ASME PTC 2:2001 pdf download DEFINITIONS AND VALUES
2.4 INTRINSIC ACCURACY
The intrinsic or inherent accuracy of a measure-ment system depends upon
(a) materials,
(b) construction,and
(c) physical condition at time of use.
wWhile the inherent accuracy of the first two itemscan be estimated from published data concerning anindividual measuring system component,the actualaccuracy at the time of use must be determined bycalibration. For this reason,all significant and primary measuring system components must be cali-brated or checked before and after PTCtests toestablish the effect of their physical condition oninherent accuracy, unless there is no cause for theircalibration to change.
2.5 IN SITu CONDITIONS
The accuracy of a measurement system as useddepends upon its ability to
(a) sense the variable to be measured,
(b) transmit energy change,
(c) apply or transduce energy,and(d) display data.
The last two items are essentially independent ofuse unless environmental conditions such as vibra-tion, temperature, humidity, etc., are of such magni-tude as to prevent normal operation. lf care is takento ensure proper environmental conditions, then thecommunication function of accuracy is simply theintrinsic accuracy of the components.
2.6OBSERVATION ACCURACY
Accuracy of observation depends primarily uponthe following two factors:
(a) accidental mistakes, e.g.,misreading of scales,parallax，incorrect log entries,failure to performsome required manipulation,etc.; and
(b) personal characteristics, e.g., ability to interpo-late between graduations,bias in observation (ten-dency to read high or low),speed of observation.These types of errors may be minimized (but nevercompletely eliminated) by selection and training oftest personnel, by selection of scales with easily-readgraduations, and by other human-factor engineering.Modern technology permits the design of instrumentsystems that will give digital printout,and the use ofthese should be encouraged to eliminate observationerror when their inherent accuracy and in situ condi-tions permit. However，digital systems may containprogramming mistakes,and these systems must bedebugged thoroughly.
2.7 SENSING ACCURACY
The accuracy of sensing depends upon the follow-ing factors:
(a)Effect of the Primary Element on the MeasuredQuantity
For example, a Pitot tube installed in a flow streamto sense a local velocity must be designed so thatits presence does not change the original velocityprofile.
3.1 PRIMARY DEFINITIONS AND SYSTEMS OF
UNITS
The dimensions of mass,length, and time arerelated to forces as follows in the various systemsof units.
Aforce of one pound applied to a mass of oneslug (also known as the geepound) will acceleratethe mass at the rate of one ft/sec2.
A force of one newton applied to a mass of onekilogram will accelerate the mass at the rate of onem/s2.
Equations written in these units will appear identi-cal. Converting measured values from the U.s. cus-tomary units to the primary pound-slug-second sys-
tem of units will simplify the expression of testresults in Systeme International (S).
By way of contrast and for clarification,a forceof one pound applied to a mass of one pound will
accelerate the mass at a rate numerically equal to“gc”ft/sec2. This fact is the origin of the appearanceof the conversion factor, gc in engineering equationsexpressed in the U.S. customary units. Note that gc
is ‘not the local acceleration of gravity at the testsite; it is simply a ratio of masses and a constant.
3.2 HSTORICAL DEFINITIONS OF UNITs OF
MEASURE
lt is often useful to be aware of the historicphysical bases for many of the units of performancemeasurement. The reader is cautioned that the nu-merical values of these physical definitions havebeen refined over the years,so that the following
historic definitions may be no longer numericallyexact. Nonetheless, the embodied physical concepts
can improve one’s understanding of a measurementor test result. The current values and definitions for
use are given starting in para. 3.3.
boiler horsepower: standardized by the ASME in1889,it was based on an engine steam rate of 30
lbm of steam per horsepower-hour at 70 psig anda feedwater temperature of 100°F.This corresponds
to 34.5 lbm/hr evaporated from and at 212°F(33,475
ASME PTC 2:2001 pdf download
