Civil Engineering Reference
In-Depth Information
• When current transformers or potential transformers are used, the CT and the PT
ratios must be taken into account. The meter readings multiplied by the meter
constant and the CT or PT ratios (or both in the case of wattmeter) give the
values of current, voltage, and power.
Typical CT ratios are 100:5, 1,000:5, 5,000:5. These values show how many
amperes flow in the primary conductor or line conductor when a 5 A current
flows in the secondary winding. For security reasons the secondary winding
must never work with no-load, and consequently it must always be connected to
a low-resistance ammeter or shorted by means of a jumper wire, screw, or switch
on a CT shorting terminal strip.
Complex metering systems can also be introduced by using current and voltage
transducers whose signals are elaborated by means of computers and transformed
into electric power and energy. The introduction of computerized systems allows
both the measurement of energy flow and the control of electric systems based on
different optimization criteria.
These systems can also be integrated conveniently with flow and other physical
parameter measurements in order to obtain complete process control. This
approach, quite common in energy-intensive industry, is becoming popular also
in many manufacturing sectors and in buildings (see Sect.
17.8
).
2.4.3 Temperature, Pressure, and Other Measurements
Devices for measuring temperatures can be grouped according to the principle of
operation and the temperature range. The commonest devices are liquid-in-glass
thermometers, bimetallic thermometers, resistance temperature devices,
thermocouples, infrared thermometers, and optical pyrometers. A correct choice
must be based on temperature ranges, environmental conditions, and uses of the
measurements for reporting or process control.
Figure
2.6
shows relationships among the Kelvin (K), Celsius (
C), Rankine
(
R), and Fahrenheit (
F) temperatures.
Devices for pressure measurement can be listed as Bourdon gage, diaphragm
gage, and manometer. They measure the difference between the absolute pressure
in a system and the absolute pressure of the atmosphere outside the measuring
device. This difference is called differential or gage pressure. If the pressure of the
system is lower than atmospheric pressure, the term vacuum pressure is used
instead of gage pressure.
The Bourdon gage consists of a curved tube closed at one end with the other end
connected to the pressure to be measured. When the pressure inside the tube is
greater than the pressure outside, the tube tends to straighten and the amount of
change in length or curvature can be translated into a gage reading.
The diaphragm gage is based on the detection of the diaphragm movement if the
pressures against its two sides are different.
