ASNT

PART 4 Units of Measure for Radiographic Testing

ORIGIN AND USE OF SI In 1960, the General Conference on Weights and Measures established the International System of Units. Le Systéme International d’Unités (SI) was designed so that a single set of measurement units could be used by all branches of science, engineering, and the general public. Without SI, this Nondestructive Testing Handbook volume could have contained a confusing mix of obsolete centimeter- gram-second (CGS) units, imperial units, and the units preferred by certain localities or scientific specialties. SI is the modern version of the metric system and ends the division between metric units used by scientists and metric units used by engineers and the public. Scientists have given up their units based on centimeter and gram, and engineers made a fundamental change in abandoning the kilogram- force in favor of the newton. Electrical engineers have retained their ampere, volt, and ohm, but changed all units related to magnetism. Table 3 lists the seven SI base units. Table 4 lists derived units with special names. Table 5 gives

Table 4 SI derived units with special names a Relation to Other Quantity Units Symbol SI Units b Capacitance farad F C/V Catalytic activity katal kat mol/s Conductance siemens S A/V Energy joule J N·m Frequency (periodic) hertz Hz 1/s Force newton N kg/m/s 2 Inductance henry H Wb/A Illuminance lux lx lm/m Luminous flux lumen lm cd·sr Electric charge coulomb C A/s Electric potential c volt V W/A Electric resistance ohm Ω V/A Magnetic flux weber Wb V·s Magnetic flux density tesla T Wb/m Plane angle radian rad 1 Power watt W J/s Pressure (stress) pascal Pa N/m 2 Radiation absorbed dose gray Gy J/kg Radiation dose equivalent sievert Sv J/kg Radioactivity becquerel Bq 1/s Solid angle steradian sr 1

Table 3 SI base units

Quantity

Unit

Symbol

degree celsius

Temperature

°C

K

Length

meter

m

Mass Time

kilogram second ampere

kg

Time a

hour

h

3600 s

s

Volume a

dm 3

liter

L

Electric current Temperature a

A K

a. Hour and liter are not SI units but are accepted for use with SI. b. Number one (1) expresses a dimensionless relationship. c. Electromotive force.

kelvin

Amount of substance

mole

mol

Luminous intensity

candela

cd

a. Kelvin can be expressed in degrees celsius (°C = K – 273.15).

CHAPTER 1

32

Part 4