Measuring instrument

When a system is isolated, it cannot be affected by its surroundings. Nevertheless, changes may occur within the system that are detectable with such measuring instruments as thermometers, pressure gauges, and so on. However, such changes cannot continue indefinitely, and the system must eventually reach a final static condition of internal equilibrium. 

The concept of equilibrium is central in thermodynamics, for associated with the condition of internal eqmlibrium is the concept of. state. A system has an identifiable, reproducible state when 1 its propei ties, such as temperature T, pressure P, and molar volume are fixed. The concepts oi state a.ndpropeity are again coupled. One can equally well say that the properties of a system are fixed by its state. Although the properties T, P, and V may be detected with measuring instruments, the existence of the primitive thermodynamic properties (see Postulates I and 3 following) is recognized much more indirectly. The number of properties for wdiich values must be specified in order to fix the state of a system depends on the nature of the system and is ultimately determined from experience. 

As normally used in the process industries, the sensitivity and percentage of span accuracy of these thermometers are generally the equal of those of other temperature-measuring instruments. Sensitivity and absolute accuracy are not the equal of those of short-span electrical instruments used in connection with resistance-thermometer bulbs. Also, the maximum temperature is somewhat limited. 

The principal classes of flow-measuring instruments used in the process industries are variable-head, variaBle-area, positive-displacement, and turbine instruments, mass flowmeters, vortex-shedding and iiltrasonic flowmeters, magnetic flowmeters, and more recently, Coriohs mass flowmeters. Head meters are covered in more detail in Sec. 5. 

Golding, E.W., Electrical measurements and measuring Instruments. The English Language Book Socieiy and Sir Isaac Pitman Sons Ltd, London. 

M to 9 Direct acting indicating analogue electrical measuring instruments and their accessories 1248 1 to 9 BS 89-1 to 9 ... 

These assemblies are fitted with switching devices (breakers, switches, fuse switches and contactors etc.) and control and measuring instruments, indicating, regtilating and protective devices etc. to transform the assemblies into composite units, called control centres to perform a number of functions in the field of distribution and control of electrical power. Some of these functions may be one or more of the following ... 

Direct acting recording electrical measuring instrument and their accessories - BS 90/1993 -... 

This is not material in voltage transformers, as neither the voltage measuring instruments nor the protective relays will carry any inrush current during a switching operation or a fault. No short-time rating is thus assigned to such transformers. 

This is the ratio of instrument limit primary current to the rated primary current. Consequently a high SF will mean a high transformation of the primary current and can damage instruments connected to its secondary. For measuring instruments therefore it is kept low, as it is required to measure only the normal current and not the fault current. 

As in lEC 60051, the measuring instruments are required to have an overcurrent factor of not more than 120% for two hours for instruments of all accuracy classes, 200% for 0.5 second for class 0.5 or less, and 1000% for 0.5 second for class I accuracy and above. Overcurrents or durations longer than this may damage the instruments. 

I When the system voltage is linear (an ideal condition that would seldom exist) but the load is non-linear The current will be distorted and become non-sinusoidal. The actual current /, (r.m.s.) (equation (23.2)) will become higher than could be measured by an ammeter or any other measuring instrument, at the fundamental frequency. Figure 23.13 illustrates the difference between the apparent current, measured by an instrument, and the actual current, where / = active component of the current... 

In all electrical measurements, current and voltage measuring instruments with two terminals are employed. The object being measured similarly has two termi-... 

Table 3-2 Survey of measuring instruments for corrosion protection measurements...

Point (a) only concerns simple metal electrodes and needs to be tested for each case. Point (b) is important for the measuring instrument being used. In this respect, polarization of the reference electrode leads to less error than an ohmic voltage drop at the diaphragm. Point (c) has to be tested for every system and can result in the exclusion of certain electrode systems for certain media and require special measures to be taken. 

Fig. 3-8 Circuit for determining the response time of measuring instruments with internal resistance R...

Filters have a time constant r = R x C which increases the damping of the measuring instrument. The time constant depends on the required attenuation and the interfering frequency, but not on the internal resistance of the measuring instrument. The time constants of the shielding filter are in the same range as those of the electrochemical polarization, so that errors in the off potential are increased. Since the time constants of attenuation filters connected in tandem are added, but the attenuation factors are multiplied, it is better to have several small filters connected in series rather than one large filter. 

Fig. 3-14 Circuit of a measuring instrument for determining the apparent soil resistance by the Wenner method.

According to Ref. 32, the functioning of impressed current cathodic protection stations should be monitored every 2 months, and the stray current protection station every 1 month. If protection installations are provided with measuring instruments for current and potential, this supervision can be carried out by operating staff, so that the readings are recorded and sent to the technical department for... 

Air pollution meteorology came of age and, by 1980, mathematical models of the pollution of the atmosphere were being energetically developed. A start had been made in elucidating the photochemistry of air pollution. Air quality monitoring systems became operational throughout the world. A wide variety of measuring instruments became available. 

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