SI system of measurement

Base units The seven units from which all other units in the SI system of measurement are derived. 

Step 8. Initially convert the pressure drops into the SI system of measurement units of bar . 

OBJECTIVE To learn the English, metric, and SI systems of measurement. 

Because chemists must be able to communicate their measurements to other chemists all over the world, they need to speak the same measurement language. This language is the SI system of measurement (from the French Systeme International), commonly referred to as the metric system. There are actually minor differences between the SI and metric systems, but for the most part, they re interchangeable. 

Standard uncertainty. Some of the universal constants, such as the speed of light in vacuum, form the basis of the SI system of measurement, hence their estabhshed value is considered exact. There are other constants, such as the basis of the atomic mass unit, which are not fundamental in nature but play an important practical role. Therefore, their value has also been adopted as exact. 

The SI system of measurement is used in science. It has seven base units the meter (length), kilogram (mass), second (time), kelvin (temperature), mole (amount of substance), ampere (electric current), and candela (luminous intensity). 

Point T in Figure 1.12 marks the meeting point of the three different phase boundaries and is known as the triple point. The triple point occurs at a unique temperature, volume, and pressure for a material, and at this point all three phases (solid, liquid, and gas) may coexist. The triple point of water is particularly important as it provides the definition of the kelvin (K), the SI unit of thermodynamic temperature. In 1954, the General Conference on Weights and Measures Conference Generate des Poids et Mesures, GCPM), an international organization responsible for the SI system of measures, defined the kelvin unit as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. ... 

We recognize that the International System of Units (SI) has become the fundamental basis of scientific measurement worldwide and is used for everyday commerce in virtually every country except the United States. As painM as it may be for those of us who have learned and practiced the British or US Customary system of units, we feel that it is time to put aside the units of the industrial revolution and adopt the SI system of measurement in all aspects of modern engineering and science. For this reason, SI units have been adopted as the primary system of units throughout this book. However, it is recognized that US customary or British units are stiU widely used in the United States and some use of them is provided herein for the benefit of those who still relate closely to them. Dimensional constants specific to the British system, such as gc, have been left out of the formulae. 

The International System of Units (SI) provides a coherent system of measurement units, and all the physical quantities required for refrigeration and air-conditioning can he derived from the basic standards ... 

It may be noted that the pressure measuring devices (a) to (e) all measure a pressure difference AP(— Pj — P ). In the case of the Bourdon gauge (0, the pressure indicated is the difference between that communicated by the system to the tube and the external (ambient) pressure, and this is usually referred to as the gauge pressure. It is then necessary to add on the ambient pressure in order to obtain the (absolute) pressure. Even the mercury barometer measures, not atmospheric pressure, but the difference between atmospheric pressure and the vapour pressure of mercury which, of course, is negligible. Gauge pressures are not. however, used in the SI System of units. 

In the SI system of units the distance is measured in meters, mass in kilograms, and the force in Newtons. 

Science cannot be performed without an accurate system of measurement, which is globally standardized and compulsory. Units and standards of measurement are agreed upon and harmonized on an international basis by the Bureau International des Poids et Mesures in Sevres, France, and by the International Organization for Standardization in Geneva, Switzerland. The units and standards are then laid down in national laws. Nearly all countries have accepted the Systeme International d Unites (SI units) as their system of measurement. This also applies to countries that had been accustomed to use British units like Australia, Canada, South Africa and the United States. In Britain, SI units are official from January 2010. The valid standards are available from the competent bureaus, for example Bureau International des Poids et Mesures, www.bip.fr National Measurement Institute (Australia), www.measurement.gov.au National Institute of Standards and Technology (NIST, USA), www.physics.nist.gov/ Pubs/SP811/... 

SI the international system of measurement units, including units such as the metre, the kilogram, and the mole from the French Systeme intemationale d unites (Review)... 

The metric system, or Systeme International d Unites (SI system as it is commonly known), is the predominant system of measurement in the world. In fact, the United States is one of only about three countries that do not commonly use the metric system. The metric system attempts to eliminate odd and often difircult-to-remember conversions for measurements (5,280 feet in a mile, for example). It is a decimal-based system with standard terminology for measurements of length, volume, and mass (weight). It also uses standard prefixes to measure multiples of the standard units. 

SI units of measurement, used by scientists around the world, derive their name from the French Systeme International d Unites. Fundamental units (base units) from which all others are derived are defined in Table 1-1. Standards of length, mass, and time are the meter (m). kilogram (kg), and second (s), respectively. Temperature is measured in kelvins (K), amount of substance in moles (mol), and electric current in amperes (A). 

Although SI is the internationally accepted system of measurement in science, other units are encountered. Useful conversion factors are found in Table 1-4. For example, common non-SI units for energy are the calorie (cal) and the Calorie (with a capital C, which stands for 1 000 calories, or 1 kcal). Table 1-4 states that 1 cal is exactly 4.184 J (joules). 

All systems of measurement / V I are based on arbitrary standards. The SI system is as arbitrary as the English system, but the relationship of one unit to another is more systematic and easier to remember in the metric system. 

We will use SI units throughout this text as much as possible. When another system of measurement is much more common, however, we will use the more conventional or convenient unit. 

It is of main importance to set up a total uncertainty budget following the GUM and EURACHEM guidelines for combined uncertainty to identify the main sources of uncertainty [40, 41]. Results from different laboratories or from the same laboratory at different times have to be comparable with conbdence. This is achieved if all laboratories are using the same stated reference. In many cases, this is achieved by establishing a chain of calibrations leading to primary national or international standards, ideally the SI units of measurement [79]. The SI system provides an international infrastructure for realizing comparable measurements by the use of traceable measurements. 

To communicate effectively, scientists rely on a standard system of measurement. As you have learned in previous studies, this system is called the International System of Units (Le systeme international d unites, SI). It allows scientists anywhere in the world to describe matter in the same quantitative language. There are seven base SI units, and many more units that are derived from them. The metre (m), the kilogram (kg), and the second (s) are three of the base SI units. You will learn about two more base units, the mole (mol) and the kelvin (K), later in this book. 

The current International System of Units (SI) is a metric system of measurement which has been adopted internationally by the General Conference of Weights and Measures and is described in an International Standard... 

The net quantity of contents (in terms of weight or mass, measure, or numerical count) shall be separately and accurately stated in a uniform location upon the principal display panel of that label, using the most appropriate units of both the customary inch/pound system of measure, as provided in paragraph (3) of this subsection, and, except as provided in paragraph (3)(A)(ii) or paragraph (6) of this subsection, the SI metric system ... 

When describing a measurement, you normally state both a number and a unit (e.g. the length is 1.85 metres ). The number expresses the ratio of the measured quantity to a fixed standard, while the unit identifies that standard measure or dimension. Clearly, a single unified system of units is essential for efficient communication of such data within the scientific community. The Systeme International d Unites (SI) is the internationally ratified form of the metre-kilogram-second system of measurement and represents the accepted scientific convention for measurements of physical quantities. 

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