测控专业英语第二章

2 Measurement of non-physical quantities
• Non-physical quantities found in non-technical fields are very difficult or even impossible to measure, mainly because they are part of very complex systems (people, organizations ,societies ,etc.) . Cardinal measurements can therefore very rarely be made when dealing with the non-physical characteristics, states or phenomena in such systems. When measuring complex systems or objects, the measurand often also depends on all kinds of other quantities .Usually ,the nature and magnitude of these dependencies are unknown .Furthermore, these parasitic quantities ,which influence the measurement result ,are not constant during the measurement, making it impossible to correct any errors in the measurement .This means that the measurement is no longer selective ;other factors are also being measured. Non-physical quantities are usually a part of a living organism or an organization of living beings .It is essential to the nature of an organism or an organization that they maintain (social , cultural , political) interactions with their environment .For this reason , it is usually not possible to perform an isolated measurement in the same way we can with inanimate things (putting in a thermostat, hooking up to a fixed supply voltage, etc.). It is , for example, not very practicable to isolate a section of the population from the rest of the world , for the sake of an economic measurement conducted to verify the relationship between the scarcity and the price of consumer goods .The object of the measurement continues to interact with its surroundings, to an extent and in a manner that remains unknown. These interactions obscure and corrupt the measurement results to some unknown degree ,depending on the sensitivity to these interference.
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Capacitance It is possible to construct a capacitor from four coaxial cylinders , whose capacitance only depends on the length of the cylinders . Such a capacitor is particularly suitable as a standard of capacitance , since only the length has to be determined accurately . With the aid of optical interferometry this can be done extremely accurately . These so-called Thompson-Lampard cylinder capacitors can achieve an inaccuracy of less than 10^-8. A disadvantage ,however ,is the fact that the capacitance is small (approximately 1.9pF per metre ). For lower-order standards other configurations of electrodes are used ,which provide larger capacitance values (10~1000pF),but also come larger uncertainties . Inductance Accurate standards of inductance are difficult to realise . This is caused by the many parameters that determine the relatively complex of a coil ,all of which influence the accuracy of the inductor .Furthermore ,power losses occur ,due to wire resistance ,proximately effects and eddy currents ,which add to inaccuracy ,Currently a available standards of inductance have an inaccuracy of about 10^-5 . Frequency The standard of frequency is based on quantum mechanical effect that electrons in an atom can only occupy one of a limited number of energy levels .If an electron jumps to a higher or a lower energy level ,the difference in energy E of the photon that is absorbed or a emitted , respectively ,is related to the frequency of the photon by the expression E=h f0.When atoms are irradiated with electromagnetic energy of frequency f0,many electrons will pass to higher energy levels .
Measurement of Physical Quantity
The physical quantities listed bellow are often used in test and measurement technology .The first 2 are commonly called as electrical quantities and the other are commonly called as electrical parameters . Usually the sensors receive the information of quantities under measurement (QUM) and convert it into variation of electrical parameters or electrical potential, which will normally be conditioned ,and then converted into the electrical quantities ,specially the current ,and sent to the A/D converters . Electrical potential difference The primary standard for electrical potential difference used to be provided by an electrochemical standard cell (the Weston standard cell ).The voltage of a Weston cell is approximately 1.01860 V at 20`C,with an inaccuracy of 3X10^-6 under optimal conditions. Optimal conditions means a variation of temperature less than 10^-3 K, no load, no vibrations or jolting . The cell must remain in an upright position. A Weston standard cell has a very long thermal after-effect. After the cell has been heated to 30`C it can take 6 months before it is totally stable within 0.3u V of the original value at 20degrees .Furthermore ,a Weston cell will age , resulting in an increase in the internal resistance (R1~500-1000欧)，and a small decrease of a few uV in the output voltage (during the first years ). Electrical current Electrical current is standardized by measurement with an instrument called a ‘current balance’. This device measures the electromagnetic force between two current carrying coils (one fixed ,one moving ) by balancing it with the force of gravity ,acting on a known mass . The force between the coils is given by F=I^2dM/dx , in which M is the known mutual induction of the coils and x is the known distance between them . The differential quotient dM/dx is determined by known geometry of the coils . Electrical resistance Standards of electrical resistance are resistors wound of special alloy wire giving a minimal temperature effect .An example of such an alloy is evanohm , consisting of 74% nickel ,20%chromium and 6%aluminium and iron . This alloy is frequently used for high resistance standards (10kQ). For low resistance values (1Q) manganine (86%Cu ,12%Mn ,2%Ni ) or constantan (54%Cu ,45%Ni ,1%Mn ) is often used .Higher –order resistance standards are kept at very accurately stabilised temperature by thermostats .