Copyright Goodheart-Willcox Co., Inc. May not be reproduced or posted to a publicly accessible website. Chapter 5 Measurement 81 5.7.2 SI In SI units, pressure is measured in pascals (Pa). A pascal (Pa) is the amount of force of one newton pushing on one square meter of area. Since a pascal is a small unit, the HVACR fi eld normally uses the kilopascal (kPa) as a unit of pressure. The kilopascal is equal to 1000 pascals. The term bar was also used and still may be referenced in some aspects of the mechanical trades. Although this measurement is not an SI standard, in modern usage it is equivalent to 100 kPa. Originally, bar was meant to express the pressure of the atmosphere at sea level and was equivalent to 1 atmosphere (atm). A millibar (mbar) is 1/1000 of a bar. Vacuum measurement in the SI system uses the micron (μm), which is 1/1000 of a millimeter or 1/1,000,000 of a meter. In countries other than the United States, the micron is referred to as the micrometer. The term torr is also used, which represents 1000 microns. A digital vacuum gauge reads in microns and has a range of 760,000 μm (atmospheric pressure) to 0 μm (perfect vacuum). Ritchie Engineering Co., Inc. – YELLOW JACKET Products Division Some pressure gauges show both the US Customary and SI scales, such as having psi on the outermost scale and kPa on the inner side of that scale. However, refrigerant gauges more commonly show only one type of pressure unit along the outer scale, and the numbers on the inner scales correspond to the saturated temperatures of different refrigerants for that particular pressure reading. Mastercool Inc. 5