Service Application Manual SAM Chapter 620-82 Section 13B ASHRAE SI (METRIC) GUIDE FOR HEATING, REFRIGERATING, VENTILATING AND AIR CONDITIONING INTRODUCTION It is the decision of the Educational and Examining Board of the Refrigeration Service Engineers Society to make its membership aware of the SI conversions. A lot of time and effort have been put in to prepare this ASHRAE Guide and RSES is thankful to ASHRAE for granting permission to reprint this guide. Measurement practices all over the world are in various stages of transition to the modern metric system, Systeme International d' Unites. This system, known as SI, was developed by an international treaty organization for measurement, which has been in existence since 1875. First released in 1960, the new system represents the culmination of efforts begun in about 1900 to improve the metric system. It is important to understand that SI is the modernized version of the metric system. Some of the previous metric units and practices are not necessarily compatible with SI. Only authorized SI units or authorized exceptions may used. In addition to US Customary Units, there are Imperial (British) units, cgs units, and other metric units, which will be phased out in favor of the new international system. As can be understood, the retention of individual preferences for some units in various countries and in various disciplines has been and may remain an impediment to the realization of the full benefits of SI. In June 1975, ASHRAE adopted SI and a timetable. In February 1977, the Board of Directors modified the timetable to read as follows: ASHRAE documents published after January 1, 1976 shall be prepared using only SI (Metric) units, or shall be prepared using dual units, i.e.,SI (metric) units and conventional units, with the sequence of units left to the discretion of the author or editor. Exceptions in HANDBOOK volumes, Standards, and special publications may be authorized by the responsible ASHRAE staff in coordination with the ASHRAE Metric Committee. After July 1, 1977, the use of SI units first, followed by conventional units, is encouraged in the preparation of ASHRAE publications, unless the material is already prepared in SI units only. Exclusive use of SI (metric) units shall be required in ASHRAE publications when it is determined by the Board of Directors to be in the best interest of the membership. During the transition period, the Metric Committee will continue to disseminate educational materials to prepare the membership for the eventual exclusive use of SI units. For the sake of brevity, this guide covers primarily information directly applying to the heating, ventilating, air conditioning and refrigerating industry. The reference standard covering SI in ASHRAE publications is ASTM E 380-76, Standard for Metric Practice (American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103). For further information on subjects not covered in detail here, and for conversion values with more significant digits, refer to ASTM E 380. I. UNITS AND SYMBOLS The International System (SI) consists of seven base and two supplementary units, which can stand alone, and a number of derived units, which are combinations of base units or base and supplementary units. Some derived units have special names and symbols. 1 Service Application Manual SAM Chapter 620-82 Section 13B ASHRAE SI (METRIC) GUIDE FOR HEATING, REFRIGERATING, VENTILATING AND AIR CONDITIONING BASE UNITS Quantity Name Symbol length metre m mass kilogram kg time second s electric current ampere A thermodynamic temperature kelvin K amount of substance mole mol luminous intensity candela cd SUPPLEMENTARY UNITS Quantity Name Symbol plane angle radian rad solid angle steradian sr II. USE OF BASE AND DERIVED UNITS In SI there is one and only one unit for each physical quantity. There are the base and supplemental units, which may be modified by prefixes as indicated in Section III. All the derived units are defined by simple equations using the base units. This is known as a coherent system and is the simplest measurement system to use. It is incumbent upon the user to protect the basic simplicity of the system by adhering strictly to the approved units or authorized exceptions. Angle The correct unit for the plane angle is the radian. The degree and its decimal fractions may be used, but use of the minute and second is discouraged. Area The SI unit of area is the square metre (m2). Large areas are expressed in hectares (ha) or square kilometers (km2). The hectare is restricted to land or sea areas and is equal to 10,000 m2. Energy The correct unit of energy is the joule (J). However, the kilowatt hour (3.6 megajoules) is widely used as a measurement of electric energy. As kilowatt hour will eventually be replaced by the