![]() ![]() In the American/English system a Newton is about 0.225 lb and a square meter is 10.76 sq ft, so 1 psi = 6895 pascals or 6.8 kPa. A Newton is a force that will give a mass of 1kg an acceleration of 1m/s 2. The Pascal is the defined unit of measure for Pressure in the SI metric system of measurement and is defined as 1 Newton/meter 2. Now the European community has standardized on the SI (System International) system of measurement with the unit for pressure being the Pascal (Pa). With all these units based on different physical parameters, there was never a clean conversion from the American/English pressure units to the European/metric equivalent units. For low pressures and vacuum measurements "inches of water" was used in America and TORR (mm of Mercury) was used in the European system. Some countries used the BAR, others used kg/cm2 where 1 kg/cm2 = 14.22 psi and 0.98 BAR. Adding to the confusion is the fact that the metric world used several different units for pressure. Atmospheric pressure is 14.7 psi at sea level whereas the metric system uses the BAR for atmospheric pressure and 1 BAR is equal to only 14.5 psi. Pressure also seems to be an especially difficult unit to cross to metric. ![]() How could you build a house without 2x4x8's or describe temperature with 32° Hot in ☌elsius or Cold in ☏ahrenheit? The same seems true for velocity/speed (although if 60 km/hr was faster than 60 mph it might have caught on). When I went to engineering school, we learned both sets of units (English and Metric) and could readily convert between them in calculations but in public life, the metric system never caught on in America. We still use the Fahrenheit system for temperature, inch/foot/yard/mile units for length, cup, pint, quart, gallon for volume, miles per hour for velocity, and pounds per square inch for pressure. Here in the United States, we differ from the rest of the world by holding onto the pre-metric (English) units in public life. Temperature and pressure are certain foundational units that are enshrined in everyone’s daily life. This is an interesting anomaly, I do not know the background but suspect measurement “accuracy” at the time such as with Fahrenheit determining body temperature to be 100° rather than what we now today is actually 98.6°. Atmospheric pressure was determined and given the unit BAR (1 BAR = 14.5 psi). On Continental Europe, the unit of weight is the gram and the unit for area the meter, but g/m 2 was not a workable unit so kg/cm 2 was used. In this system, Atmospheric pressure was 14.7 psi at sea level. In England and America, the unit of force was the pound and the unit for area was square inch, hence, the Pressure unit became pounds per square inch (psi). Pressure is a more complex parameter defined as “force per unit area”. The Celsius Temperature scale was 0° at the freezing point of water and 100° at the boiling point of water. The Fahrenheit temperature scale was set with 0° was the coldest you could get an ice salt slurry and 100° was body temperature. All units were originally derived from readily available physical standards that laboratories around the world could duplicate. ![]()
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