Industry instrumentation : Units and conversion tables for process control

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The International System of Units (SI Units)

The International System of Units (SI units) is rapidly becoming the most commonly used in the world. However, English units are still in common use in instrumentation within the United States. Hence this table has been arranged to facilitate conversion into SI units by placing these conversions first in each list.

The measurement of such quantities as force, pressure, mass, and weight has in the past often been made through the convenient use of the "force of gravity" without concern for the variation of this force from one location to another-which was normally insignificant in application, though does vary even in the minor difference in elevation between the top of the hill and the bottom . However, as the process industries have spread geographically, and as processes have required more sophisticated control, the difference in gravity between the points of calibration and use of an instrument has become significant more often. Moreover, even though the values were very close, the practice of ignoring them was fundamentally wrong.

Most pressure and differential pressure instruments use forms of springs as elements. Springs measure force directly, independent of the effects of gravity. Thus most modem pressure instruments will read the same for the same pressure regardless of local gravity. They are gravity-independent. However, the pressure standards used to calibrate them are often gravity-dependent, since they depend upon the weight of (the force of gravity on) a column of water or mercury or fixed masses ("weights") in deadweight testers. Thus, for higher accuracies, pressure instruments must be calibrated in the location at which they are to be used, or account taken of the difference in gravity between the point of calibration and the point of use, even though the instruments themselves are gravity-independent.

In pneumatic transmission of pressure measurements, the measured variable and the output signal of the transmitter will be determined at the same local gravity. Therefore, if the same type of pressure standard is used for both input and output, both gravity-dependent or both gravity-independent, the effect of local gravity will be cancelled out. In electronic transmission, on the other hand, the output signal is a current or voltage unaffected by gravity. Thus it may be necessary to use a gravity-independent standard on the input -or to apply a correction factor for local gravity.

It has long been most common to use the same name for units of mass (the invariant quantity of matter in a body) and weight (the force of gravity on that mass, which varies with location). Similarly, the names of pressure units, force per unit area, usually contain the name of a mass unit instead of a force unit. As long as variations in gravity with location are insignificant, this practice does not cause much difficulty. However, the effects of local gravity value are becoming more significant in comparison with now desired instrument accuracy. And the fact that in the SI system the unit of force is the Newton, while the kilogram is reserved strictly for mass, makes it important to designate whether a non-SI unit such as the pound is being used to describe mass or force (or weight). This is done by adding the suffix "-force" to the name of the unit. Thus, in non-SI units, the mass of a body is properly described in pounds, but its inertial force should be described in pounds-force. Common non-SI pressure units should always be designated as pounds-force per square inch, kilograms-force per square centimeter, etc. The letter symbols for these force units also contain the suffix "-f" as in lbf, kgf, etc, However, the two most common non-SI pressure units named above are usually abbreviated psi and kg/CM2. These customs are reflected in the table.

The suffix "-mass" is sometimes added to non-SI units actually used as mass units. However, this practice is decreasing as more people grasp the basic distinction between mass and force or weight.

In the measurement of "weight," the effect of local gravity is usually compensated for automatically. Either a "no-springs" type of scale is used, in which the unknown mass is balanced against known masses ("weights"), thus canceling out the effect of variations in local gravity; or a spring type scale is used, calibrated in place using known masses ("weights"). Thus the quantity often called "weight" is actually the mass, and must be properly described in mass units , the kilogram in SI units or the pound and ounce in English units. The only change from past practice needed is to label the quantity as "mass" instead of "weight."

Great care has been taken to avoid errors, no responsibility can be taken for complete accuracy.

HOW TO USE THIS TABLE: Find the unit you want to convert from listed in capital letters at the left-hand margin and multiply it by the number indicated to arrive at the unit listed to the right of the number. Where appropriate, additional information has been provided.

EXAMPLE:

Additional information { NOTE: There are several definitions of Btu, differing only past the third significant digit .If four or more significant digits are needed, refer to the appropriate handbook.

DERIVING UNITS: Many categories have several units related by a power of ten (e.g., Pa and kPa) or by a factor of 60 (e.g., ft/s, ft/min, and ft/h). Generally, conversion factors are provided for only the proper Sl unit or the unit most easy to use. There are several shortcuts to deriving units not listed; following is one reliable method.

Suppose you have a volume per unit time of 1 cfh and you want to express that in m³/s . Look up Cubic Feet Per Hour and read, "Divide by 60 and refer to Cubic Feet Per Minute." Look up Cubic Feet Per Minute and find the conversion factor to m³/s (cfm x 4.7195 x 10^(-4) = m³/s). String these together and get

(1 cfh

Suppose you have a volume per unit time of 1 m³/s  and you want to express that in cfh. Look up Cubic Metres Per Second and find that the closest conversion factor for cfh is cfm

(m³/s x 2.1189 x 10³ = cfm). Then look up Cubic Feet Per Minute and find the conversion factor to cfh (cfm x 60 = cfh). String these together and get 

SCIENTIFIC NOTATION: Remember when using a calculator that positive powers move the decimal point to the right of the 1 (e.g., 10³ = 1000.0) and negative powers move the decimal point to the left of the 1 (e, g., 10^(-3) = 0. 0010).

PRESSURE UNITS: Where density is specified or implied, it is based on the following:

  • Density of water at 60°F = 62.3707 Ib/ft³
  • Density of mercury at O°C = 13.5955 g/cm³

MAGNITUDE OF FIGURES: Where practical, conversion factors over 10^4 or under 10^(-4) have not been included. (Factors have been included for all proper SI units no matter what the multiple.) However, avoid mixing prefixes within one document-or equation (e.g., don't use kPa, Pa, and MPa together).

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