makes the math easier than dealing with negative numbers all the time, but it is merely a convention. As a person who learned about electricity when vacuum tubes were still the most used active electronic devices, I can not be convinced that something with a positive charge is emitted from the plate of a vacuum tube and arrives at the cathode. That means that I must declare that I am using, "electron flow" in order to communicate effectively.

"There are 12,000 B.T.U.s in a ton of air conditioning."

"The freezing point of water is the standard method to find zero degrees Celsius."

"The standard for atmospheric pressure is the height of a column of mercury."

"The standard for blood pressure is the height of a column of mercury."


Normal blood pressure is defined as a systolic BP between 100 and 120 mm Hg and a diastolic BP below 80 mm Hg (in adults over age 18). http://medical-dictionary.thefreedictionary.com/blood+pressure

Pressure in the compressed artery is estimated by the column of mercury it balances when the cuff is inflated. See also blood pressure, manometer.
Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier.

A normal range for systolic pressure is usually considered to be 100-140 mmHg and for diastolic pressure below 90 mmHg. See arteriosclerosis; hypertension; hypertensive retinopathy.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann


Devices which register pressure using a mercury column were the routine choice for many years, and are still the “gold standard” for blood pressure readings. http://ec.europa.eu/health/scientific_committees/opinions_layman/sphygmomanometers/en/index.htm


It is my contention that if you try to derive any of these constants and your answer is not the same as the convention, you must have made an error. Your water might not be pure, your thermometer might not be correct, and atmospheric pressure might interfere with a blood pressure measurement if you did not adhere strictly to the conditions agreed upon to make these measurements.

One example might be testing an electrical circuit to see if it works. If it doesn't work, you must have made a mistake because electrons will obey the laws of physics regardless of what you intended when you designed and built the circuit. In the same vein, water can not be wrong about its freezing temperature and mercury can not be wrong about how much it weighs.

There are very many ways to find a, "wrong" answer, but the standards can not be wrong. They were declared to be the standards and that's all there is to it.

If you don't believe the standards are correct, you can try to be seated at the next conference on weights and measures. Until then, we have to work with the standards that have been declared, WBahn.

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