Questions for self-verification

1. What is the measurement? What is the difference between the direct and indirect measurement?
2. What is the indirect measurement? Give the examples.
3. What are the measurements in a closed series? Give the examples.
4. What are the simultaneous measurements? Give the examples.
5. What is the method of measurement? What is the difference between the direct evaluation method and the comparison methods of measurements?
6. What is the differential measurement method? What are its advantages? What examples of its using for electrical and non – electrical measurements can you adduce?
7. What is the null-method of measurement? What are its advantages? What examples of applying this method for electrical and non-electrical measurements can you adduce?
8. What is the substitution method of measurement? What are its advantages? What examples of applying this method for electrical and non-electrical measurements can you adduce?
9. What is the coincidence method of measurement? What are its advantages? What examples of this method application for electrical and non-electrical measurements can you adduce?

Measurement instruments and their varieties

 A measurement instrument (MI) is a technical device, intended for measurements, that has the standardized, metrological features, reproduces and (or) retains a unit of a PQ, value of which is accepted as invariable (inside the prescribed error limits) during the known time interval.

In this defenition you should pay your attention to the two basic distinctions. First of all, it must be a technical device, the second – its metrological features must be normalized and legitimated. For example, the measurement of distance by steps is not legitimated, because a legitimated, with normalized metrological features (first of all, permissible errors) technical instrument, such as a tape – measure has not been applied.

Material measure is a measurement device, which reproduces and (or) retains the physical quantity of the specified size.

For example, a weight is a mass material measure; a measurement resistor is a material measure of the electrical resistance; a measurement generator is a material measure of the electric oscillation frequency.

Indicating (measurement) instrument is a measurement device, in which a visual signal of measuring information is created.

For example: ammeter, ohmmeter, manometer, thermometer and others.

Measurement transducer is a measurement device, which realizes a measurement transformation. For example, a shunt converts a current under measurement into a voltage drop; a thermocouple converts a temperature under measurement into an electromotive force, etc.

Auxiliary measurement instrument is а measurement instrument of the quantities, which influence on metrological characteristics of other MIs during its use or verification.

 For example, EMF of the Weston standard cell depends on its temperature.

When the Weston standard cell applied, for example, in direct current potentiometer, for accurate EMF determination according to known formulas or tables, a thermometer, used necessarily, is an auxiliary MI.

A measurement installation is a complex of functionally united MIs (material measures, indicating instruments, measurement transducers) and an auxiliary equipment, that is intended for producing signals of measurement information in form, which is usable for direct perception by observer and that is situated in one place.

 For example, an installation for verification of ammeters and voltmeters is a special table with the power sources with suitable transformers and regulators, reference instruments or terminals for their link up, commutation facilities in its curbstones, etc.

A measurement system  is a complex of measurement channels, measurement devices and other technical facilities, united to create signals of measurement information about several PQs under measure- ment.

As an example of such system is the transmission system of measurement information from space objects.