megajoule or gigajoule, kwh should not be used for any new applications. DERIVED UNITS Quantity Name Formula Symbol acceleration - angular radian per second squared rad/s2 acceleration - linear metre per second squared m/s2 area square metre m2 2 Service Application Manual SAM Chapter 620-82 Section 13B ASHRAE SI (METRIC) GUIDE FOR HEATING, REFRIGERATING, VENTILATING AND AIR CONDITIONING watt millimetre per square metre degree Celsius W•mm/m2•°C conductivity, thermal, k or, watt per metre kelvin W/m•K density - energy joule per cubic metre J/m3 density - heat flux watt per square metre W/m2 density - mass kilogram per cubic metre kg/m3 energy, enthalpy - work joule N • m J energy, enthalpy - specific joule per kilogram J/kg entropy - heat capacity joule per kelvin J/K entropy - specific joule per kilogram kelvin J/kg•K force newton kg•m/s2 N frequency - periodic hertz 1/s Hz frequency- rotating revolutions per second (pref.) r/s inductance henry Wb/A H moment of a force newton metre N•m potential, electric volt W/A V power, radiant flux watt J/s W pressure, stress pascal N/m2 Pa resistance, electric ohm V/A Ω velocity - angular radian per second rad/s velocity-linear metre per second m/s viscosity - dynamic pascal second Pa•s viscosity-kinematic square metre per second m2/s volume cubic metre m3 volume, specific cubic metre per kilogram m3/kg Force The correct unit of force is the newton (N). Do not use the word weight and do not use kilogram force. The newton is also used in combination units that include force. pressure or stress N/m2 = Pa (pascal) work N•m = J (joule) power N•m/s = W (watt) Mass The correct unit of mass is the kilogram (kg). Among the base and derived units of SI, the unit of mass is the only one whose name, for historical reason, contains a prefix. Names of decimal multiples and submultiples of the unit mass are formed by attaching prefixes to the word gram. Do not use the word weight, as this could be confused with force. The multiple—megagram, (Mg) or tonne (t) is the appropriate unit for measuring large masses. 3 Service Application Manual SAM Chapter 620-82 Section 13B ASHRAE SI (METRIC) GUIDE FOR HEATING, REFRIGERATING, VENTILATING AND AIR CONDITIONING Pressure The correct unit of stress or pressure which is force per unit area is the newton per square metre. This unit has been given the special name, pascal (Pa). No other units are acceptable. There is no equivalent symbol for psig or psia, so if there is a possibility of misunderstanding, spell out Pa absolute or Pa gage. Temperature The correct unit of temperature is the kelvin, which is equal to a degree Celsius (formerly called degree centigrade). The thermodynamic temperature (absolute temperature) is related to Celsius as follows: t = T-T0 where t = degrees Celsius T = thermodynamic temperature T0 = 273.15°K by definition Time When expressing rates, the correct unit of time is the second. Do not use the minute or hour. In some cases of long cycles it may be necessary to use the day, week, month, or year. Exception: revolution per minute may be used, but revolution per second is the correct and preferred unit. Volume The correct unit for volume is the cubic metre. The cubic decimetre, which has the special name litre (l), is a regularly formed submultiple of the cubic metre. This is the correct and convenient unit to replace gallon and cubic foot. Litre per second would therefore replace gpm and cfm. A smaller correct unit is the millilitre per second. Note that the litre is restricted for use only with liquids and gases and for the volume of a vessel. III. PREFIXES Prefixes indicate orders of magnitude in steps of 1 000. This provides a convenient way to express large and small numbers and to eliminate non significant digits and leading zeros in decimal fractions. The following are the most used prefixes. Prefix Symbol Represents giga jig'a (i as in jig; a as in about) G 1 000 000 000 109 mega (as in megaphone) M 1000 000 106 Kilo (kill-oh) k 1 000 103 milli (as in military) m 0.001 10–3 micro (as in microphone) µ 0.000 001 10–6 nano (nan-oh) (an as in ant) n 0.000 000 001 10–9 126 000 watts becomes 126 kilowatts 0.045 metre becomes 45 millimeters 65 000 metres becomes 65 kilometers 4 Service Application Manual SAM Chapter 620-82 Section 13B ASHRAE SI (METRIC) GUIDE FOR HEATING, REFRIGERATING, VENTILATING AND AIR CONDITIONING The following prefixes should be avoided, except that centimeter is used for anatomical measurements, cloth, and clothing sizes. Prefix Symbol Represents hecto (heck-toe) h 100 102 deka deck'a (a as in about) da 10 10¹ deci (as in decimal) d 0.01 10–1 centi (as in centipede) c 0.01 10–2 To realize the full benefit of the prefixes when expressing a quantity by numerical value, choose a prefix so that the number lies between 1 and 1 000.